Maritime Pine, Its Biological and Silvicultural Traits for the Basis of Natural Resources: An Overview

*Teresa Fidalgo Fonseca, Ana Cristina Gonçalves and José Lousada*

## **Abstract**

Maritime pine (*Pinus pinaster* Aiton) is a forest tree species with a high representation in southwestern European countries, in particular Portugal, Spain, and France. The species traits and their flexibility and plasticity are of importance both for timber and to the sustainability of the forest systems. Extensive research has been made on the maritime pine systems and productions. The aim of this study is to review the state-of-the art on the knowledge of the species, their forest systems, and their productions, to identify vulnerabilities and to summarize tools to help its management. The specific objectives of this review are: i) characterizing maritime pine, its distribution, genetic material and provenances, the biotic and abiotic disturbances, the diversity and sustainability of its forest systems; (ii) its management, encompassing the silvicultural systems and practices; (iii) to list existing growth models, simulators and decision support systems; and (iv) present information on wood technology, including sylvotechnology, wood properties, and their use.

**Keywords:** species traits, distribution, silviculture, models, wood technology

## **1. Introduction**

Maritime pine (*Pinus pinaster* Aiton) is a conifer with a large area of distribution and of particular value, namely in terms of provisioning, regulating, and supporting ecosystem services. In Europe, its main distribution occurs in the Southwest Atlantic region (Portugal, Spain, and France), and to a lesser extent in other regions of Mediterranean influence. It has also been successfully introduced to other continents. One major benefit of the maritime pine forests, inherently associated with its expansion, is wood production and the supply of timber. The species plasticity and rusticity associated with its many functions, from production to protection, is linked to its wood quality and yields, make it a specie of primordial importance in several countries. Currently, it is prone to a suite of abiotic and biotic disturbances (e.g., fire, drought, pests, and diseases), which can act simultaneously or not. The forest system and production sustainability have to be thought holistically, with the selection of the better-suited management systems and sites to promote optimized yields and wood quality. The aim of this review is to provide information on the state of the technical knowledge of maritime pine and its forest systems.

The objectives are fourfold: (i) distribution and ecology of maritime pine (Section 2); (ii) silviculture (Section 3); (iii) models, simulators, and decision support systems (Section 4), and (iv) wood technology (Section 5).

## **2. Distribution and ecology of** *P. pinaster*

Maritime pine (*P. pinaster* Aiton) is an evergreen conifer species belonging to *Pinaceae* and *Pinus* genera. It is a plastic specie characterized by its fast growth, shade intolerance, and being rustic ([1, 2] and references therein). Its area of distribution ranges from Portugal to Greece and from Morocco to Tunisia, whether as continuous ancient or recent areas (**Figure 1**, [3]). The specie is reported as native in France, Italy, Spain, Morocco, and Portugal [4]. It can be found outside its natural range in Australia, New Zealand, South Africa, Chile, Argentina or Uruguay [5], Turkey, the Balkans, United Kingdom, and Belgium (**Figure 1**, [3]). Its distribution is probably associated with the species traits' plasticity and wood quality. The specie's prolific seed production, wind-dispersed seed, and rapid growth rate, support the qualification of the species as an aggressive colonizer in some of the countries where it was introduced [4].

Maritime pine develops for a range of mean annual temperature between 13 and 15°C, and 8 to 10° C in the colder months, mean annual precipitation larger than 800 mm (100 mm in the dry season), altitudes up to 800 m. It has low sensitivity to autumn and winter frosts, but high to spring ones, and has a high sensitivity to snow. It prefers soils of light texture, with good drainage and with a depth larger than 30 cm, where root systems develop better but do not tolerate, calcareous, saline, hydromorphic, and compacted soils. Its ability to grow in shallow and

**Figure 1.** *Area of distribution of maritime pine (source: Caudullo et al. [3]).*

*Maritime Pine, Its Biological and Silvicultural Traits for the Basis of Natural Resources… DOI: http://dx.doi.org/10.5772/intechopen.102860*

nutrient-poor sites is due to not being very demanding regarding mineral nutrition and by establishing ectomycorrhizal associations that improve its ability to uptake nutrients in soils with pH less or equal to 5 [1, 2, 6–8]. The root system consists of superficial roots, which ensure the stability of the tree and support the fine roots, responsible for the absorption of water and nutrients, and deep roots, which ensure the attachment to the soil and the tree's access to water from deeper groundwater levels [6]. It provides good anchorage regardless of soil water content, except when in full saturation in sandy soils [9]. Nonetheless, the lower the nutrients' availability the lower the potential growth of the trees [10, 11]. It reaches 30–40 m in height [12], its longevity is between 80 and 300 years [1, 2] and it is shade intolerant [2]. It resists well the summer water deficits, characteristic of the Mediterranean region, as due to the high sensitivity of the stomata to water deficit it is able to maintain tissue hydration at adequate levels [13, 14]. Its imminently pioneering character is notorious in the success of its use in the fixation of coastal dunes formed by sands poor in organic matter, minerals, and water retention capacity [2].

In France, maritime pine occupies an area of 1015 thousand hectares, with the Landes having the largest monospecific area. While it represents 5% of the metropolitan French forested area, it is the most harvested species with 6.7 Mm3 /year of removals [15]. It is also widely distributed in northwest Spain, in the Autonomous Communities of Galicia and Asturias and the province of León, and is the most important coniferous tree species in terms of both surface cover, with an area of 433,754 ha, and wood production [16, 17] with a volume harvested in 2017 of 3.4 Mm3 [16]. In Portugal mainland, its distribution extends along a coastal strip of low altitude from North to South as well as in the inner North and Central regions, up to an altitude of 700–900 m mainly under Atlantic climatic influence, and mostly in the Southwest to North aspects. It is the most represented conifer species in northern and central Portugal, occupying an area of 713.3 thousand hectares and a growing stock of 67 Mm3 [18]. Wood availability is estimated at 1.8 Mm3 , in 2018, with a consumption of 4.2 Mm3 [19]. Typical stands are shown in **Figures 2**–**4**.

**Figure 2.** *A mature stand of* Pinus pinaster *(Mata Nacional de Leiria, Portugal).*

**Figure 3.**

*Natural regeneration of* Pinus pinaster *after clearcutting (Mata Nacional de Leiria, Portugal).*

**Figure 4.** *Adult stand of* Pinus pinaster *(Vale do Tâmega, Portugal).*

The importance of maritime pine is not confined to its area, but it is also related to its economic returns and goods and services its stands and forests provide. Maritime pine major products (wood and resin) have a wide variety of uses, involving a complex forestry-industrial sector and integrating, in addition to the set associated with the transformation of wood, a range of enterprises processing

*Maritime Pine, Its Biological and Silvicultural Traits for the Basis of Natural Resources… DOI: http://dx.doi.org/10.5772/intechopen.102860*

non-woody forest raw materials, with emphasis on resinous products. Its contribution to the national economies is relevant. For example, in Portugal, this sector has 8516 companies and is responsible for 57,843 employees (representing 88% of industrial companies and 81% of employment in the Forestry Sector) and generate 1225 € million of Gross Value Added, €4348 million of Turnover, and €1876 million of exports (3.1% of national exports of goods) [19, 20].

In Portugal, it is the main wood-producing species for general purposes, which, in addition to a medium wood density, combines good strength characteristics and easy working. According to [20], of the 4.5 million m3 of *P. pinaster* wood consumed in 2019 in Portugal, 1.82 million m3 corresponded to timber wood, 1.07 million m3 to pellets, 0.68 million m3 to wood panels, 0.56 million m3 for pulp and paper, 0.20 million m3 for biomass, and 0.15 million m3 for poles, pilings, posts, and sleepers. In addition, it has also to be highlighted the production of resin extracted from this species, which in the last 8 years has ranged from 6000 to 8000 t per year [20].

The importance of this sector goes far beyond the purely economic aspects, as its stands are essential for the populations life quality, with a direct impact on the quality of air, soil, and water and, in general terms, in the surrounding ecosystem. For example, *P. pinaster* forests constitute the largest carbon reservoir in the Portuguese forest (90.3 Gg CO2) and also the most carbon stored per hectare (119.4 t CO2/ha) [20].

Maritime pine stands, due to its low crown cover, result of its shade intolerance, enable the development of an herbaceous and shrub understorey. This understory encompasses a suite of species resulting in moderate to high species richness. Also, it serves as shelter and reproduction spots for several bird, mammal, and reptile species [21]. Diversity is also enhanced by the different stand structures, from pure even-aged to mixed uneven-aged [22–24].

The sustainability of the pine stands and their productions are dependent on their resilience to disturbances, which include type, intensity, and frequency. Silvicultural practices are disturbances of low intensity and high frequency, with the aim of promoting growth. In general, its effects promote the system sustainability. Inversely, high intensity and low-frequency disturbances, such as fires or storms, may endanger the system sustainability [25]. Maritime pine stands are prone to fires, especially when a well-developed understory promotes the continuity of the vertical profile of the stand. The effects of forest fires on forest stands in general, and on maritime pine in particular, are twofold: the destruction of the stand and effects on soil. The resilience of the stand is linked to the regeneration which in turn is associated with the intensity of destruction (total or partial), type of regeneration (sexual or asexual reproduction), and the availability of seeds (whether in the soil or in the tree crowns). Maritime pine regenerates by seed (it is not able to sprout) and as long as seed is available, stand regeneration occurs [26]. It is well known the effect of vegetation on soil conservation and reduction of erosion risk, which is especially relevant in climates subject to high-intensity rainfalls, such as the Mediterranean climate. Also, vegetation, especially the arboreal, gives a primordial contribution to the maintenance and improvement of the soil's physical, chemical, and biological properties, thus contributing to maintain and improve site quality ([27] and references therein). Maritime pine stands are frequently in sites of low quality, many times in steep slopes areas with high-intensity rainfalls [28, 29]. Thus, its sustainability can be enhanced by disturbances of low intensity and high frequency, such as the silvicultural practices (thinning and pruning) that can prevent those of high intensity and low frequency, such as fires. Maritime pine is also vulnerable to wind damage [30]. Extreme wind events associated with severe extratropical cyclones (storms) have caused extensive damage in Europe. In France, Nouvelle-Aquitaine region, the damage of Martin and Klaus storms affected predominately maritime pine (37 million m3 ), which correspond to 15% and 32% of the maritime pine standing volume in the region in the former and latter storm, respectively [31]. The uprooting of trees and stem breakages have been reported for the species in Portugal [32–34], which may result from soil characteristics and individual tree social status, and the critical turning point at the base of the stem was correlated to tree size and particularly to stem weight or volume [35].

Among the biotic agents affecting the species, the pine processionary moth, *Thaumetopoea pityocampa* (*Lepidoptera*, *Thaumetopoeidae*) is referred to as the most serious pest in the Mediterranean region [4, 36]. The species is susceptible to *Bursaphelencus xylophilus*, the nematode that causes the pine wilt disease [4, 37], and to root rot pathogen *Heterobasidion annosum* [38]. Bark beetles (*Ips sexdentatus*, *Orthotomicus erosus*, *Tomicus piniperda* and *T. destruens*) are also referred to as the main biotic agents causing economic losses to the species [37].

The maritime pine stands sustainability is also linked with climatic change. The increase in temperature and decrease of precipitation may result in a trend to its northwards distribution [28, 29]. Also, it seems that there will be a trend towards a longer dry season in the Mediterranean. One way to mitigate its effects is by reducing density through thinning in maritime pine stands and/or with mixed stands [39, 40] of maritime pine with other conifer or broadleaved species (Section 3.1).

## **3. Silviculture of maritime pine (***P. pinaster***)**

#### **3.1 Forest systems**

Maritime pine is managed in high-forest stands [2] (see **Figures 2** and **4**). The structure is most frequently even-aged, whether from natural [41–44] (**Figure 3**) or artificial [2, 40, 45, 46] regeneration. Traditionally, maritime pine is managed in pure stands. The preference for even-aged stands is related to easier management, promotion of wood quantity and quality [2, 40, 47–51], and disturbances, mainly fire or harvest events that usually result in one regeneration cohort shortly after disturbance, if seed is available [41–44].

The uneven-aged structure is less frequent [22, 24, 42] probably due to the specie traits. Uneven-aged stands are more frequently developed with shade-tolerant species. Yet, uneven-aged stands have been successfully developed with shadeintolerant species with few cohorts (1 to 4) [52–54]. Several studies compare and discuss even and uneven-aged stands of maritime pine [55–57]. Uneven-aged stands of maritime pine are frequently originated from natural regeneration, whether as pure [22, 42, 47, 58] or mixed stands [23, 24, 59, 60].

The advantages of mixed stands in what concerns the stands' sustainability while attaining similar or better yields than pure stands [52] enhanced the spread of maritime pine mixed stands. Examples are: *P. pinaster* and *P. sylvestris* [46, 50, 60]; *P. pinaster* and *P. pinea*, *P. sylvestris*, *P. halepensis* or *P. nigra* [39]; *P. pinaster* and *Quercus pyrenaica* [61]; *P. pinaster* and *P. radiata* [45]; *P. pinaster*, *Castanea sativa* and *Quercus robur* [23, 24]; and *P. pinaster* and *Eucalyptus* spp. [62]. While some mixed stands are originated from plantations [45] others are the result of natural regeneration [24, 42, 62]. Overall, mixed maritime pine stands have higher diversity [24, 50]; soil fertility is enhanced [50]; have a higher water holding capacity [63], and higher yields [60].

The development of maritime pine is determined by four broad factors; water availability, aerial growing space availability, tree "social" status (based on tree's height relative to surrounding trees), and silviculture practices. Maritime pine stands in the Mediterranean climate are constrained by the available water.

#### *Maritime Pine, Its Biological and Silvicultural Traits for the Basis of Natural Resources… DOI: http://dx.doi.org/10.5772/intechopen.102860*

Several references [59, 61, 64–69] indicate that growth occurs mainly in spring and autumn as a result of precipitation [67]. A study on the effect of precipitation on water uptake in maritime pine, stresses the effects of the temporal variability of rainfall and site on the water availability [67]. As the water absorption by maritime pine individuals does not occur immediately after the rainfall but has some delay in time [67, 68], it is better explained by a set of events of rainfall [67]. Also, summer precipitation (from May to September) seems to have low contribution to the absorption of water for two reasons: the precipitation amount is low and it is partially lost through evaporation. In mixed stands of *P. pinaste*r and *Quercus pyrenaica*, spring growth of maritime pine is promoted in the early spring because leaf area is available prior to the oak's [59, 61] and due to the maritime pine root system, which is able to develop in depth thus exploring a large volume of soil [70, 71]. Also, when under water deficit, maritime pine ceases growth both in spring and fall [59, 61, 64]. In fall, trees are able to grow if water is larger than what is needed for the rehydration [59, 64]. The geographic origin along with the climate influences the tree growth reaction to drought, with higher growth under Atlantic climates than under the Mediterranean ones, which is related to the xeric climate adaptation of the species [72].

All species have, to a lower or wider extent, plasticity which enables individuals to adapt to the available growing space, by maintaining or increasing light intersection, water and nutrients absorption, and reducing competition. Species plasticity results in the variation of tree allometry, which enables the maintenance of growth. Crown plasticity can be the result of stand structure and/or climatic conditions [52]. For maritime pine individuals the increase in density results in the reduction of crown size due to crowding, when individuals do not have enough aerial growing space, or when branch abrasion occurs. These phenomena constrain the lateral growth of the crown and being maritime pine shade-intolerant, the lower crown under shade dies, resulting in the regression of the crown [39, 73]. Drought also affects crown allometry. In sites prone to drought its crown tends to have a large volume. The larger crown volume can be explained by the stands' low density, being trees in free growth thus expressing the growing habits characteristic of the specie; and as the main limiting factor is water; it is expected that belowground competition is higher than that above ground. As a consequence, the crown competition and the variability in its allometry are weaker on dry sites and stronger on humid ones [39]. Likewise, the increase of aridity decreases productivity both in pure and mixed stands, whether for volume [73] or for biomass [74].

Individual tree social status influences tree allometry and growth. In pure stands, the individuals in the lower social status (dominated) have lower sizes and growth rates, due mainly to the lower availability of growing space, light in particular. In mixed stands of *P. pinaster* and *P. sylvestris*, it was found a negative effect on dominated maritime pine individuals, probably due to the shade casted to those individuals. Inversely, in the admixtures of *P. pinea* and *P. pinaster*, and *P. nigra* and *P. pinaster,* the effects on the dominated trees were positive, which can be attributed to the different crown architecture of the species [39]. In *P. pinaster* and *P. sylvestris*, pure and mixed stands [60], maritime pine crowns in mixtures had smaller volumes (related to the specie shade intolerance), than in pure stands, and high competition for light was also found. Inversely, *P. sylvestris* tends to keep its lower branches (as it is more tolerant to shade). Also, maritime pine tends to increase its height growth to enable the individuals to reach the upper canopy layer, and, thus to reach sunlight. This results in the ascension of its crowns, which is enhanced by the crown regression (i.e., the death of the shaded lower branches) and by the development of branches with steep angles in relation to the stem. The different behavior of the two species might promote the stand vertical stratification and the optimization of

the available canopy space [60]. The former and the higher capacity to hold water off the mixed stands [63] may, at least partially, explain the increase in productivity [60], stocking, and total organic carbon [75] found in mixed stands when compared to pure maritime pine stands.

Defoliation in maritime pine individuals results in the reduction of growth, of −0.9% of increment in basal area per 1% reduction of leaf area. For 15–30% of defoliation, the reduction of growth is considerable [49]. In a drought study, Rodriguez-Vallejo [40] observed that leaf area reduction due to drought resulted in the reduction of tree growth and that in natural stands was lower than in plantations. The reduction of growth due to leaf area reduction is related to the decrease of transpiration, hydraulic conductivity, and increase in xylem embolisms as well as competition for water. Thinning reducing competition may mitigate drought impacts on tree vigor and growth in maritime pine plantations [40].

Differences in tree allometry can also be assessed based on the configuration of the tree stem profile and have a direct influence on stem volume. Calçada-Duarte [76] points to a large number of geometric volume shapes for the species, varying from paraboloid to a solid of intermediate features of cone and neiloid (stem form with high tapering), which can result in stem volume differences greater than 25% for trees with equal values of diameter at breast height and total tree height.

#### **3.2 Silvicultural practices**

The most frequent silvicultural practices in maritime pine stands are thinning and pruning. Thinning is used to regulate stand density. The goal is to maintain the best trees, that will reach the end of the production cycle and remove those that have lower growth rates (dominated), less desired stem shapes, or are dead or diseased [77] while providing intermediate economic revenues. The most frequent thinning method is from below (e. g., [2, 47]). This method is used because it is suited for shade-intolerant species and for sites with periodical drought season [77], which is the case of the maritime pine stands in the Mediterranean basin with an annual summer drought period. Thinning is of importance in these stands due to its effects on tree and stand growth; wood quality and quantity, especially when associated with pruning; and system sustainability, particularly to disturbances such as fire and drought. Due to its shade intolerance, their release should be done early in stand development [2, 78].

The thinning intensity can be based on empirical rules or defined by objective criteria, being usual to use of Wilson's spacing factor [79] or Hart-Becking spacing index (H-B), widely used in France for coniferous trees (e.g., [80]), and Stand Density Index [81], the latter based on the self-thinning theory law. Density regulation based on SDI relies on the assumption that in monospecific even-aged populations of trees experiencing complete crown closure, mortality is density-dependent. The natural trajectory of the number of trees per tree size was defined by Luis and Fonseca [82] and revised by Enes et al. [44]. The use of relative values of SDI is suitable for management purposes, as it provides information on the appropriate number of living trees for given tree size, according to the management aims (e.g., optimum growth-density interval, maximization of stand volume, or maximization of mean tree size).

Arellano-Pérez [47] in maritime pine pure even-aged stands, used thinning from below with two intensities, light (removal of 20% of basal area) and heavy (removal of 40% of basal area), and compared them with unthinned plots. The authors observed that growth in diameter was the largest in the heavy thinning plots while total and crown base were similar in all treatments. Six years after thinning basal area was the largest in unthinned plots. The fuel load was lower in

#### *Maritime Pine, Its Biological and Silvicultural Traits for the Basis of Natural Resources… DOI: http://dx.doi.org/10.5772/intechopen.102860*

thinned plots, but that of the understorey had a slight increase in the thinned plots. Thinning reduces the probability of active crown fire probability but increases passive one. Overall, according to Arellano-Pérez [47] thinning did not affect fire severity and reduced potential fire risk. The effect of density on maritime pine growth is related to competition for growing space. The higher the density attained, the lower the growth, especially in diameter [58, 83]. Stands with high density are exposed to longer periods of hydric stress, especially during the drier months. Inversely, in low density stands, individual trees develop larger (deeper and wider) root systems, thus reaching water stored in the lower soil layers [83].

Nunes et al. [84] in a thinning from below experiment in maritime pine pure even-aged stands with intensity ranging from light to heavy, highlighted its importance in diameter growth while height growth was not affected. Another study in a mixed stand of *P. pinaster* and *Quercus pyrenaica* [59] observed the highest radial increments with heavy thinning intensity. The difference between treatments corresponded to the spring growth (earlywood) and was constrained especially by water availability, *i.e.*, under drought, there was a reduction of radial growth. Inversely, the autumn radial growth (latewood) does not seem to be affected by thinning, probably because it is highly dependent on the precipitation amount [59].

Pruning is a silvicultural practice frequently associated with thinning. Its main goal is to form a knot-free wood stem as high as possible, the reduction of the knotty stem core (both in number and size) and to stop juvenile wood growth [2, 85]. Pruning is recommended for two reasons: to reduce the knots number and size, which is one of the most derogatory wood features when used for nobler applications (e.g., veneer, plywood, structural elements, and furniture), both in the wood appearance characteristics and their mechanical resistance [86, 87]; and the removal of the less photosynthetically efficient branches (frequently the lower), enabling an increase of the carbohydrate availability, thus increasing growth [88, 89]. Yet, pruning removes both dead and live branches, the latter reducing also leaf area, which may also reduce photosynthesis and thus growth [90]. Hevia et al. [45] evaluated the effect of light (12–15% crown removal) and heavy (29–37% crown removal) pruning in young (7–11 years old) pure evenaged stands of maritime pine, and compared the results with unpruned trees. The higher the pruning intensity is, the greater will be the reduction of diameter growth, while lower effects were detected for height growth. Similar results were attained by Courdier et al. [91]. The effects of pruning intensity are related to species traits, namely the architecture of the crown, leaf surface area, photosynthesis, shade tolerance, and growth rates; but also, to edaphic and climatic site characteristics [45]. Hevia et al. [45] observed that the increase of growth post pruning was related to site index, relative spacing index, age, and tree diameter, as well as stand structure prior to pruning. The authors mentioned that the better the site, the older the trees, and the larger the diameter, the higher the growth in diameter and height. The post pruning growth seems to be also linked to the reserves in carbohydrates; the larger the reserves the higher the growth ([45] and references therein).

#### **3.3 Stand regeneration**

The regeneration of a stand is linked to its forest system. Clear cutting is associated mainly with artificial regeneration while clear-cutting with standards, clearcutting by strips and/or patches, and shelterwood systems are frequently linked to natural regeneration [92, 93]. The most frequently used regeneration systems in maritime pine stands are clear-cutting, clear-cutting with standards, and clearcutting by strips [2, 46].

Natural regeneration encompasses a set of sequential steps, namely seed production, seed dispersal, germination, and seedling establishment. Maritime pine trees are self-fertile. Wind pollination helps to spread their pollen grains from the male sexual organs (cone) to the female ones. Flowering, fruiting, and seed production are dependent on the tree development stage, stand density, and climate. Maritime pine individuals start to fruit at about 10–15 years old, with a periodicity of masting cycles of 3–5 years [2]. Trees with larger dimensions produce higher cone yields. Trees with larger dimensions tend to be in the upper layer of the canopy, are more vigorous and the light crow area is larger, all of which contribute to the increase of cone production [43, 94]. The reduction of density through thinning, reducing competition, and promoting the increase of crown area, especially the outer one where flowering and fruiting occur, increases fruit yield [43, 94].

Cone full development needs 2 years to be achieved [2] and climate, especially precipitation, determines the number of mature cones per year [43, 95]. For maritime pines stands the seed production per year is enough to regenerate the stands, in spite of its interannual variability [94, 96]. Its seeds are mainly wind dispersed; thus, wind direction and intensity are key factors in its dispersal, which occurs in the summer, from June to August [2]. The mean and the maximum dispersal distances of the seed are circa 14–25 m and 54 m, respectively [97].

Germination is related to seed germination rate and predation both before and after dispersal. Maritime pine germination occurs either in spring or autumn [2] and it is dependent on nutrient availability as the seed have few reserves; water, the increase in water stress reduces the germination and survival rates; and light environment, as germination and early development of seedlings is promoted by semi-shade environments that reduce light intensity and soil temperature, and increase soil moisture [78, 94, 98]. Guignabert et al. [94] mentioned that drought in summer was the primary cause of death in seedlings, mainly due to the increase of the deficit in vapor pressure and transpiration of seedlings. Partial cutting reduced water stress, thus promoting seedling survival [94] and a crown cover of about 32% had higher germination and survival rate of seedlings when compared with a crown cover of circa 5% [98].

Guignabert et al. [94] comparing seedlings with partial cutting clearcutting observed that seed production and dispersal were not limiting factors to regeneration. Inversely, the storage and conservation of seed in the seedbank constrained germination because of the high predation after dispersal; harvest residues and litter layer did not allow seeds to reach the soil; the capacity of germination of seeds was lower on clearcutting, and the germination rate was high in the first year after seed rain (previous year to harvest) and drastically reduced in the two following years.

Seed predation is a primordial factor in maritime pine regeneration. Predation before dispersal occurs when fruits are in the maturation early stages, while predation after dispersal takes place in the ground prior to germination, mainly by birds and insects. Post dispersal seed predation happens mostly in autumn and winter and depends on seed and predators' number, frequently having a trend towards a high spatial and temporal variability [99]. Ruano et al. [96] observed that predation reduced seed of maritime pine from 400,000–500,000 seeds/ha to 10,000 seeds/ ha, and that the seed predation rate increased with the decrease of quantity of seed.

#### **3.4 Stand structure dynamics**

Stand structure dynamics is determined by the initial species composition and proportions and structure. The differences in stand structure, even if they are small, may be, and many times are, enlarged in time [25]. These differences are

#### *Maritime Pine, Its Biological and Silvicultural Traits for the Basis of Natural Resources… DOI: http://dx.doi.org/10.5772/intechopen.102860*

visible both in the estimates of the stand variables and their precision and accuracy, which reinforces the need to develop flexible models that accommodate the variability of growth patterns and interactions between individuals for the variability in stand structure [52]. Alegria [42] and Alegria and Tomé [22] developed growth models for maritime pine uneven-aged stands. In both studies, the authors referred that the existing models (developed for even-aged stands) are not able to accommodate the differences in structure, and the new models outperformed the existing ones. Gómez-García [100] developed height-diameter functions for *P. pinaster* mentioning that mixed models were able to accommodate the variability in tree allometry as well as the limitations on the available data. Riofrío et al. [46] developed height-diameter functions for *P. pinaster* and *P. sylvestris*, pure and mixed even-aged stands. The model was able to accommodate the different patterns between trees and species, and account for the different species traits, allometry, and interactions. Also, the authors reported that these models had better performance than those existing for pure even-aged stands.

In maritime pine even-aged stands, rotation can be defined for a target age or diameter. Rotation age varies between 35 and 45 years [2], though longer rotations have been used, for example in coastal dunes of Mata Nacional de Leiria (see **Figure 2**), of 70 years for timber and 100–140 years for protection [101]. The target diameter is defined according to the use of wood with 7–14 cm of diameter at breast height for panels and pulp; 14–20 cm for timber and > 35 cm for veneer wood and large dimension timber [2]. **Figure 5** presents *P. pinaster* wood logs, after logging.

Stand structure, tree growth, and silvicultural practices have a key role in wood quantity and quality. High stand density, especially in the early stages of development, promotes height growth in maritime pine stands, which shortens the period of juvenile growth of wood enabling trees to develop mature wood at early stand development stages [87, 102, 103], as well as reducing stem taper and promoting stem straightness that reduces the amount of reaction (compression) wood, thus reducing the undesirable characteristics for most wood uses [104]. However, as it is a fast-growing specie and shade-intolerant, release through non-commercial or

**Figure 5.** Pinus pinaster *wood logs.*

commercial thinning should be prescribed [2]. The reasons for the early release of competition are twofold. The release will increase diameter growth and tree mechanical stability. The mechanical tree stability is frequently accessed with the h/d ratio (ratio between total tree height and diameter at breast height, with both variables in the same units). Mechanical stability is attained for h/d lower than 85 ([105] and references therein). As already referred due to its shade intolerance maritime pine individuals, when in dense stands lose their lower branches [2] whether due to shading or branch abrasion, originating the crown regression and reduction of growth [25]. Two structure indices can be used as proxies of potential photosynthetic ability, vigor, and growth: crown ratio (cr: percent of crown length in relation to total height), which is also used for mechanical stability assessment; and linear crown ratio (lcr: percent of the crown in relation to stem diameter). For good vigor and growth cr ≥ 30% and lcr > 50%, while for a good mechanical stability cr ≥ 50% ([105] and references therein).

Spatial tree arrangements have also a determinant role in wood quality. In irregular spacing, especially in dense stands, trees can develop eccentric and tortuous or leaned stems, which reduce mechanical stability, in particular to wind and snow, and depreciate wood quality due to compression wood [85, 104].

Stem taper determines the quantity and quality of wood. Theoretically, trees in free growth tend to have stems more conical while those with narrower spacing tend to be more cylindrical. Also, maximum radial growth is higher near the crown base where carbohydrates are more available due to mechanical stress [85]. Thus, density should be suited to the development of cylindrical stems. Wood quality is also determined by the presence of branches and juvenile wood. Early pruning indicated for maritime pine [2] enables to increase in the length of the cylindrical stem, reduces the knotty stem core, and promotes the formation of mature wood [2, 85, 87, 102, 103]. Pruning in the early stand development stages, with few highintensity interventions enables an easier and faster recovery of the tree growth. The goal is to attain a knotty stem core of 1/3 or less of the diameter at breast height at the end of the production cycle [2].

Annual radial growth and its variability also determine the quantity and quality of timber. The goals are attaining a radial growth as large and as constant as possible, that maintains good wood technological properties. Thinning, redistributing the growing space by the better-suited trees that are foreseen to reach the end of the production cycle, enables to achieve the two aforementioned goals. Thinning from below and selective (Schädelin) thinning can be used [2]. In the former the trees removed are predominantly the dominated ones, thus maintaining the upper canopy. The latter is characterized by the selection of the future trees which are released from completion in thinning. This results in a trend towards higher growth rates in the latter [77]. Regarding thinning intensity, the higher the larger the radial growth, but also increases annual radial growth variability [77]. Thus, the option is between thinning of lower intensity and higher frequency or of higher intensity and lower frequency.

When the objective of forest stands is the production of quality wood, it is advisable that they be installed with reduced spacing. With this practice, the height growth is promoted (in detriment to diameter growth), in order to release the influence of the crown at the lower levels of the stem as soon as possible, reducing the amount of juvenile wood in the stem and promoting the early development of the mature wood (of better quality) in the lower levels of the stem [87, 102, 103], which are the most valuable due to their larger dimension in diameter. At the same time, the stem taper is reduced and its straightness is increased, thus also reducing the amount of compression wood, which presents undesirable characteristics for most wood uses [104].

*Maritime Pine, Its Biological and Silvicultural Traits for the Basis of Natural Resources… DOI: http://dx.doi.org/10.5772/intechopen.102860*

A profile of a radial section of maritime pine wood is shown in **Figure 6**.

## **3.5 Growth rate** *vs.* **wood quality**

Given the great importance of the effect of the growth rate on wood quality, this topic has been studied for a long time, without, however, maintaining a great controversy, even allowing any bibliographic review to be forwarded to support any of the preconceived views. Initially, it was generally accepted that, in softwoods, rapid growth was associated with low densities, but this idea was based on a simple analysis of the cross-section of the stem by comparing the wide rings with low density, located in the center of the tree (juvenile wood), and the narrow rings with high density, located close to the bark (mature wood). However, the effects of ring width and age were confounded, so that most of the problems thought to be related to wide rings were, after all, due to the age of wood formation, that is, due to juvenile wood versus mature wood [106]. Regardless of the ring width, the juvenile wood is characterized by presenting a low density, which contrasts with the high density of the mature wood. Although the juvenile wood of softwood normally presents wide rings, the narrow rings of the juvenile wood also have low densities, as well as the wide rings of the mature wood show high densities [102]. Thus, the true effect of growth rate on density (as well as on other properties) can only be well evaluated in rings of the same age [106]. Currently, it is consensual that it is the occurrence of juvenile wood (age of the growth rings) and not the growth rate in diameter (ring width) that produces the worst quality wood.

**Figure 6.** *Radial section of* Pinus pinaster *wood.*

Numerous studies carried out with resinous species in Portugal and Spain have repeatedly demonstrated the absence of correlation between ring width and wood quality characteristics [107–117] which are sufficiently clear to stop fearing, for this species growing in these regions, any hypothetical antagonism between the vigor and the wood quality. In this regard, also worth mentioning the work carried out by Fernandez-Golfin and Diez [107] on the influence of the ring width on the wood density and other physical-mechanical properties of wood in different species (among which *P. pinaster*). In addition to corroborating the reduced predictive capacity of ring width for wood density, the authors draw attention to the fact that the first research teams on wood technology were North European, so the most widespread wood quality standards came from studies carried out in these latitudes with slow and homogeneous species, as a result of reduced interannual variability. However, according to these authors, the woody material produced in Southern Europe is characterized by an enormous variability in the ring width, essentially induced by the great variability of precipitation, which, in this region, is the main limiting factor for growth. Thus, "The wood of these species and origins must be classified according to standards that take into account their growth characteristics and not using standards made to classify other species and/or provenances. In this sense, the use of the ring width as a limiting factor of wood quality (imposed by many European classification standards) only results in an unfounded technical barrier to wood from fast-growing species and/or from European southern climates, and open the doors to slow-growing species from more northern regions" [107].

## **4. Growth models, simulators, and decision support systems (DSS) for maritime pine (***P. pinaster***)**

The importance of the maritime pine, both in area and yield, has led to the development of a large number of growth and production models to support the management of this forest resource. The first growth models for maritime pine - in the form of Yield Tables - were developed in Portugal, for the Leiria National Forest, by Santos Hall [118], and in Spain, in the 1940s, by Echeverría and De Pedro [119] in the Atlantic area. In France, the production tables developed by Décourt and Lemoine [120] for the pinewoods of the SW region (Landes) were the first models published for the specie. Significant development of models followed, attesting to the interest shown in this field of modeling applied to the species by researchers and technical experts. The evolution of the models since the production tables reflected the state of the art in the respective research area at the time, and documents the contemporary approach to forest growth prediction. In general, the models that have been proposed are empirical, at the stand or tree level, aiming at the application to pure and regular *P. pinaster* stands. The Dryads model [121], for unevenaged, pure or mixed stands of *P. pinaster* and hardwoods (*Castanea* spp. and *Quercus* spp.), the PBIRROL model [122], for uneven-aged stands, should be highlighted here, due to their distinctive application, as well as the tests performed with the hybrid models, physiologically based of FOREST-BGC [123] and 3-PG [124], calibrated for the species by Lopes [125] and Alexandre [126]. Additional information about growth models can be found in Fonseca [127] and Bravo et al. [128]. Fonseca [127] presents a list of 30 models developed for the species in Portugal, and Bravo et al. [128] summarize the main models developed for the Atlantic and the Mediterranean maritime pine forests in Spain. The FORMODELS database (available at http://www.iefc.net/formodels\_database\_forest\_modeles\_liste/**) contains** a comprehensive list of 20 models developed for the species for different ranges


*Maritime Pine, Its Biological and Silvicultural Traits for the Basis of Natural Resources… DOI: http://dx.doi.org/10.5772/intechopen.102860*


#### **Table 1.**

*Simulators and web products for Pinus pinaster.*

of applicability in Portugal, Spain, and France, most of them referring to growth models and a few of other categories (biomass, mushrooms and fire behavior).

In this section we identify the simulators available for the species, presenting the references as to authorship or their reference documents and availability to users. Some of the models are hosted on platforms, namely, the CAPSIS (Computer-Aided Projection of Strategies in Silviculture) platform, see [129], the platform "Qforestry" (Quantitative forestry), the web-based application to simulate alternatives for sustainable forest management SIMANFOR [130], and the "sIMfLOR" platform, where the StandSim.dd simulator is located (**Table 1**) [141].

Although each model has its own specificities, the models produced to describe the dynamics of growth and several of them make it possible to anticipate the results of silvicultural options or management scenarios, according to predefined objectives or those to be achieved.

To support forest management, optimization models are used, usually anchored in Decision Support Systems (DSS), with the objective of obtaining optimal solutions for a given objective - usually wood production - subject to a set of constraints. Examples of optimization models for *P. pinaster* are found in Pasolodos-Tato [142], Fonseca [143], Rodil [144], and Petucco et al. [145]. In terms of supporting decision, Costa et al. [146] and Garcia-Gonzalo et al. [147] present case studies of DSS to generate management plans aimed at the production of wood for common lands and national forests, respectively, in Portugal. Other references are Falcão and Borges [148] and Garcia-Gonzalo et al. [148, 149].

## **5. Wood traits**

#### **5.1 Anatomy**

Concerning the anatomical characterization, *P. pinaster* wood shows particularly longer tracheids than most resinous woods, which gives it great axial cohesion during its mechanical performance in use. For example, while *P. pinaster* wood presents an average tracheids length of 4.35 ± 0.50 mm [150], *P. nigra* and *Cupressus lusitanica*, also growing in Portugal, present average values of 3.74 ± 0.45 mm and

*Maritime Pine, Its Biological and Silvicultural Traits for the Basis of Natural Resources… DOI: http://dx.doi.org/10.5772/intechopen.102860*

1.60 ± 0.16 mm, respectively [151, 152], *P. sylvestris* 1.73 ± 0.12 mm in Finland [153] and *Picea abies* with average values of ~2.75 mm [154], much lower than *P. pinaster*. Another important anatomical wood feature is the dimension of the lumen diameter of the earlywood tracheids, which in maritime pine is approximately 33 μ, a significantly higher value than that of *Picea abies* (27μ) and *P. sylvestris* wood (29 μ) [155]. This characteristic is reflected in the good performance of *P. Pinaster* wood in its drying behavior and preservation treatments.

### **5.2 Physical properties**

The usual air-dry wood density values of approximately 0.566 g/cm3 in 30-year-old trees are worth mentioning [156], but which can reach average values of 0.657 g/cm3 at 70 years old [150]. These values are identical to those of *P*. *nigra* (0.588 ± 0.096 g/cm3 ) [116, 117] and *P*. *sylvestris* (0.588 ± 0.101 g/cm3 ) [114, 115] with identical ages and growing in Portugal, but higher than *P*. *sylvestris* wood from Sweden, France and the Czech Republic (0.391–552 g/cm3 ) [157–160], *Picea abies* (0.410–516 g/cm3 ) [157], and *Abies balsamea* (0.351 g/cm3 ) [161].

Another important aspect is that the difference between the wood density of the earlywood and the latewood is not very high, which results in a considerable homogeneity of density within rings [108, 151], with very advantageous repercussions in terms of its workability, namely in its transformation into sheet to plywood and veneer and in the easiness of receiving connection elements (e.g., nails, screws).

The fact that *P. pinaster* wood has a relatively high density, has consequently a great dimensional instability caused by the gain or lose water during the wood drying (sorption/desorption processes), which results in tangential shrinkage values (T) between 9.1% at 10.1%; Radial (R) between 4.7% and 6.0%; Axial (L) between 0.0% and 1.0% and volumetric (V) between 14.5% and 16.7% [156, 162]. This aspect may be particularly critical in situations where wood is used outdoors, heavily exposed to adverse weather conditions. Comparatively, in softwoods it is common to find lower shrinkages, whose mean T values are usually between 5.6% and 8.3%; R between 3.1% and 5.3%, and V between 9.4% and 13.4% [163, 164]. In this way, it is imperative not only special care during the drying process but also that it only be applied after its moisture content is stabilized in the air. Additionally, it is also recommended to periodically apply insulating products (*e.g.*, paints, varnishes) to reduce these shrinkages [87, 103].

#### **5.3 Chemical properties**

In relation to chemical properties, the wide range of studies carried out on this theme has been unanimous in demonstrating a reduced variability, not only between different conifers species but even between trees of the same species. This lack of variability is notable not only in terms of variations in the macromolecule contents (cellulose, hemicelluloses, and lignin), but also in terms of the elemental chemistry. The only difference that is sometimes identified is related to the extractive content of some species, whose range values are usually from 1.5 to 5% [165–167]. In the case of maritime pine in Portugal, it usually presents relatively higher contents, between 4.2% and 9.6% [113, 168–170].

Even so, these values for *P. pinaster* are lower than those reported for the *P. sylvestris* (10.7–15.4%) and *P. nigra* wood (6.6–12.9%) growing in Portugal [115, 117]. In terms of the use of *P. pinaster* wood, these high extractive values give it some natural resistance to biodegradation (but do not prevent the need to apply preservative products in situations of outdoor use) but may cause some problems in surface finish operations.

Regarding the elementary chemistry contents, several studies have shown that the woody biomass of the *P. pinaster*, not only contain high heating value (HHV), between 20.15 and 21.60 Mj/kg, but also low undesirable elements contents, such as N, S, K, Na, Ca, Mn, Ni, Cr, Cu, F, Cl, and ashes [171–177]. Thus, the *P. pinaster* wood is one of the most suitable types of biomass for energy purposes, namely through combustion processes, given the high HHV and the low risk of sintering and corrosive effect of chloride salts and HCl on metal parts in furnace and boiler, that occurs when the halogen elements (F and Cl) are high [178–186]. Likewise, the low values of N and S also indicate a reduced risk of formation and release to the atmosphere of NOx and SOx [180, 187–190].

#### **5.4 Genetics and breeding**

Although the studies on genetic improvement of *P. pinaster* in Portugal had started in the 60s of the last century, they were focused on the characteristics of growth, form, and resistance to pests and diseases, and only in the last 25 years did the first study on the genetic control and improvement of the wood qualitative characteristics. At the moment, there is enough knowledge to recognize the existence of high genetic variability (essential to ensure good genetic gains through an improvement program) for some wood characteristics. For example, there was a high genetic control of the characteristics associated with wood density (heritability between 0.60 and 0.98), much higher than that verified for the growth characteristics in diameter (between 0.15 and 0.17), height (0.34), as well as for other wood features, such as lignin content (0.34), Radial Modulus of Rupture (0.34) and Radial Modulus of Elasticity (0.30) [108, 110, 111, 113]. Furthermore, when analyzed separately, the earlywood (formed in spring) exhibits much greater genetic dependence and is controlled over several years by the same set of genes, being the one that better results will provide in the future selection and improvement programs. In the opposite situation, the latewood, showing the lowest and most unstable heritability values, reveals that this type of wood is more strongly affected by environmental conditions than the earlywood [108, 110, 111, 113].

With regard to ring width, no adverse genetic correlations were detected between this and the wood density components. The fact that ring width is genetically and consistently positively correlated with the ring density, earlywood density, latewood percentage, and negatively with the heterogeneity index, allows us to contest, once again, the erroneous idea, but unfortunately still deeply rooted in the thinking of many researchers and wood users, that trees with higher radial growth (higher ring width) produce lower wood quality, namely lower density and latewood percentage in xylem [109, 110, 113].

These results should be sufficiently enlightening for us not to fear, for this species, any possible antagonism between the vigor and wood quality. On the contrary, it is expected that selection by the ring width will have a correlated effect in a slight increase in ring density, earlywood density, and latewood percentage (which should make it possible to reconcile good radial growths with high density), but not being accompanied by any significant changes in the latewood density, which will indirectly allow to increase the homogeneity of the growth rings. This fact is one of the most valued attributes by some of the wood processing industries. For example, the greater the homogeneity within the rings, the easier and more profitable will be the production of veneers, the greater its mechanical strength, the easier it receives the connecting elements (nails and screws), and the lower the risk of wood cracking [109, 113].

One of the places where the genetic improvement of P*. pinaster* is most advanced is in Australia, which began in 1957 and is currently in its fourth phase. The first

#### *Maritime Pine, Its Biological and Silvicultural Traits for the Basis of Natural Resources… DOI: http://dx.doi.org/10.5772/intechopen.102860*

phase was the establishment of a preliminary test of provenances that took place between 1964 and 1984 which revealed that in the growing conditions of West Australia, the provenances from Leiria (Portugal) were the most vigorous, confirming, once again, the superiority of the Atlantic provenances for growth [191]. The development in height and diameter at 10 and 20 years old was much higher in the 2 origins from Leiria, compared to those from Corsica, Landes, and Italy and, in terms of volume, the origins from Leiria presented a value greater than twice that of any of the other provenances. Furthermore, the provenances from Leiria were also the most resistant to drought (0.8% mortality, compared to 9.7% for the Landes and 10.1% for Corsica), but little to frost and with frequent stem bifurcations. The provenances from Corsica were superior in the stem straightness, while those from Leiria did not differ significantly from those from Spain and did not show a good performance in this parameter.

In the second phase of the improvement program, an attempt was made to combine, in the same individuals, the vigor characteristic of the provenances from Leiria with the stem straightness of those from Corsica, having crossed these two provenances. However, the hybrids obtained by this cross kept these 2 characteristics apart in the same individuals: either a high vigor, or a good stem configuration, but never both, simultaneously [191].

Faced with this setback, the next phase aimed to improve the stem shape while maintaining its high vigor, using material from 86 selected trees in the Leiria pine forest, which provided considerable genetic gains. According to Butcher and Hopkins [192] and Hopkins and Butcher [193] at this stage of the program, an increase in total volume production of +36% was obtained, which represents, by itself, an average increase of about 3.5 m3 ha−1 year−1 and which, complemented by a significant improvement in the stem quality by increasing their straightness by around 40% and by reducing the size of the branches by 25%, allows for an even greater increase in the total volume of usable wood.

For the fourth phase of the program, which is still in progress, the main objectives were to improve the characteristics of the branches (reduction of the insertion angle and size) and to increase the wood density, having been selected the best individuals from the best families obtained in the previous phase of the program that showed good configuration of the stem and crown, and whose average density of juvenile wood was equal to or greater than 0.430 g/cm3 [193–195].

Thus, the current knowledge about the properties and characteristics of *P. pinaster* wood allowed to identify it as a type of wood with potential for a wide range of uses, which go beyond those with less added value (packaging, pallets, and briquettes). In fact, this wood has suitable characteristics for more noble applications, such as structural applications, floors, carpentry and furniture, veneer, particleboard and plywood, poles, and sleepers.

## **6. Conclusions**

Maritime pine is a plastic species widely distributed. Its traits and stand structures as well as the quantity and quality of its wood allow a wide range of uses. The stands are managed for wood, non-woody products, and services, thus recognizing its importance both economical and as a provider service demanded by society, thus contributing to its well-being.

The large representation of the species, particularly in southern Europe, has allowed advanced research on silvicultural systems and cultural practices, and their effects on wood properties, providing clarification on less well-perceived aspects of wood quality, particularly when considering the development of the species in the

Mediterranean region. In parallel with silvicultural studies, several growth models and simulators have been developed and proposed to support management.

The challenges facing the species in the future are known, including severe weather conditions, especially drought, rural fires, storms, pests, and diseases. In addition, the systems are under pressure due to the high demand for woody material. From the extensive review carried out on maritime pine, it is noticed these challenges are part of research conducted or underway and of joint initiatives through international research projects (e.g., ForManRisk, https://formanrisk.eu/) to ensure the definition and update of management guidelines for the sustainability of maritime pine systems in the long term.

## **Acknowledgements**

Thanks are due to the Pinus Competence Center (CCPB), Pinus Center (Centro Pinus), and International Union of Forest Research Organizations (IUFRO), namely Division 1 (Silviculture), unit 1.01.10 Ecology and Silviculture of Pine, for promoting fruitful discussions on the silviculture and management of pine forests that have contributed to the organization of this chapter.

## **Funding**

For the author integrated with the research center Forest Research Centre (CEF), the research was financed by National Funds through the Portuguese funding agency, FCT (the Portuguese Foundation for Science and Technology), within project UIDB/00239/2020. For the author integrated with the MED research center, this work is funded by National Funds through FCT—Foundation for Science and Technology under the Project UIDB/05183/2020. For the author integrated with the CITAB research center, it was supported by National Funds by FCT—Portuguese Foundation for Science and Technology, under the project UIDB/04033/2020.

## **Conflict of interest**

The authors declare no conflict of interest.

*Maritime Pine, Its Biological and Silvicultural Traits for the Basis of Natural Resources… DOI: http://dx.doi.org/10.5772/intechopen.102860*

## **Author details**

Teresa Fidalgo Fonseca1,2,3\*, Ana Cristina Gonçalves4 and José Lousada1,5

1 Department of Forestry Sciences and Landscape Architecture (CIFAP), University of Trás-os-Montes and Alto Douro, Vila Real, Portugal

2 Forest Research Center (CEF), School of Agriculture, University of Lisbon, Lisboa, Portugal

3 IUFRO Division 1 - Unit Ecology and Silviculture of Pine, Austria

4 Department of Rural Engineering, School of Sciences and Technology, MED—Mediterranean Institute for Agriculture, Environment and Development, Institute of Research and Advanced Education (IIFA), University of Évora, Évora, Portugal

5 Center for the Research and Technology of Agro-Environmental and Biological Sciences (CITAB), University of Trás-os-Montes and Alto Douro, Vila Real, Portugal

\*Address all correspondence to: tfonseca@utad.pt

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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## **Chapter 2**

Complexity of Regeneration Dynamic at the Ecocline between Mixedwood and Coniferous Domains of the Southernmost Boreal Zone in Eastern North America

*Yassine Messaoud*

## **Abstract**

To explain the ecocline between the southern mixedwood and the northern coniferous bioclimatic domains dominated, respectively, by balsam fir (*Abies balsamea* (L.) Mill.) and black spruce (*Picea mariana* (Mill.) B.S.P.), 59 field sites and 7010 sample plots (from the Quebec Ministry of Forests, Wildlife, and Parks), with no major disturbances, were selected throughout the two bioclimatic domains. Regeneration (seedlings and saplings), mortality (difference between seedlings and saplings) of balsam fir, and black spruce (saplings) were examined, accounting for parental trees, main soil type (clay and till), summer growing degree-days above 5° C (GDD\_5), and total summer precipitation (May–August; PP\_MA). Balsam fir regeneration was more depended on parental trees and soil type than black spruce. Balsam fir mortality was related to seedling competition, species composition of the canopy, and the soil type. GDD\_5 and marginally PP\_MA were beneficial and detrimental for respectively balsam fir and black spruce regeneration. The ecocline mixedwood/coniferous bioclimatic domains was attributed to a northward gradual decrease of balsam fir regeneration and increase of its mortality, due to cooler temperatures, shorter growing seasons, and decrease of the parental trees. However, balsam fir persists above this ecocline, where parental trees populations and good establishment substrates occur.

**Keywords:** balsam fir, black spruce, natural regeneration, mortality, ecological conditions, climate change

## **1. Introduction**

In North America, the southern limit of the continuous boreal zone decreases in latitude from Alaska eastward [1, 2] and reaches its southernmost (*circa* 48°N) in

eastern Canada, between eastern Ontario and western Quebec. At this point, this is where the boreal zone reaches its most southerly limit worldwide, except for pockets at high elevations [3]. In Quebec, the area is composed of two bioclimatic domains that are characterized by different late-successional species in mesic sites: the southern balsam fir—paper birch (*Betula papyrifera* Marsh.) bioclimatic domain (hereafter, referred to as mixedwood forest) with some species reaching their northern distribution limit, such as sugar maple (*Acer saccharum* Marsh.), yellow birch (*Betula alleghaniensis* Britton), red pine (*Pinus resinosa* Ait.), white pine (*Pinus strobus* L.) and red maple (*Acer rubrum* L.) [4, 5], and the northern black spruce—feather moss bioclimatic domain (hereafter, referred to as coniferous forest [6]. Trembling aspen (*Populus tremuloides* Michx.), paper birch, and jack pine (*Pinus banksiana* Lamb.) are abundant immediately after fire in both bioclimatic domains.

The ecocline between the mixedwood and coniferous bioclimatic domains occurs at *circa* 49°N, which represents the shift in dominance for the two species instead of their range limit. Indeed, black spruce extends farther south into the temperate forest zone, where it reaches its southern limit at *circa* 40°N [7], while balsam fir reaches its northern limit at 54°N [8].

Balsam fir and black spruce have contrasting ecological traits. In fact, balsam fir is more shade-tolerant but less cold-tolerant than black spruce. Furthermore, balsam fir does not have seed bank, while black spruce cones containing seed remain in the canopy for several years [9]. In addition, balsam fir is less resistant to fire than black spruce given that the former has a thinner bark [10]. Therefore, extreme fire is not conducive to the regeneration of balsam fir, and the availability of seeds depends to a large extend on the living parental trees in protected areas from fire. Moreover, balsam fir is less adapted to saturated soil water conditions compared with black spruce [11].

Regeneration dynamics are closely dependent on seed availability and recruitment potential, which may be limited by many factors [12, 13]. Tree seedlings, the first stage of regeneration, are specially dependent on seed source (parental trees) and soil substrates that are suitable for their establishment [14]. Since their root systems are shallower and less extensively developed, seedlings may be unable to explore and exploit soil resources compared with later regeneration stages, which make them particularly sensitive or susceptible to spatiotemporal variability in microenvironments and the variability of regeneration niches [15].

Climate is well known to directly and indirectly limit regeneration dynamics [16, 17]. As a direct effect, low temperatures and diminished precipitations can reduce seedling survival in northern Holarctic forests [18, 19]. As indirect effects, low temperature reduces the organic matter decomposition (low soil fertility) and soil evaporation (water surplus and paludification), unsuitable substrates for establishment of many species [20]. In addition, forest fires also influence regeneration, depending upon the species' fire tolerance [21].

At the leading woody species range expansion, regeneration becomes more temperature-limited as latitude and elevation increase, thereby decreasing seedling abundance due to lower seed inputs and higher mortality that prevents them from establishing further northward [22]. Yet, fewer studies have been conducted at the transition between abutting, closed-canopy forest ecosystems [23]. In North America, some studies on tree regeneration dynamics [23, 24] have been conducted at the ecocline between the boreal and temperate zones that lies further to the south. The former is dominated by balsam fir, while the latter is dominated by sugar maple. These studies showed that although balsam fir did not reach its tailing range [2], regeneration is better adapted to colder climate, yet it is limited by sugar maple

*Complexity of Regeneration Dynamic at the Ecocline between Mixedwood and Coniferous… DOI: http://dx.doi.org/10.5772/intechopen.101565*

litter thickness. In contrast, sugar maple is limited at its leading range by soil acidity and cold temperature.

The objective of the study is to determine if the location of the ecocline between the mixedwood and coniferous bioclimatic domains is explained by the contrast regeneration dynamic between the two dominant tree species. We expect a decrease of balsam fir regeneration and increase of its mortality in the coniferous bioclimatic domain compared with mixedwood domain, due to unfavorable climate and substrate conditions. What makes this study unique is that it is conducted not only between two forested areas, but also at the shift between two species dominance. In addition, we verify to what extent the maintenance of balsam fir and black populations is linked with parental trees and regeneration potential driven by climate and site conditions. This chapter was derived from the two projects conducted by Messaoud *et al.* [25, 26].

### **2. Study design**

#### **2.1 Survey area**

The area was located in western Quebec (Canada) and is a part of the Quebec and Ontario Clay Belt, formed by lacustrine sediments left by the former proglacial lake Barlow-Ojibway ([27]; **Figure 1**). The altitude ranges from 300 and 400 m asl, with low hills scattered in a flat landscape. The area has a continental climate, with cold winters and warm summers. Yet, climate differs between the two bioclimatic domains (**Table 1**).

#### **Figure 1.**

*Study area showing the field sites (a) and sample plots for balsam fir (b) and black spruce (c) stands that were established for the collection of forest inventory data by Ministry of forest, fauna, and parks of Quebec (MFFPQ). The bold red line that runs from east to west depicts the boundary between the southern mixedwood and northern coniferous bioclimatic domains in western Quebec (after [6, 25, 26]).*


**Table 1.**

*Climatic variables for the western bioclimatic domains.*

#### **2.2 Sampling design**

#### *2.2.1 Field design*

Fifty-nine closed-stand sites were selected along a latitudinal gradient throughout the ecocline between the mixedwood and coniferous domains (**Figure 1a**). These sites are located on public land, easily accessible from nearby roads, with no major disturbance, had a moderate moisture regime [28], and the surface deposits consist of either clay or till.

In each site, a transect was established perpendicular to the nearby road, where five circular plots (40 m2 ) were randomly selected independently from each other. Within each plot, balsam fir dynamics were surveyed by counting the number of seedlings (diameter at breast height, DBH < 2 cm), saplings (DBH between 2 and 9 cm inclusively), and mature trees (≥10 cm DBH). DBH of the mature trees was measured. The data from all five plots in each site were combined to compute an average number of seedlings, saplings, and mature trees to account for plot variability. These data were subjected to a basic negative exponential model, which may properly reflect the depletion of balsam fir populations over a very short period of time, for example, from seedlings to saplings [29]. This model assumes a consistent balsam fir mortality rate for different age classes at a given site. As a result, we computed the difference between seedling and sapling numbers, which were also reported as percentages, to assess absolute mortality between two age classes in each site.

#### *2.2.2 Inventory design*

A total of 7010 sample plots (400 m<sup>2</sup> each) characterized by closed-stand and no major disturbance were provided by the Quebec Ministry of Forest, Wildlife, and Parks (MFFPQ) and used to test the natural regeneration of balsam fir and black spruce in the mixedwood and coniferous domains (**Figure 1b** and **c**; [30]). Balsam fir was more common in the mixedwood bioclimatic domain than black spruce, which was more common in the coniferous ones. Within each sample plot, balsam fir and black spruce saplings (2 cm ≤ DBH < 10 cm) were counted and measured including all mature trees, irrespectively for species (DBH ≥ 10 cm) and downloaded from the database, as well as with latitude, longitude, elevation, and soil type (clay or till, which are the dominant parent materials in both forests). The basal area (m<sup>2</sup> /ha) of mature trees in each plot was estimated and then converted to a percentage of total mature trees. To classify the evolution of the sites toward black spruce or balsam fir, a threshold was used as follows: when the proportion is 60% or more of black spruce, the site is considered to be developing toward black spruce dominance. Whereas when the proportion of balsam fir was ≥40% of the coniferous component, the site is considered to be developing toward balsam fir

dominance. This threshold was set based on the competitive ability and shade tolerance of the two species; under similar condition, balsam fir is generally more competitive and more shade-tolerant than black spruce [2, 10]. In addition, disturbed sites and sites with <20% balsam fir or spruce in the canopy and jack pine sites were excluded from the analyses [11].

## **2.3 Climate**

Geographic locations (latitude, longitude, and elevation) of each sample plot or site were used to extract climate variables using the BIOSIM11 modeling software (h ttps://cfs.nrcan.gc.ca/projects/133). Climate variables concerned cumulative growing degree-days >5°C (GDD\_5) and total summer precipitation (May to August, PP\_MA, mm). The GDD\_5 threshold is the temperature at which plant growth begins. The influence of climate on regeneration was determined using averages of climatic parameters corresponding to climate normals for the years 1981–2010 [31].

### **2.4 Statistical analyses**

A MIXED procedure of SAS software (V 9.1, SAS Institute Inc., Cary, N.C., USA) was used to estimate the parameters of Eqs. (1) and (2), adjusted to field and inventory data, respectively:

$$\begin{array}{l} \mathbf{Y}\_{\vec{\eta}} = \rho\_0 + \rho\_1 \mathbf{B} \mathbf{D}\_{\vec{\eta}} + \rho\_2 \mathbf{P}\_{\vec{\eta}} + \rho\_3 \mathbf{S}\_{\vec{\eta}} + \rho\_4 \mathbf{G}\_{\mathbf{P}-\vec{\eta}} + \rho\_5 \mathbf{G}\_{\mathbf{T}-\vec{\eta}} + \rho\_6 \mathbf{G}\_{\mathbf{S}-\vec{\eta}} + \rho\_7 \mathbf{B} \mathbf{D}\_{\vec{\eta}} \mathbf{S}\_{\vec{\eta}} \\ \quad + \rho\_8 \mathbf{B} \mathbf{D}\_{\vec{\eta}} \mathbf{G}\_{\mathbf{P}-\vec{\eta}} + \rho\_9 \mathbf{B} \mathbf{D}\_{\vec{\eta}} \mathbf{S}\_{\vec{\eta}} \mathbf{G}\_{\mathbf{P}-\vec{\eta}} + \mathbf{e}\_{\vec{\eta}} \end{array} \tag{1}$$

where *Y* is the dependent variable indicating the germination or mortality rate (in percent) for bioclimatic domain *i* and site *j*. *BD* indicates the bioclimatic domain, *P* is the sample plot, *S* is the soil type, GP is the parental tree basal area (m2 ha�<sup>1</sup> ), GS is the other tree species basal area (m2 ha�<sup>1</sup> ), and <sup>ε</sup>*ij* (ε*ij* � N(0, <sup>σ</sup><sup>2</sup> )) is a normally distributed error term. The influence of soil on regeneration or mortality between bioclimatic domains was represented by the interaction between bioclimatic domain and soil type. The influence of parental tree basal area on regeneration or mortality was represented by the interaction between *BD* and GP. For deduction purposes, the bioclimatic domain was considered as a random variable. To consider for distinction in total basal area that could differentially influence the regeneration or mortality, GT was included in the model as a covariate. Climate influences on regeneration were evaluated employing a different model including GDD\_5 and PP\_MA, together with their interaction with the bioclimatic domain. All comparisons were conducted using *t* tests with significant difference being declared at *p* < 0.05.

$$\begin{aligned} Y\_{ijk} &= \beta\_0 + \beta\_1 B \mathbf{D}\_i + \beta\_2 \mathbf{P}\_{\vec{\eta}} + \beta\_3 \mathbf{S} \mathbf{P}\_i + \beta\_4 \mathbf{S}\_{\vec{\eta}k} + \beta\_5 \mathbf{S} \mathbf{B} \mathbf{A}\_{\vec{\eta}k} + \beta\_6 \mathbf{T}\_{\text{BA}\_{\vec{\eta}k}} + \beta\_7 B \mathbf{D}\_{\vec{\eta}k} \mathbf{S}\_{\mathbf{B}} \mathbf{A}\_{\vec{\eta}k} \\ &+ \beta\_8 \mathbf{F}\_i \mathbf{S}\_{\vec{\eta}k} \mathbf{T}\_{\mathbf{B}} \mathbf{A}\_{\vec{\eta}k} + \varepsilon\_{\vec{\eta}k} \end{aligned} \tag{2}$$

where *Y* is the response variable indicating the sapling number for the *i*th bioclimatic domain, *j*th sample plot and the *k*th species. The explained fixed effect variables are *BD* for the bioclimatic domain, *P* for sample plot, *SP* for species, *S* for soil type, and S\_BA for parental tree basal area. To consider for total basal area of a given sample plot, affecting somehow regeneration, total sample plot basal area (T\_BA) was included as a covariate. To evaluate how regeneration was affected by a combination of numerous site factors, we also included in the model interactions between bioclimatic domain, soil type, and parental trees or total basal area. The model

intercept is *β*0, while *β*1–*β*<sup>7</sup> are the parameters to be estimated for the explained fixed effects and their interactions. The error term, ε*ijk*, was expected to be normally distributed (ε*ijk* � N(0, <sup>σ</sup><sup>2</sup> )). Before analyses, and to meet the assumption of homoskedasticity of the residuals, sapling number and basal area were subjected to natural logarithmic transformation. The main categorical influences as bioclimatic domain, species, and soil type were evaluated using the PDIFF option of the LSMEANS statement. The interaction factors including categorical and continuous variables were evaluated by contrast analyses using the ESTIMATE statement. Altogether, an influence was considered significant for *p* < 0.05 based on t-tests of the fixed effects. To test the influence of climate on regeneration, relationships between regeneration and GDD\_5 and PP\_MA were achieved using correlation analyses.

For a given species, an index that was named "anomaly of sapling abundance" (ASA) was assessed at the sample plot scale, as the difference between plot sapling abundance (SAP) and mean sapling abundance of the total study area (SAM). Subsequently, ASA was plotted against the percentage basal area of parental trees (SBA) to calculate a threshold of percent parental tree basal area from which a species maintains itself within the overall mean of the total study area (same value as the mean) or "overflows" (above this mean). A comparative method was carried out at the level of bioclimatic domain and another one controlling for both bioclimatic domains and soil type, using Eq. (3):

$$\begin{cases} \mathbf{A\_{SA}} = a + b \mathbf{S\_{BA}} \\ \mathbf{SAp} = \mathbf{S\_{AM}} \mathbf{S\_{BA}} = \text{mean threshold} \\ \mathbf{SAp} \mathbf{S\_{AM}} \mathbf{S\_{BA}} = \text{overflow} \end{cases} \tag{3}$$

In Eq. (3), the coefficients a and b are derived from a regression analysis. Prior to analyses, we tested the possible multicollinearity between the explaining variables. The results showed that for both data sources, the Pearson correlation values between different explaining variables were mostly below 0.600 or not significant (α < 0.05; **Table A1**).

### **3. Results and discussion**

#### **3.1 Seedling dynamic**

Balsam fir seedlings abundance was positively and strongly linked with parental tree basal area regardless for the soil type (**Figure 2a**), indicating the importance of the seed source of its parental trees, which agrees with a previous study [24]. As well, the proximity of parental trees appears to be crucial for effective regeneration, suggesting that the large size of balsam fir seed decreases their dispersal capacity [10, 32]. Furthermore, for a similar parental tree basal area, balsam fir regeneration was higher in the mixedwood than in the coniferous domain. Controlling for the soil type, balsam fir showed overall significantly higher regeneration on till than on clay soils (*p* = 0.022; **Figure 2a**). In the mixedwood bioclimatic domain, there was no evidence of the soil type effect on the regeneration. In contrast, a significantly higher regeneration was observed on till compared with clay soils in coniferous bioclimatic domain (*p* < 0.001). On till soils, basal area of other tree species had no effect on balsam fir regeneration, while it decreased and increased balsam fir regeneration in the mixedwood and coniferous bioclimatic domains, respectively (*p* = 0.016; **Figure 2b**), indicating a less suitability for balsam fir regeneration. Clay soils exhibit low temperatures and high water-holding capacity, slowing the organic matter decomposition, which leads to lower nutrient availability [33].

*Complexity of Regeneration Dynamic at the Ecocline between Mixedwood and Coniferous… DOI: http://dx.doi.org/10.5772/intechopen.101565*

#### **Figure 2.**

*Abundance of balsam fir seedlings (normal log of the number of seedlings per hectare (log abundance)) with basal area (m2 ha<sup>1</sup> ) of (a) parental trees and (b) other tree species according to bioclimatic domain and soil type. The dark green triangles and solid lines represent clay soils, while the dark green circles and dashed lines indicate till soils in the mixedwood bioclimatic domain. The light green triangles and solid lines indicate clay soils, while the light green circles indicate till soils in the coniferous domain [25].*

The anomaly of balsam fir seedling abundance, representing the deviation from the mean of the total seedling abundance of the study area, was significant and positive in both bioclimatic domains except on clay soil in coniferous bioclimatic domain, where it was negative (**Figure 3**). Messaoud *et al.* [9] found in the same region a higher balsam fir reproductive capacity in the mixedwood than in the

#### **Figure 3.**

*Anomaly of balsam fir seedling abundance (normal log of the number of seedlings per hectare) according to bioclimatic domain and soil type. An anomaly represents a deviation from the mean of the total seedling abundance of the study area (positive = overflow, and negative = lower seedlings than the average). Inset panels indicate the basal area (m2 ha<sup>1</sup> ) between the two bioclimatic domains (MXD, mixedwood; CNS, coniferous) for (a) parental trees of balsam fir and (b) other tree species. The letters at the top of the histograms indicate nonsignificant (same letters) or significant (different letters) differences between the mixedwood and coniferous bioclimatic domains. The lowercase letters show the comparison between till and clay soils within each bioclimatic domain (modified from [25]).*

coniferous domains. For a similar balsam fir basal area, there are more seed trees involved in the regeneration in the mixedwood than in the coniferous domains. However, we found a highest positive value on till soil in coniferous bioclimatic domain and secondly on clay soil in mixedwood bioclimatic domain. Controlling for the bioclimatic domain, anomaly values were higher on clay than on till soil in mixedwood bioclimatic domain, although it was not significant. Yet, the difference was strongly significant in coniferous bioclimatic domain, which corresponded with the lower parental trees basal area (results not shown). However, in mixedwood bioclimatic domain, the importance of the parental trees was higher on till than on clay soil. Thus, our findings confirm the importance of the substrate for the regeneration success, besides the strong importance of parental tree seed source (**Figure 3a** and **b**). Indeed, in coniferous bioclimatic domain, where climate is colder, clay appears to be detrimental for seedlings establishment because of its water surplus and cold temperature [33], whereas till soil became more suitable for seedling success. Note that water surplus (clay) appears to have greater effect than a possible water deficit due to more water evaporated till on the shallow seedling roots. This is obvious since forest canopy shading protects seedlings from a high soil evaporation and prevents from a high water demand of seedlings, whereas it might add the negative effect of low temperature and water surplus.

In addition, there was not only a positive relationship between balsam fir seedling mortality rate and parental tree basal area (*p* = 0.045), but also an unexpected higher mortality in the mixedwood compared with the coniferous domain (*p* < 0.004; **Figure 4a**) regardless for the soil type. Balsam fir seedling mortality declined with increasing basal area of other tree species (*p* = 0.020; **Figure 4b**). Thus, balsam fir mortality appears to be related to its seedling density

#### **Figure 4.**

*Mortality rate of balsam fir seedlings according to bioclimatic domain and soil type. The inset panels indicate the basal area (m<sup>2</sup> ha<sup>1</sup> ) between the mixedwood and coniferous bioclimatic domains for (a) parental trees of balsam fir and (b) other tree species. The dark green triangles and solid lines indicate the mixedwood bioclimatic domain, while the light green circles and solid lines indicate the coniferous domain. The letters at the top of the histograms indicate nonsignificant (same letters) or significant (different letters) differences between the mixedwood and coniferous bioclimatic domains. The lowercase letters show the comparison between till and clay soils within each bioclimatic domain (modified from [25]).*

#### *Complexity of Regeneration Dynamic at the Ecocline between Mixedwood and Coniferous… DOI: http://dx.doi.org/10.5772/intechopen.101565*

(competition) associated with the high seed source (parental trees). Understory shrubs and herbaceous plants are quite nonexistent in our site (personal observation), excluding any competition other than between balsam fir seedlings. In the mixedwood bioclimatic domain, seedlings are in similar shading environments; therefore, the competition between balsam fir seedlings may be for water and nutrients owing to similar quantity of soil resources being shared between a higher number of balsam fir seedlings. However, the other species had no effect or decreased mortality respectively in mixedwood and coniferous bioclimatic domains, probably due to the abundance of deciduous tree species, which may promote balsam fir establishment, notably in coniferous bioclimatic domain [34]. Another unexpected finding is that mortality rate was higher in mixedwood than in coniferous bioclimatic domains, less obvious on till soil, adding the effect of the competition between the seedlings (**Figure 4**). Moreover, higher mortality on till soils may also result from dryer conditions characterizing till soils, which easily release water during drainage or through evaporation as mentioned above [35].

The effect of climate on balsam fir seedling dynamic demonstrated a positive relationship with the growing degree-days above 5°C (GDD\_5; *p* < 0.001; **Table 2**) regardless for the soil type, while total summer precipitation (PP\_MA) had a negative effect on clay soil. As well, a decline of GDD\_5 was observed in the coniferous bioclimatic domain compared with the mixedwood domain (**Figure A1**). Thus, lower balsam fir regeneration found in the coniferous bioclimatic domain could be linked to decrease of GDD\_5 and increase of PP\_MA, which worsen the water surplus of clay soil, hence showing an additional soil effect on regeneration. To test the effect of drought on regeneration, we used the interaction between GDD\_5 and PP\_MA [36]. The results demonstrated a positive and significant relationship with balsam fir regeneration irrespectively of the soil type, indicating a positive effect of drought. In the study area, balsam fir seedlings were in the shade of the canopy trees, where soil moisture is pretty much higher than in opened area. Furthermore, the positive effect of drought appears to prevent seedlings from a water surplus occurring on clay soil. In addition, balsam fir seedlings occurred mostly on a particular substrate such as woody mounds and thin moss cover [37], known to have a high water capacity, which may explain the absence of the PP\_MA effect and the positive effect of drought on seedling abundance [38, 39].

#### **3.2 Sapling dynamic**

Balsam fir sapling abundance was significantly higher in the mixedwood than in the coniferous bioclimatic domain, while it was opposite for black spruce (**Table 3**). Moreover, this tendency was also similar for both species irrespectively of soil type. The abundance of balsam fir sapling was greater on till than on clay soils in mixedwood, whereas it was alike within the coniferous bioclimatic domain. In


#### **Table 2.**

*Person correlation (r) between balsam fir seedlings abundance and climate variables summer growing degreedays above 5°C (GDD\_5) total summer precipitation (May to August; PP\_MA), and the interaction between the two climate variables accounting for soil type (till or clay).*


*Superscripts indicate nonsignificant (same letter) or significant (different letters) differences between mixedwood and coniferous domains. Uppercase letters are comparisons between bioclimatic domains, while lowercase letters are comparisons between soil types within bioclimatic domain.*

#### **Table 3.**

*Sapling abundance of balsam fir and black spruce (means, standard errors in parentheses) according to bioclimatic domain and soil type.*

contrast, black spruce sapling abundance was higher on clay than on till in both bioclimatic domains. Our results demonstrated a significant positive role of the parental trees as seed sources on regeneration abundance for both species (*p* < 0.001). Balsam fir sapling abundance and parental tree basal area exhibited a positive relationship on both soil type, regardless of bioclimatic domain (**Figure 5a**). We noticed that although the model demonstrated no significant

#### **Figure 5.**

*Abundance of balsam fir and black spruce saplings (ln(stems/ha)) with basal for parental trees (a and b) and for the stand (c and d), according to bioclimatic domain and soil type. The dark green solid and dashed lines respectively indicate clay and till soils in the mixedwood domain, while the light green solid and dashed lines respectively indicate clay and till soils in the coniferous domain [26].*

*Complexity of Regeneration Dynamic at the Ecocline between Mixedwood and Coniferous… DOI: http://dx.doi.org/10.5772/intechopen.101565*

relationship, sapling abundance was greater on till than on clay soils in the mixedwood bioclimatic domain. As well, the relationship between saplings abundance and parental tree basal area was stronger in the mixedwood than in the coniferous bioclimatic domain on clay soils. With respect to black spruce, the results indicated a decline sapling abundance linked to increasing parental tree basal area, excluding on till and barely on clay soils in the mixedwood bioclimatic domain, where the relationship between sapling abundance and parental tree basal area was positive (**Figure 5b**). The relationship between sapling abundance and parental tree basal area was also stronger on clay soils. Furthermore, greater abundance of black spruce sapling arose in the coniferous domain (*p* = 0.027). For balsam fir and regardless of soil type, total basal area positively impacted sapling abundance in the mixedwood, but negatively influenced it in the coniferous bioclimatic domain, although the general distinction prevailed nonsignificantly (**Figure 5c**). The relationship between black spruce sapling abundance and total basal area was negative, irrespectively of soil type or bioclimatic domain (**Figure 5d**). The negative impact of total basal area on regeneration seemed to be significantly stronger on till soils in the mixedwood (*p* < 0.001) and coniferous (*p* = 0.003) bioclimatic domains. In differentiating both bioclimatic domains, the negative influence of total basal area on abundance of black spruce sapling was more grounded in the coniferous than in the mixedwood bioclimatic domain, regardless of the soil types (*p* < 0.001). This indicates that with the increase of the parent tree base area, the influence of soil type on black spruce regeneration is greatly reduced, which may be due to the convergence of soil temperature and organic layer thickness conditions between the forest domain and the soil type. Therefore, stands dominated by black spruce seem to change its own microclimate and soil conditions by increasing soil moisture and lowering the soil temperature, which is conducive to promoting its seedling abundance. However, on clay soils in the coniferous bioclimatic domain, black spruce regeneration is more likely to be negatively affected, due to soil moisture saturation and soil temperature decrease, which both lead to paludification [31, 40].

**Figure 6** shows a different regeneration pattern between balsam fir and black spruce. Balsam fir parental tree basal area was higher in mixedwood bioclimatic domain, while the opposite was true for black spruce in coniferous bioclimatic domain, which overlapped with regeneration abundance. Again, the presence of nearby seed trees has been previously reported as an important factor explaining regeneration abundance [41, 42]. For comparable parental tree basal area between both bioclimatic domains, regeneration for balsam fir was higher in the mixedwood domain, with black spruce demonstrating higher regeneration in the coniferous domain. As well as to parental tree influences on regeneration abundance, their potential seed production appears to play a crucial role in regeneration. As noticed before, balsam fir of comparable basal area has been demonstrated to produce fewer seeds in the coniferous than in the mixedwood bioclimatic domain [9], resulting in lower subsequent regeneration in the coniferous domain. In the same study, Messaoud *et al.* [9] found that black spruce showed similar seed production between the two bioclimatic domains. Thus, the regeneration pattern seems to be related to the reproductive capacity of both species. However, the reproductive capacity is not enough for the regeneration success. Indeed, balsam fir saplings abundance is still below the average of the total saplings in the study area on the clay soil, more pronounced in coniferous than in mixedwood bioclimatic domains, while it was only positive in mixedwood bioclimatic domain on the till soil. For black spruce, regeneration abundance was positive regardless of the bioclimatic domains and soil type, except on till soil in mixedwood bioclimatic domain. The negative effect of clay soils on balsam fir has already been reported in a previous

#### **Figure 6.**

*Anomaly of balsam fir and black spruce sapling abundance ln(stems/ha)), according to bioclimatic domain and soil type. The anomaly represents the deviation from mean abundance of saplings for each species across the entire study area (positive = sapling excess, negative = sapling deficiency). Inset panels indicate percentage of basal area between the two bioclimatic domains (MXD = mixedwood, CNS = coniferous) for parental trees of balsam fir (a), black spruce (b), and the absolute values of the basal area for other tree species (c), and for the stand (d). The dark and light green histograms indicate the mixedwood and coniferous domains, respectively. The letters at the top of the histograms indicate nonsignificant (same letters) or significant (different letters) differences between the mixedwood and coniferous bioclimatic domains. The lowercase letters show the comparison between till and clay soils within each bioclimatic domain (modified from [26]).*

research [11]. Negative influences of clay soils on balsam fir appear be related to higher water content often characterizing clay soils, having lower temperatures and lower rates of evaporation. Saplings of balsam fir growing on clay are bound to encounter oxygen deprivation in the rooting zone due to waterlogging and reduced gas exchange, conditions that are transcendently found in the coniferous domain. It has been additionally mentioned that lower temperatures in clay soils seem to promote increased organic matter accumulation, unfavorable for balsam fir establishment and survival [11, 33]. This suggests that site species composition, local climate, or soil characteristics are also factors, determining regeneration success, especially for balsam fir. In addition, the hard link between parental trees and regeneration for balsam fir on the one hand and the noticed inconsistency noted for black spruce on the other proposes that balsam fir is more dependent on parental tree proximity compared with black spruce. Contrasting large balsam fir seeds falling beneath or close to their parent tree, the small size of black spruce seeds allows them to spread further (effective distances of 20–80 m; [10, 40] from the parent tree. The basal area of other species on sites that are primarily composed of balsam fir or black spruce can be used as a proxy for forest composition, which could influence sapling abundance to some extent. In fact, basal area of other tree species dominated by deciduous species was greater in the mixedwood than in the coniferous bioclimatic domain (**Tables A2** and **A3**). The presence of deciduous species, such as paper birch or

#### *Complexity of Regeneration Dynamic at the Ecocline between Mixedwood and Coniferous… DOI: http://dx.doi.org/10.5772/intechopen.101565*

trembling aspen, two dominant deciduous species in our study area, may favor suitable conditions (e.g., higher soil temperatures, increased organic layer decomposition, and impeded paludification), hence increasing sapling survival, especially for the balsam fir [34]. Indeed, our results confirmed the higher sensibility of balsam fir regeneration to the environment condition than black spruce. In addition, large balsam fir seeds can be protected from seed predation and competition with herbaceous plants by a broadleaf litter layer, as long as it arises from the small leave trees such as birch and aspen [24, 43, 44]. In contrast, thicker broadleaf litter layers appear to be detrimental for small seeded tree species, such as black spruce, because their seeds contain fewer nutritional reserves for germination and sufficient root elongation to penetrate the mineral soil through the litter layer [45]. Thus, the lower density of other species in the coniferous domain could trigger a population shift from warmer balsam fir conditions to colder conditions to which black spruce and associated species are better adapted [10]. We would posit that a higher diversity of forest composition exerts two major effects upon the regeneration: facilitation for balsam fir and exclusion for black spruce. Yet, the effects of exclusion were linked more indirectly to the negative effects of broadleaf litterfall on black spruce regeneration, as previously mentioned.

To maintain mean regeneration, balsam fir required on clay soil at least 40% and 44% of parental trees in the mixedwood and coniferous bioclimatic domains, respectively. However, the requirement was only 27% and 37% of parental trees on till soil in the mixedwood and in the coniferous domain, respectively (**Figure 7a**). Conversely,

#### **Figure 7.**

*Dark green and gray circled values pertain to the respective clay and till soils inside a given forest domain. Scatter plots and trend lines depict relationships between the anomaly of sapling abundance and percent basal area of parental trees. The dark green solid and dashed lines respectively indicate clay and till soils in the mixedwood domain, while the light green solid and dashed lines respectively indicate clay and till soils in the coniferous domain.*

#### *Conifers - Recent Advances*

black spruce required 82–84% of parental trees to maintain mean regeneration, except on clay soils in the coniferous bioclimatic domain (73%; **Figure 7b**). This indicates that balsam fir required more parental trees (seed source) to maintain mean regeneration in coniferous than in mixedwood bioclimatic domains. This contrasts with black spruce, which requires higher numbers of parental trees than does balsam fir, although the percentage was slightly lower in coniferous bioclimatic domain, especially in clay soil. This might be explained by lower black spruce seed inputs compared with those of balsam fir, due the smaller semi-serotinous cones constituting an aerial seed bank for former, which can release small quantities of seed continuously but episodically with the occurrence of fire [9, 46]. Another explanation may be related to higher seedling mortality within the low light understory for the less shade-tolerant black spruce. Another interesting finding is that black spruce regeneration required an equivalent percentage of parental trees basal area, except on clay soil in coniferous bioclimatic domain, where the percentage was the lowest. Conversely, balsam fir regeneration required more parental trees basal area on clay than on till soils regardless of the bioclimatic domain, adding the stronger role of the substrate on the balsam fir regeneration success compared with black spruce.

The effect of the climate on regeneration showed a contrasting effect of GDD\_5, with a positive and negative relationship on balsam fir and black spruce regeneration, respectively (**Table 4**), illustrating greater adaptation to warmer environments that was shown by balsam fir compared with black spruce [10]. The effect of GDD\_5 was stronger and positive because clay is known to be colder, with a greater water-holding capacity than till soils [33, 47], which are not only warmer, but are subject to greater rates of moisture evaporation [48]. Although, saplings have deeper roots than seedlings, saplings are tall individual, making them less shaded by the forest canopy. This may explain the positive and the absence of the PP\_MA on balsam fir saplings on till and clay soils, respectively. Another explanation is that the absence of significant effect of PP\_MA on balsam fir saplings on clay soil could be due to the contrasting effect of the clay. Indeed, in mixedwood bioclimatic domain, clay soil prevents balsam fir regeneration from the water stress due to warmer temperature, while it has a negative effect in coniferous bioclimatic domain due to its lower temperature and higher amount of water, unfavorable condition for balsam fir establishment. The negative effect of GDD\_5 on black spruce regeneration irrespectively of the soil type confirmed its lower tolerance to higher temperatures than balsam fir [10], which are found mostly in southern locations. More, the effect of PP\_MA on black spruce regeneration was negative on till and not significant on clay soils. On clay soils, black spruce demonstrated its adaptation to cold soil temperatures and higher water content [49]. The negative relationship between PP\_MA and black spruce regeneration found on till soils appears to be due to competition for water with coexisting deciduous species, since till soils are subject to greater water drainage and higher rates of evaporation. The interaction between GDD\_5 and PP\_MA, indicating the drought effect, showed a significant positive and negative relationship for balsam fir and black spruce


#### **Table 4.**

*Person correlations (r) between species saplings abundance and climate variables according to the soil type.*

#### *Complexity of Regeneration Dynamic at the Ecocline between Mixedwood and Coniferous… DOI: http://dx.doi.org/10.5772/intechopen.101565*

regeneration, respectively of the soil type (**Table 4**). This indicates that drought did not influence negatively on balsam fir regeneration, while it did on black spruce regeneration. Therefore, balsam fir seems to be more drought-tolerant compared with black spruce, explaining the adaptation of balsam fir to warmer conditions compared with cooler and moister conditions for black spruce [10]. Unexpectedly, the positive effect of drought on balsam fir regeneration was more obvious on till than on clay soils (*r* = 0.189 vs. 0.129), probably due to the higher occurrence of till soil in central and eastern parts of the study area, where precipitation increases eastward (**Figure A1**). Thus, balsam fir was less adapted to the occurrence greater soil moisture levels. Another explanation is that since black spruce is less shade-tolerant than balsam fir [10], black spruce regeneration occurs under lower forest cover and, thus, is more exposed to drought conditions [44, 50]. Although, levels of PP\_MA were comparable between the two bioclimatic domains, drought seemed to arise more frequently in the warmer mixedwood than in the cooler coniferous bioclimatic domain (**Figure A1**).

The ecocline between the two bioclimatic domains of eastern North America constitutes a shift from balsam fir to black spruce dominance rather than the northern limit of balsam fir, which extends further north (54°; [7]). This clarifies the persistence of scattered balsam fir populations in the matrix of black spruce in coniferous bioclimatic domain, where reduced regeneration did not deal with the stability of such populations, just as long as a minimum parental tree basal area remained to support mean regeneration. The insight provided by the current study agrees with prior findings indicating that these few balsam fir populations can seemingly exist for a long time in the absence of severe disturbance such as fire [8]. Indeed, wildfire is a major disturbance in the boreal forest, and it can have a significant impact on the composition and dynamics of the vegetation at any particular place [21, 51]. Balsam fir is well known to be fire-intolerant species because its thin bark offers weak protection against fires [10]. Unlike black spruce, balsam fir cannot maintain a seedbank in the tree crown. Thus, its low abundance has been linked to large and intense fire regimes [10]. Furthermore, large fires may kill the parental trees of balsam fir, compromising regeneration success [52]. In contrast, black spruce is well adapted to fire because of its thick bark, which offers efficient protection against fire. Also, black spruce benefits more from large fires, owing to its aerial seed bank that persists in serotinous cones remaining in the canopy for many years [9], until a fire opens them to liberate seeds.

## **4. Conclusion**

The ecocline between the mixedwood and coniferous bioclimatic domains of boreal zone in eastern North America represents a shift of the balsam fir and black spruce dominance below and above this ecocline. Thus, a decline of this dominance strongly impacted the regeneration success for both species, especially for balsam fir. Soil type played a different role below and above this ecocline depending on the species, the bioclimatic domain, climate, and at the lesser extend the regeneration stage (seedlings vs. saplings). However, in coniferous bioclimatic domain, clay soil (lower temperature and higher water holding), increase of black spruce, and decrease of deciduous stand tree basal area, lower air temperature affected negatively balsam fir regeneration. Unexpectedly, morality rate was higher in mixedwood than in coniferous bioclimatic domains regardless for the soil type and higher on till than on clay soils in coniferous bioclimatic domain. This highlights the importance of the competition as the regeneration density increases. In addition, mortality rate declines with increasing the basal area of other tree species in coniferous bioclimatic domain, adding the importance of

deciduous species promoting balsam fir regeneration in more limiting environment. The results also confirmed the contrasting adaptation of balsam fir and black spruce to the temperature and drought conditions through their regeneration dynamic. To maintain regeneration equivalent to the mean for the entire study area, balsam fir and black spruce required, according to the soil type, respectively 4–10% more and 2–10% less parental trees in the coniferous than in the mixedwood bioclimatic domain. This is related to the lower adaptation of balsam fir to lower temperatures and shorter growing seasons in coniferous bioclimatic domain. Moreover, the parental trees basal area requirement for the black spruce regeneration success was pretty much less affected by the bioclimatic domain and soil type, except on clay soil in coniferous bioclimatic domain, where it was the most successful. Conversely, this requirement for the balsam fir success was associated to the bioclimatic domain and to the soil type, but higher on clay soil regardless of the bioclimatic domain, confirming the lower adaptation of balsam fir to the more water-holding soil compared with black spruce. Nonetheless, the threshold of parental tree basal area required for species regeneration to be equal to the mean study area is lower for balsam fir compared with black spruce, irrespectively of the bioclimatic domains, explaining the occurrence of mixedwood balsam fir populations well above the ecocline between the two bioclimatic domains. Consequently, our research confirms that this ecocline does not reflect the northern limit of balsam fir species, as shown by scattered but viable balsam fir populations found in the matrix of the coniferous forest domain, even further north of the ecocline. Another new insight given by this study is the both spatial scales (local and landscape) that were tended to, which significantly builds our comprehension of vegetation dynamics in the boreal biome inside the setting of future global change.

## **Acknowledgements**

This work received no funding.

## **Conflict of interest**

There is no conflict of Interest.

## **Notes/thanks/other declarations**

No declaration.

## **A. Appendix**


*Complexity of Regeneration Dynamic at the Ecocline between Mixedwood and Coniferous… DOI: http://dx.doi.org/10.5772/intechopen.101565*


*BF\_BA and BS\_BA indicate the basal area for balsam fir and black spruce respectively, while Other\_BA and Total\_BA indicate the basal area for other tree species and the basal area of the stand respectively. In bold significant level (0.01 < α < 0.05, \*0.001 > α < 0.01, \*\*α < 0.001.*

#### **Table A1.**

*Pearson correlation coefficient between the different basal areas.*

#### **Figure A1.**

*Distribution of the climate variables in the study area: (a) growing degree-days; and (b) total summer precipitation (May–August). Red line indicates the boundary between the mixedwood and coniferous bioclimatic domains.*


#### **Table A2.**

*Composition of canopy species other than black spruce and balsam fir in the study area and their mean basal area (m2 ha<sup>1</sup> ) in each bioclimatic domain.*

*Complexity of Regeneration Dynamic at the Ecocline between Mixedwood and Coniferous… DOI: http://dx.doi.org/10.5772/intechopen.101565*


*The uppercase superscript on each mean value indicates a nonsignificant (same letter) or significant (different letters) difference between mixedwood and coniferous bioclimatic domains. Standard errors are in parentheses.*

#### **Table A3.**

*Average proportion of deciduous species on sites that were dominated by balsam fir or black spruce regeneration in the mixedwood and coniferous bioclimatic domains.*

## **Author details**

Yassine Messaoud University of Quebec in Abitibi-Temiscamingue, Rouyn-Noranda, Canada

\*Address all correspondence to: ymessaou@lakeheadu.ca

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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## **Chapter 3**
