**3.1 General forest stand spatial characteristics**

There are three main forest stand characteristics that have a strong influence on spatial stand features. They are stand origin, structure and composition. Stand structure relates to the vertical stratification with different tree heights occupying

### **Figure 2.**

*Forest stand origin, structure and composition have an important role on stand characteristics, influencing many functional attributes of the ecosystem. Different combinations may lead to different silvicultural systems; some are represented here: (a–d) high-forest; (e–f) coppice; (a) pure even-aged; (b) mixed two-storied; (c) pure uneven-aged; (d) mixed uneven-aged; (e) simple coppice; (f) mixed uneven-aged coppice. The figure shows the vertical and horizontal distribution (spatial pattern) of the trees within the stand.*

*Spatial and Temporal Variability Regarding Forest: From Tree to the Landscape DOI: http://dx.doi.org/10.5772/intechopen.91701*

different canopy layers. Tree origin (seminal or vegetative) and species composition (pure or mixed) will also affect the stand stratification because of their different tree height and growth pattern. The combination and levels of stand origin, structure and composition lead to different silvicultural systems. These different forest stand components have a strong influence on the ecosystem functional processes that operate on both spatial and temporal scales (**Figure 2**). They affect stand yield, as well as forest ecologic and social functions. This also means that they have different silvicultural importance. Furthermore, their natural dispersion pattern within the stand also plays an important role and may introduce additional spatial variability (**Figure 3**).

#### **Figure 3.**

extension temporal occupation. The cohort may also be referred to as singular or multiple if it results from one or more disturbance events. At the landscape level, different forestland uses may occur depending on the disturbance regime, the species, site conditions and objectives. A mosaic can be established with different compositions and structures formed by cohorts with different characteristics. Small stands do not behave like large stands since the edge is very much influenced by the

*Spatial Variability in Environmental Science - Patterns, Processes, and Analyses*

The direct and indirect effects of human disturbances on ecosystems and biological diversity are subject of debate and concern at various levels [7, 17–19]. Human activity has been affecting deeply forest and landscape characteristics for centuries or millennia (e.g., Ellenberg [20]). Patterns of land and forest use by man are also forms of spatial influence on ecosystems affecting various aspects such as connectivity or the edge effect on habitats. In many situations, we are witnessing a deterioration of habitats and destruction of biological balances at various territorial scales. Any effect on the ecological balance, or on any of its components, has

There are three main forest stand characteristics that have a strong influence on

spatial stand features. They are stand origin, structure and composition. Stand structure relates to the vertical stratification with different tree heights occupying

*Forest stand origin, structure and composition have an important role on stand characteristics, influencing many functional attributes of the ecosystem. Different combinations may lead to different silvicultural systems; some are represented here: (a–d) high-forest; (e–f) coppice; (a) pure even-aged; (b) mixed two-storied; (c) pure uneven-aged; (d) mixed uneven-aged; (e) simple coppice; (f) mixed uneven-aged coppice. The figure*

*shows the vertical and horizontal distribution (spatial pattern) of the trees within the stand.*

adjacent area.

**Figure 2.**

**118**

repercussions on the entire ecological system.

**3. The forest ecosystem in space and time**

**3.1 General forest stand spatial characteristics**

*Natural tree horizontal dispersion patterns in forest stands, seen from above. A circle may represent a tree-unit of a different origin, size or species (white and gray circles represent different tree-units). Illustrated cases of stands with 1 and 2 combination of trees. Some trees and species tend to dominate stand composition and coverage appearing in large spatial groups (a), for a certain period of time. Others tend to naturally appear scattered with an isolated pattern (b) or in small groups (c) across the stand, while others may occur with a larger coverage in the stand (d). Besides a horizontal dispersion pattern, there is also a vertical canopy or stand stratification (Figure 2) according to the species, site conditions, tree size, stand dynamics and silviculture. These different vertical and horizontal tree and species occurrences and dispersion patterns introduce possibilities for spatial variability.*

#### **3.2 Ecological succession: the forest ecosystem in time and space**

The initial concepts concerning the ecological succession were guided to relatively predictable developmental stages of the ecosystem in general, and of plant communities in particular, in a succession of stages to a certain climax state [21]. Through ecological succession, with temporal changes in the vegetation, with biotic interaction processes, facilitation and inter- and intraspecific competition, as well as changes in habitat itself, are reached at a given time, a state of equilibrium with the climate, which results in a more stable condition and functional evolution. In this classic model of Clements, the succession consists of a predictable temporal

sequence of plant communities, each modifying the environment and creating conditions for subsequent communities. This notion would be contrasted with an interpretation made by Gleason [22] where the plant communities are the result of processes of adaptation and individual development to environmental conditions, shaping the ecological continuous concept of vegetation. On the other hand, in several situations, the succession is greatly influenced by the initial plant species composition. In addition to the facilitation, other biotic processes are involved such as colonization, competition, tolerance, inhibition and survival, as well as other biotic interactions like herbivory and mutualism, which may lead to different dynamics [10, 23]. Later, other authors showed that plant communities did not behave in a simplistic way as postulated by Clements, but where the environment factors could give rise to different pathways and climax states in a given climatic region [7, 10, 17, 24–26]. On the other hand, the climax state was seen as a relatively stable equilibrium condition. Currently, the succession is mostly understood as a dynamic process of re-equilibriums and adaptations, in response to external disturbances and as a result of internal development processes of the ecosystem. The concept of succession has thus become more complex, where the prediction on the vegetation and ecosystem dynamics requires local specific information about the site characteristics, the type of disturbance, the composition and biology of the species. Many of the initial concepts included equilibrium characteristics related to the flows of energy and matter, tropic interactions and population dynamics. Complementary and alternative approaches developed concepts related to the temporal and spatial variability, the nonlinear dynamics and complex systems. On the other hand, the ecosystems are subjected to changes and adaptive processes of wider temporal scales as well as related with climate variations [11].

large changes as the system evolves [10, 11]. The disturbances and heterogeneity are

The different vegetation components of a forest stand are important for the ecosystem functioning. Feedback processes are also involved, which allow the development of self-regulation mechanisms. For example, less visible organisms (e.g., fungi) play important functions such as the formation of a good soil (decomposition, recycling of nutrients and formation of humic compounds), in a variety of biotic relationships indispensable for the ecosystem functioning. The temporal and spatial fluctuations, as well as the connections, are important aspects of the forest

The resistance and resilience concepts are related to the ecosystem dynamics, with their ability to absorb disturbances and recover to a given state. Some studies show that complexity offers greater stability to the ecosystem [10, 31–33]. The multiple interrelationships between a population and the community contribute to stability situations. The complex adaptive systems take into account the diversity and heterogeneity. They promote self-regulation, in which the reciprocal interactions within the system between the structure and processes contribute to the regulation, organization and dynamics. Different initial conditions are directed for a stable situation, becoming relatively robust for certain disturbances, where the system components adapt. On the other hand, in simplified or unstable systems,

The forest development stages provide an idea about the changes that operate on a forest stand as regards the structure, composition and ecosystem processes associated with the dynamics of a population of trees. These stages seek to provide a general framework in which certain conditions and procedures are more prevalent. They occur successively and may also involve processes that operate at different sizes and moments in the stand. These variations are related to the concept of dynamic equilibrium of the forest ecosystem. The ecosystem functionality will be linked to structural, compositional and population dynamics characteristics.

Several authors (West et al. [7]; Oliver and Larson [9]; Spies [34]) recognized

**Figure 4** shows the evolution of total biomass at different development stages, after a clearcutting. In the re-organization stage, a loss of total biomass occurs, where growth and living biomass accumulation begin. In the stem exclusion stage, the ecosystem accumulates biomass to a certain point. In the transition stage, the total biomass decreases slightly until it stops in a fluctuating way in the old-growth or durable mosaic stage. The biomass reaches a maximum at the beginning of the transition stage, decreasing and stabilizing subsequently as a result of mortality of dominant trees that are replaced by smaller trees. Carbon retention in the living and dead components of the ecosystem may also reach a maximum at this stage. Throughout these stages, a development of the stand structure occurs through

interdependent factors, creating opportunities for recolonization.

*DOI: http://dx.doi.org/10.5772/intechopen.91701*

*Spatial and Temporal Variability Regarding Forest: From Tree to the Landscape*

small disturbances may have a destabilizing and destructive effect.

the following stages in the development process of a forest stand:

• Establishment, initiation or re-organization stage

• Transition or understory re-initiation stage

• Old-growth or shifting mosaic stage

**3.3 Forest stand development stages**

• Stem exclusion stage

**121**

ecosystem dynamics.

In addition to the temporal aspects, successional processes at the landscape scale with mosaic dynamics are also important to be considered [4, 6, 27]. In this sense, both the community and the ecosystem are landscape properties responding to changes in environmental gradients. One feature is the occurrence of successional stages across the landscape and time, gaps and patches of different sizes and trees of different growing stages within a stand.

The concept of *forest ecosystem dynamics* covers several notions, namely: the ecosystem is an open system; the ecosystems and landscapes are dynamic; the disturbance is a critical element of the system; the ecosystem is controlled by biotic and physical processes that occur at different spatial and temporal scales with levels of biological hierarchy; the succession does not necessarily follow the same pathway and ends at the same point of equilibrium; the spatial pattern is important for biological diversity; the interaction between ecosystem processes and landscape dynamics is important for biodiversity; past and recent human activities have an impact on ecosystems currently perceived as natural [3, 6, 19, 28].

The dynamics of the forest ecosystem and the temporal and spatial heterogeneity are related. The successional processes, disturbances and changes in the site factors create a complex of situations where forest communities develop (dynamic patches), which can be more wide and not necessarily in equilibrium [14, 15, 29]. Biotic interactions are also important, as are results from herbivores or pathogens and may in some cases be crucial in the development of the forest stand. The spatial pattern of the forest can itself have a strong influence on population dynamics and ecosystem processes. For example, habitat connectivity has a major effect on the abundance and persistence of certain species [30]. Therefore, besides the attributes of a certain forest, it is also important to consider the stand landscape context.

More recently emerged notions related to complex systems linked to the ecosystem dynamics. Profound changes may occur from small variations of the initial conditions. In sensitive systems, small changes to the initial conditions can result in

### *Spatial and Temporal Variability Regarding Forest: From Tree to the Landscape DOI: http://dx.doi.org/10.5772/intechopen.91701*

large changes as the system evolves [10, 11]. The disturbances and heterogeneity are interdependent factors, creating opportunities for recolonization.

The different vegetation components of a forest stand are important for the ecosystem functioning. Feedback processes are also involved, which allow the development of self-regulation mechanisms. For example, less visible organisms (e.g., fungi) play important functions such as the formation of a good soil (decomposition, recycling of nutrients and formation of humic compounds), in a variety of biotic relationships indispensable for the ecosystem functioning. The temporal and spatial fluctuations, as well as the connections, are important aspects of the forest ecosystem dynamics.

The resistance and resilience concepts are related to the ecosystem dynamics, with their ability to absorb disturbances and recover to a given state. Some studies show that complexity offers greater stability to the ecosystem [10, 31–33]. The multiple interrelationships between a population and the community contribute to stability situations. The complex adaptive systems take into account the diversity and heterogeneity. They promote self-regulation, in which the reciprocal interactions within the system between the structure and processes contribute to the regulation, organization and dynamics. Different initial conditions are directed for a stable situation, becoming relatively robust for certain disturbances, where the system components adapt. On the other hand, in simplified or unstable systems, small disturbances may have a destabilizing and destructive effect.

#### **3.3 Forest stand development stages**

sequence of plant communities, each modifying the environment and creating conditions for subsequent communities. This notion would be contrasted with an interpretation made by Gleason [22] where the plant communities are the result of processes of adaptation and individual development to environmental conditions, shaping the ecological continuous concept of vegetation. On the other hand, in several situations, the succession is greatly influenced by the initial plant species composition. In addition to the facilitation, other biotic processes are involved such as colonization, competition, tolerance, inhibition and survival, as well as other biotic interactions like herbivory and mutualism, which may lead to different dynamics [10, 23]. Later, other authors showed that plant communities did not behave in a simplistic way as postulated by Clements, but where the environment factors could give rise to different pathways and climax states in a given climatic region [7, 10, 17, 24–26]. On the other hand, the climax state was seen as a relatively stable equilibrium condition. Currently, the succession is mostly understood as a dynamic process of re-equilibriums and adaptations, in response to external disturbances and as a result of internal development processes of the ecosystem. The concept of succession has thus become more complex, where the prediction on the vegetation and ecosystem dynamics requires local specific information about the site characteristics, the type of disturbance, the composition and biology of the species. Many of the initial concepts included equilibrium characteristics related to the flows of energy and matter, tropic interactions and population dynamics. Complementary and alternative approaches developed concepts related to the temporal and spatial variability, the nonlinear dynamics and complex systems. On the other hand, the ecosystems are subjected to changes and adaptive processes of wider

*Spatial Variability in Environmental Science - Patterns, Processes, and Analyses*

temporal scales as well as related with climate variations [11].

impact on ecosystems currently perceived as natural [3, 6, 19, 28].

different growing stages within a stand.

**120**

In addition to the temporal aspects, successional processes at the landscape scale with mosaic dynamics are also important to be considered [4, 6, 27]. In this sense, both the community and the ecosystem are landscape properties responding to changes in environmental gradients. One feature is the occurrence of successional stages across the landscape and time, gaps and patches of different sizes and trees of

The concept of *forest ecosystem dynamics* covers several notions, namely: the ecosystem is an open system; the ecosystems and landscapes are dynamic; the disturbance is a critical element of the system; the ecosystem is controlled by biotic and physical processes that occur at different spatial and temporal scales with levels of biological hierarchy; the succession does not necessarily follow the same pathway and ends at the same point of equilibrium; the spatial pattern is important for biological diversity; the interaction between ecosystem processes and landscape dynamics is important for biodiversity; past and recent human activities have an

The dynamics of the forest ecosystem and the temporal and spatial heterogeneity are related. The successional processes, disturbances and changes in the site factors create a complex of situations where forest communities develop (dynamic patches), which can be more wide and not necessarily in equilibrium [14, 15, 29]. Biotic interactions are also important, as are results from herbivores or pathogens and may in some cases be crucial in the development of the forest stand. The spatial pattern of the forest can itself have a strong influence on population dynamics and ecosystem processes. For example, habitat connectivity has a major effect on the abundance and persistence of certain species [30]. Therefore, besides the attributes of a certain forest, it is also important to consider the stand landscape context.

More recently emerged notions related to complex systems linked to the ecosys-

tem dynamics. Profound changes may occur from small variations of the initial conditions. In sensitive systems, small changes to the initial conditions can result in

The forest development stages provide an idea about the changes that operate on a forest stand as regards the structure, composition and ecosystem processes associated with the dynamics of a population of trees. These stages seek to provide a general framework in which certain conditions and procedures are more prevalent. They occur successively and may also involve processes that operate at different sizes and moments in the stand. These variations are related to the concept of dynamic equilibrium of the forest ecosystem. The ecosystem functionality will be linked to structural, compositional and population dynamics characteristics.

Several authors (West et al. [7]; Oliver and Larson [9]; Spies [34]) recognized the following stages in the development process of a forest stand:


**Figure 4** shows the evolution of total biomass at different development stages, after a clearcutting. In the re-organization stage, a loss of total biomass occurs, where growth and living biomass accumulation begin. In the stem exclusion stage, the ecosystem accumulates biomass to a certain point. In the transition stage, the total biomass decreases slightly until it stops in a fluctuating way in the old-growth or durable mosaic stage. The biomass reaches a maximum at the beginning of the transition stage, decreasing and stabilizing subsequently as a result of mortality of dominant trees that are replaced by smaller trees. Carbon retention in the living and dead components of the ecosystem may also reach a maximum at this stage. Throughout these stages, a development of the stand structure occurs through

The restoration of vegetation leads to a progressive reduction of soil erosion. Nevertheless, some studies show that even several years after clearcutting, having attained a full canopy coverage, which may extend for 15 years or more, may still register important decrease in the soil organic matter and nutrients until full control is restored [35, 37, 38] which in turn affect attributes such as soil water holding capacity and carbon storage (**Figure 5**). A reduction in soil thickness may also occur, which may extend for a long time after a clearcutting as a result of the effects

*Spatial and Temporal Variability Regarding Forest: From Tree to the Landscape*

The reduction of transpiration and soil water holding capacity has a pronounced

*Following a clearcutting, on a broadleaved stand (*Acer, Betula, Fagus, Fraxinus *and* Prunus*), a degradation of the soil organic matter occurs that extends up to 15 years and a loss of 51% of the initial content. The recovery*

*to initial values may take nearly 40 years (adapted from Covington [39]).*

effect on the hydrological cycle. There are frequent situations where rainfall produces flooding. Situations involving the transport of particles and soil erosion become more problematic. This stage ends when the living and dead biomass are accumulated in the ecosystem and end the decline of organic matter, and the restoration of biotic regulation of the hydrology and nutrient exportation occurs. In the stem exclusion stage, a progressive reduction of tree density typically occurs as a result of the intense competition among the trees. The more evident characteristic is a rapid accumulation of biomass with a competitive exclusion of many individuals where mortality is very much dependent on the population density. The net primary productivity can be very high and some characteristics of the population, such as leaf area, can reach a maximum. The loss of nutrients at this stage is lower due to intensive use of existing resources. The stand instability against unfavorable atmospheric events may be higher due to a high population density. The tree mortality is more intense particularly in the lower and intermediate classes of light-demanding species, so there may be a reduction of the species diversity compared to the first stage. At this stage, the canopy is relatively uniform and there are few gaps. Canopy openings as a result of death of individual trees are of small size. Variations in the growing space, the species, tolerance, age, genetics, competition, site characteristics and external factors influence the growth pattern. The density of the dominant trees decreases as its size increases. Compared to the previous stage, there is a better regulation capacity of the energy flow, hydrological and nutrients through the biotic and abiotic components of the ecosystem. Another important feature is the regulation of the chemical composition of the drain water.

in many ecosystem processes [39, 40].

*DOI: http://dx.doi.org/10.5772/intechopen.91701*

**Figure 5.**

**123**

#### **Figure 4.**

*Evolution of total biomass at different stages of stand development, after a clearcutting (adapted from Bormann and Likens [35]). Stages are delimited by changes in total biomass (living and dead biomass), assuming a natural development without exogenous disturbances.*

different ways depending on the species, site conditions and the dynamics of the stand itself.

Old-growth stands receive a special attention from the point of view of ecology, conservation and forestry, addressing aspects related to biocenosis, genetics, ecosystem, management and the landscape. The various definitions on this stage or stand type show the diversity of interest. In its definition, structural elements as well as the state of the development process should be considered. According to Spies [34], it is a forest ecosystem distinguished by the presence of very old trees for the particular soil and climate conditions in which it occurs, showing certain morphological and growth characteristics.

The first phase occurs after the occurrence of a disturbance in which new individuals are established. The structural complexity varies depending on the type of disturbance and the present biological elements. Relative fast changes occur on the forest environment, the competition level, the species dominance and the population structure. At this stage, there is a great diversity of species, which may decrease as the space is being colonized by trees [36]. A severe disturbance leads to a regression of the forest ecosystem to an earlier stage of ecological succession. The development pattern after this event is also greatly influenced by the present floristic composition. The relative importance of species can vary in time and space according to the reproduction and growth strategies, and modifications of species dominance may happen.

At this stage, as a result of the disturbance, there is a loss of biotic regulation of the system. Hydrological and biogeochemical parameters are changed and deregulated. In turn, there is a temporary increase in the availability of resources, as well as an increase in solar radiation at the soil surface. The clearcutting has a strong effect on many ecosystem processes and greatly modifies the regulation ability of radiation energy flow as well as the hydrological and nutrient cycles. With the removal of the forest cover, the microenvironment is affected, with an increase of the soil temperature as well as various processes such as the absorption of nutrients and water, transpiration, the absorption and reflection of solar radiation, the primary production and the production of litter. The system's ability to store water and nutrients is greatly affected. The decomposition of organic matter is accelerated. There is a loss of soil organic matter and an increase of soil acidity.

### *Spatial and Temporal Variability Regarding Forest: From Tree to the Landscape DOI: http://dx.doi.org/10.5772/intechopen.91701*

The restoration of vegetation leads to a progressive reduction of soil erosion. Nevertheless, some studies show that even several years after clearcutting, having attained a full canopy coverage, which may extend for 15 years or more, may still register important decrease in the soil organic matter and nutrients until full control is restored [35, 37, 38] which in turn affect attributes such as soil water holding capacity and carbon storage (**Figure 5**). A reduction in soil thickness may also occur, which may extend for a long time after a clearcutting as a result of the effects in many ecosystem processes [39, 40].

The reduction of transpiration and soil water holding capacity has a pronounced effect on the hydrological cycle. There are frequent situations where rainfall produces flooding. Situations involving the transport of particles and soil erosion become more problematic. This stage ends when the living and dead biomass are accumulated in the ecosystem and end the decline of organic matter, and the restoration of biotic regulation of the hydrology and nutrient exportation occurs.

In the stem exclusion stage, a progressive reduction of tree density typically occurs as a result of the intense competition among the trees. The more evident characteristic is a rapid accumulation of biomass with a competitive exclusion of many individuals where mortality is very much dependent on the population density. The net primary productivity can be very high and some characteristics of the population, such as leaf area, can reach a maximum. The loss of nutrients at this stage is lower due to intensive use of existing resources. The stand instability against unfavorable atmospheric events may be higher due to a high population density. The tree mortality is more intense particularly in the lower and intermediate classes of light-demanding species, so there may be a reduction of the species diversity compared to the first stage. At this stage, the canopy is relatively uniform and there are few gaps. Canopy openings as a result of death of individual trees are of small size. Variations in the growing space, the species, tolerance, age, genetics, competition, site characteristics and external factors influence the growth pattern. The density of the dominant trees decreases as its size increases. Compared to the previous stage, there is a better regulation capacity of the energy flow, hydrological and nutrients through the biotic and abiotic components of the ecosystem. Another important feature is the regulation of the chemical composition of the drain water.

#### **Figure 5.**

different ways depending on the species, site conditions and the dynamics of the

*Evolution of total biomass at different stages of stand development, after a clearcutting (adapted from Bormann and Likens [35]). Stages are delimited by changes in total biomass (living and dead biomass), assuming a*

*Spatial Variability in Environmental Science - Patterns, Processes, and Analyses*

The first phase occurs after the occurrence of a disturbance in which new individuals are established. The structural complexity varies depending on the type of disturbance and the present biological elements. Relative fast changes occur on the forest environment, the competition level, the species dominance and the population structure. At this stage, there is a great diversity of species, which may decrease as the space is being colonized by trees [36]. A severe disturbance leads to a regression of the forest ecosystem to an earlier stage of ecological succession. The development pattern after this event is also greatly influenced by the present floristic composition. The relative importance of species can vary in time and space according to the reproduction and growth strategies, and modifications of species

At this stage, as a result of the disturbance, there is a loss of biotic regulation of

deregulated. In turn, there is a temporary increase in the availability of resources, as well as an increase in solar radiation at the soil surface. The clearcutting has a strong effect on many ecosystem processes and greatly modifies the regulation ability of radiation energy flow as well as the hydrological and nutrient cycles. With the removal of the forest cover, the microenvironment is affected, with an increase of the soil temperature as well as various processes such as the absorption of nutrients and water, transpiration, the absorption and reflection of solar radiation, the primary production and the production of litter. The system's ability to store water and nutrients is greatly affected. The decomposition of organic matter is accelerated.

the system. Hydrological and biogeochemical parameters are changed and

There is a loss of soil organic matter and an increase of soil acidity.

Old-growth stands receive a special attention from the point of view of ecology, conservation and forestry, addressing aspects related to biocenosis, genetics, ecosystem, management and the landscape. The various definitions on this stage or stand type show the diversity of interest. In its definition, structural elements as well as the state of the development process should be considered. According to Spies [34], it is a forest ecosystem distinguished by the presence of very old trees for the particular soil and climate conditions in which it occurs, showing certain mor-

stand itself.

**Figure 4.**

phological and growth characteristics.

*natural development without exogenous disturbances.*

dominance may happen.

**122**

*Following a clearcutting, on a broadleaved stand (*Acer, Betula, Fagus, Fraxinus *and* Prunus*), a degradation of the soil organic matter occurs that extends up to 15 years and a loss of 51% of the initial content. The recovery to initial values may take nearly 40 years (adapted from Covington [39]).*

In the transition stage, some authors recognize that in some types of forest there may be steps or sub-stages (understory release, maturation, early transition, and oldgrowth and late-transition old-growth) [41]. Two major steps may occur: a transition phase and a steady-state or shifting gap phase. Gradual changes occur in the population, the structure and development process, which together may have a very variable duration. The living biomass and diversity of forms reach a maximum. The initial group of trees disintegrates gradually, the mortality of lower trees increases, and a new group of trees may gradually grow in gaps. Some authors such as Oliver and Larson [9] recognize a stage of re-initiation, where a new group of trees grows in the understory. A transitional phase to an old-growth is developed where initial trees are also present. Compared to the previous stage, a progressive decrease of total biomass up to a more or less stable level occurs. The amount of dead wood tends to be more or less stable, fluctuating around a certain value. Species diversity increases where endogenous disturbances become more important. The death of trees leads to changes in microclimate conditions and resources. Canopy gaps promote the availability of resources, which are used by pre-existing or new regeneration. The occupation that occurs will promote stand stratification. This stage presents a great stability and resilience of the ecosystem to destabilizing events.

**4. Tree-level variability**

pattern, while others are more scattered.

*DOI: http://dx.doi.org/10.5772/intechopen.91701*

**5. Forest ecosystems in the landscape**

tainability of the forest.

**125**

**Figure 6.**

*and variability.*

Trees have various attributes such as the species, age, size, anatomical features and the dispersion or occurrence pattern (**Figure 3**). Another dimension is related to the function that a given tree may have depending on its characteristics, location and silvicultural options (**Figure 6**). This aspect introduces an additional element of variability. These functions may be related to aspects such as: production; protection; education; regeneration; biodiversity; and aesthetics. In turn, different species present distinct natural dispersion patterns. Certain species occur on an aggregate

*Spatial and Temporal Variability Regarding Forest: From Tree to the Landscape*

The presence of certain trees with particular biodiversity objectives and provid-

ing tree-related microhabitat structures is also an important aspect to consider (*habitat trees*). These are living or dead trees with singular anatomical characteristics or providing ecological niches of interest to a wide range of various life forms including rare and endangered species. Anatomical features such as tree size, snags, branching variations, broken top, dead branches, stem cracks, fork crack, rotten wood and stem cavities are of interest. In some cases, these might be remarkable and monumental or veteran trees. Different studies have shown that the presence of large trees, cavernous and dead trees, standing or down, has an important contribution to biodiversity [13, 42]. These microhabitats support a complex biological network, providing food, shelter and reproduction space, contributing to the ecosystem functioning. Certain species are particularly associated with these habitats, being important conservation components (e.g., saproxylic fungi and insects).

The forest has vital importance at the landscape scale, going far beyond the production of materials and energy, called *tradable goods*. The other functions of the forest in the landscape are the conservation of water, soil and biodiversity. These supporting ecosystem services are interrelated with other forest-related ecosystem services, such as climate regulation, bioclimatic comfort and other cultural services (landscape contemplation, recreation and cultural heritage). The quality of the ecosystem services provided depends on the principles of landscape planning, followed by the technicians and policy-makers, which will also determine the sus-

*Representation of some functions attributed to trees. Diverse tree characteristics may provide different functions*

The old-growth or shifting mosaic stage is characterized by a pattern of relatively small disturbances, resulting in gaps of different sizes, which create conditions for the establishment of new trees and growth of trees from the lower and middle layers. The aggregation and dynamics of these small disturbances, and tree response from a larger spatial scale, result in a very small change state. Hence, some authors also designate this stage as a durable state or dynamic mosaic [7, 35]. A longer period of time is required for the establishment and development of this stage. In most cases, it is not present or occurs incompletely as a result of logging activities or frequent disturbances. The disturbance pattern, climate fluctuations and other external factors affect also the stand development. Some structural features are present in this stage, such as old and large trees, dead standing and down trees, trees of variable size and age, and a diverse understory. According to several authors, the total biomass remains relatively stable with little fluctuations over time. Slight variations of biomass occur between different parts of the ecosystem, the living biomass, dead wood, floor organic matter and the soil organic matter, with development interactions and balances. The environment conditions do not differ much from the last transition stage.

At this stage, there is a progressive elimination of old dominant trees and the development of dominant trees of different ages. These processes may lead to the formation of a population with a high degree of differentiation and structure. The stand may contain different tree species, which develop in different microclimate conditions. The stand may present a considerable biological diversity. At this stage, there is also a horizontal diversification, with different structural units.

The diversity of habitats increases as the ecosystem includes various states of development. Certain species have a greater abundance and development at this stage, due to their low rate of colonization and growth, as with certain lichens, fungi and tree species. Many species are dependent for their survival of dead wood or other structural features of the stand only present in this development stage.

Regarding the hydrological and biogeochemical cycles, dynamic oscillations occur as a result of occasional disturbances. Nevertheless, the ecosystem taken as a whole is relatively stable and resilient through different processes. There is a stabilization of the total biomass and storage capacity, regulating the export of nutrients. This stage corresponds to a relative equilibrium condition in relation to growth and mortality, the hydrological and biogeochemical state. The forest ecosystem has a great resilience, able to absorb disturbances and persist within certain limits.

*Spatial and Temporal Variability Regarding Forest: From Tree to the Landscape DOI: http://dx.doi.org/10.5772/intechopen.91701*
