**2. Disturbances and forest ecosystems dynamics**

A better understanding of the forest ecosystem dynamics has allowed a broader comprehension about the influence of disturbances in the development of the forest stands and the landscape, therefore supporting the most appropriate forest management decisions.

All forest ecosystems are subject to disturbances, which may be of a different type and affect their characteristics and functioning. Natural disturbances are part of the dynamics of a forest ecosystem. The role of different disturbances, both spatial and temporal, is recognized as part of the forest development. Not only small disturbances are considered, but also major disturbances and even climate change, with their specific characteristics and occurrence.

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

A disturbance is any event that affects or disrupts a particular ecological process or ecosystem development; modifies the population structure; and changes the availability of a particular resource or the physical environment [6, 10, 14, 15]. A disturbance may be essentially described according to its type, frequency, magnitude or severity, extent and return period. The relative importance of each disturbance varies according to their characteristics and the type of forest.

The disturbance *type* is one of the most important characteristics of a disturbance regime. Disturbances may be biotic or abiotic, natural or anthropogenic, as well as endogenous or exogenous. Endogenous disorders are an integral part of the autogenic ecosystem development process. The potential to create endogenous disturbances varies with the species, the forest stand and development. The control and intervention on destabilizing forces are important for the development and stability, and the ability to minimize certain effects can be assessed, such as loss of water and nutrients.

The *magnitude* may affect more or less the existing plant mass. Some disturbances may destroy all vegetation, while others may leave some trees or other vegetation, which will influence the recovery process, depending on the number of remaining trees, species and position in the canopy. Major exogenous disturbances (fire, storm and clearcutting) result in a reduction or elimination of primary production and have different consequences in terms of biomass and export nutrients. A major disturbance may have an appreciable effect on the subsequent development of the forest ecosystem. For example, a fire may destroy a large part or all of the biomass and suppress primary production. At the same time, nutrients removed by volatilization and leaching can increase soil erosion. A clearcutting also removes a significant amount of nutrients present in the exploited material. A storm with the loss of many trees affects also the primary production; however, the biomass may remain in the system. Soil erosion, which can occur as a result of intensive logging, soil tillage or fire, has a strong negative impact on the ecosystem. A clearcutting or fire leads to the destruction of important hydrological, nutritional and biological soil properties. A clearcutting, particularly on steep slopes and thin soils, may lead to long-time changes on soil structure and the ecosystem biogeochemistry.

The disturbances *frequency* can be relatively variable and depend on the influence of various factors, both natural and anthropogenic. Typically, larger scale natural disturbances occur over longer periods of time. Disturbances may occur regularly or irregularly in time and space, which will be reflected in the stand characteristics and development. The *time* and *duration* of a disturbance are also important characteristics that may affect the ecosystem response.

The disturbance *extent* influences on the composition and structure of the stand, affecting the microclimatic conditions and colonization capacity from the surrounding areas. A disturbance may intervene at wide range of spatial scales (tree, stand and landscape).

Other relevant elements to consider in stand development are related to the initial conditions after a disturbance, the residual material and stand structural characteristics. A disturbance can create gaps of different size and shape, which may affect stand characteristics and dynamics. A given disturbance may affect different tropic and biological levels. The ecological effects due to disturbances and vegetation development vary with species. The resilience and the community type that are established after a disturbance are highly dependent on the ecosystem characteristics, the site conditions and the species that survive after a disturbance. Plants have different anatomical and physiological characteristics, with different adaptation and regeneration mechanisms, which allow them to face and survive certain disturbances.

A group of trees that develops after a disturbance is designated in some literature as *cohort* [9, 16]. The age range of the cohort may vary according to the

ecological processes and the disturbances regime [6]. The spatial heterogeneity in forested landscapes may derive from different causes, including changes in ecological conditions, biotic interactions, developmental stages, land uses and

*Forest management considering different spatial scales: tree, forest stand and landscape. Fluxes and*

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

In this chapter, the forest is assessed considering the variability at different spatial scales: the landscape, the forest population and the tree (**Figure 1**). A forest, of different size, is viewed as a part of the landscape, which might be structured according to a physiographic or natural model. A forest stand is considered a management unit that can be differentiated with a certain number of site and

The understanding of the forest ecosystem's functioning and dynamics has improved over the past decades [7–13]. A better knowledge of their dynamics is important to forestry, providing a better decision support of the most appropriate

A better understanding of the forest ecosystem dynamics has allowed a broader

All forest ecosystems are subject to disturbances, which may be of a different type and affect their characteristics and functioning. Natural disturbances are part of the dynamics of a forest ecosystem. The role of different disturbances, both spatial and temporal, is recognized as part of the forest development. Not only small disturbances are considered, but also major disturbances and even climate change,

comprehension about the influence of disturbances in the development of the forest stands and the landscape, therefore supporting the most appropriate forest

disturbances.

**Figure 1.**

vegetal characteristics.

management decisions.

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practices to achieve certain objectives.

*interconnections occur among these spatial scales.*

**2. Disturbances and forest ecosystems dynamics**

with their specific characteristics and occurrence.

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 adjacent area.

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

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

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

**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

*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*

variability (**Figure 3**).

**Figure 3.**

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*possibilities for spatial variability.*

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 repercussions on the entire ecological system.
