**4. Tree-level variability**

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

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

stability and resilience of the ecosystem to destabilizing events.

much from the last transition stage.

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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

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,

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.

there is also a horizontal diversification, with different structural units.

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 pattern, while others are more scattered.

The presence of certain trees with particular biodiversity objectives and providing 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).

#### **Figure 6.**

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