The process of differentiation is accentuated by tree-to-tree variations in vigour, microsite quality and the time of establishment relative to other trees. Differentiation is expressed as differences in total height, crown size and stem diameter throughout the life of the stand. Individual tree variation in growth promotes the differentiation of trees into stratified crown classes (i.e., dominant, codominant, intermediate and overtopped classes). As a result, weak trees are forced to relinquish growing space enabling stronger trees to increase their consumption of site resources, and continue to thrive.Ĭompetition for growing space may result in three types of tree and stand responses: differentiation and stratification, mortality and repression. Their crowns gradually decline in size to the point where they are unable to maintain their rate of height growth. At this point the survival of these trees of lower vigour is threatened because they cannot increase in height fast enough to stay ahead of the receding base of their crowns. When inter-tree crown contact occurs, the branches of the more vigorous trees gradually overstep the lateral edge of the crowns of weaker trees. The lowermost branches eventually die and the bases of the crowns of adjacent trees begin to lift at the same rate. The upper point of inter-tree crown contact moves upward and shading of the lower branches intensifies as crown expansion continues. This period ends when the branches of adjacent trees meet and interlock in an attempt to use the same resources.
#Stand the space in time free#
During the period of free growth, height growth is unimpeded and the tree crown covers the entire stem in the absence of constraining factors such as brush and pests. The dynamics of crown competition in even-aged coniferous stands are reasonably well understood. On moist to wet sites, crown competition for light is of primary importance. On dry sites, below-ground competition may limit growth to the point that crown closure does not occur. This leads to intense inter-tree competition. Site resources constrain the growth of trees in the main canopy as soon as the crowns and root systems attempt to utilize the same elements. Inter-tree competition is a growing space related factor central to the stand development process. The biology and dynamics of the stand (i.e., the change in stand development over time) are far more complex after the trees begin to compete for growing space. Stand volume production is proportional to the number of trees occupying the site during this period. This phase of tree growth is referred to as the period of free growth. In even-aged stands, trees are unrestrained by space from establishment until they begin to compete for the site resources. The relationship between tree growth and growing space is complex. Trees without adequate growing space grow poorly those that fail to meet their minimum requirements for growth will die. A tree must continue to grow in size and acquire more space if it is to continue to thrive. The relative importance of both components of growing space vary with species, site, and tree and stand developmental stage. Growing space, therefore, refers to a tree's share of total above-ground and below-ground site resources, not just the physical space. Neither factor can be completely described by physical space alone. It is more difficult to measure an individual tree's share of total site resources and the influence of neighbouring trees on its consumption of site resources. Trees in a stand occupy physical space, measured in terms of the crown dimensions above ground, and the root spread below ground. The concept of growing space is helpful in explaining the stand development process. Microsite differences, pests, natural disturbances and management interventions further alter the inherent growth variation among trees. This inherent variation is responsible for large differences in the productive potential of individual trees, with the exception of species like aspen, which form clonal stands. For example, the weakest trees in a monoculture typically increase in height at about one-half the rate of the strongest trees. Genetic variation is very large within and among species for tree characteristics such as height growth, and processes such as photosynthesis. The rate and tree-to-tree variability of height growth are the principal mechanisms driving the processes of stand development. Stand development begins at the earliest point of stand establishment and influences the pattern of tree growth, stand structure and timber production throughout its life. Stand development, or stand dynamics, is the process of structural change that occurs in stands over time. Developing Stand Density Management Regimes
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