|GUIDELINES for||. . .|
|Developing Stand Density Management Regimes|
Stand development, or stand dynamics, is the process of structural change that occurs in stands over time. 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. The rate and tree-to-tree variability of height growth are the principal mechanisms driving the processes of stand development.
Genetic variation is very large within and among species for tree characteristics such as height growth, and processes such as photosynthesis. For example, the weakest trees in a monoculture typically increase in height at about one-half the rate of the strongest 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. Microsite differences, pests, natural disturbances and management interventions further alter the inherent growth variation among trees.
The concept of growing space is helpful in explaining the stand development process. Trees in a stand occupy physical space, measured in terms of the crown dimensions above ground, and the root spread below ground. 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. Neither factor can be completely described by physical space alone.
Growing space, therefore, refers to a tree's share of total above-ground and below-ground site resources, not just the physical space. The relative importance of both components of growing space vary with species, site, and tree and stand developmental stage. A tree must continue to grow in size and acquire more space if it is to continue to thrive. Trees without adequate growing space grow poorly; those that fail to meet their minimum requirements for growth will die.
The relationship between tree growth and growing space is complex. In even-aged stands, trees are unrestrained by space from establishment until they begin to compete for the site resources. This phase of tree growth is referred to as the period of free growth. Stand volume production is proportional to the number of trees occupying the site during this period. 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.
Inter-tree competition is a growing space related factor central to the stand development process. 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. This leads to intense inter-tree competition. On dry sites, below-ground competition may limit growth to the point that crown closure does not occur. On moist to wet sites, crown competition for light is of primary importance.
The dynamics of crown competition in even-aged coniferous stands are reasonably well understood. 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. This period ends when the branches of adjacent trees meet and interlock in an attempt to use the same resources. The upper point of inter-tree crown contact moves upward and shading of the lower branches intensifies as crown expansion continues. The lowermost branches eventually die and the bases of the crowns of adjacent trees begin to lift at the same rate.
When inter-tree crown contact occurs, the branches of the more vigorous trees gradually overstep the lateral edge of the crowns of weaker trees. 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. Their crowns gradually decline in size to the point where they are unable to maintain their rate of height growth. 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.
Competition for growing space may result in three types of tree and stand responses: differentiation and stratification, mortality and repression.
Individual tree variation in growth promotes the differentiation of trees into stratified crown classes (i.e., dominant, codominant, intermediate and overtopped classes). Differentiation is expressed as differences in total height, crown size and stem diameter throughout the life of the stand. The process of differentiation is accentuated by tree-to-tree variations in vigour, microsite quality and the time of establishment relative to other trees. The rate of differentiation later accelerates with inter-tree competition.
As even-aged monocultures develop, the disadvantaged trees (due to factors such as vigour, microsite, age) drop from the upper to the lower crown positions, and eventually die. The diameter distribution (i.e., number of trees by diameter class) continues to widen as stands age, and may shift from being normally distributed to one that is skewed toward smaller diameters. In most species, however, the development of highly skewed diameter distributions are limited by the mortality of smaller trees. In stands with a clumped distribution of trees, crown differentiation and stratification begin at different times in different parts of the stand, adding to the variability in tree size and growth rate.
In stands of mixed species, the variation in tree size is even more pronounced because of the wider range of inherent rates of juvenile height growth. Species in stand mixtures tend to differentiate into distinct layers or strata when differences in height growth are large. This initial stratification can persist if the slower-growing species are shade tolerant, or if sufficient sunlight can pass through the foliage of trees in the upper stratum.
The initial stratification may change over time if the component species have different patterns of height growth. For example, the height growth of paper birch slows dramatically after about age 40. Other species in the stand may subsequently overtake and surpass the birch in height. The stratification can also be altered by insects and diseases, which may preferentially damage and weaken one species or stratum in the mixture.
Diameter distributions in mixed-species stands generally reflect the height stratification. The species in the upper and lower canopy occupy the larger and smaller diameter classes, respectively. If the slower-growing species are shade tolerant, diameter distributions are often skewed toward smaller sizes. Broadleaf species sometimes exhibit a different height-diameter ratio than conifers and consequently appear lower down in the diameter distribution than they are in the height distribution.
Canopy stratification patterns can be altered if the species with slower juvenile height growth have an advantage in early stand development. They may regenerate in advance of the faster-growing species, or density control measures may free them from competition during the juvenile phase of slow growth, thereby ensuring they do not lag far behind the faster-growing species at the time of stand canopy closure. Silviculture treatments undertaken at or shortly after establishment can create single-layered stands of species that would otherwise naturally form stratified canopies.
Regular mortality occurs when competition for growing space reduces the size and productive capacity of the crown to the point where it cannot support the basic respiration needs of the tree (respiration > photosynthesis). That is, the site-limited resources are redirected from the overtopped and dying trees to the survivors. This process will be initiated simultaneously throughout the stand if tree spacing and other factors are relatively uniform. Otherwise, it may start in the denser sections and spread to areas of lower density. The onset of regular mortality signals full site occupation or utilization. Irregular mortality is caused by pests, wind and other factors, and may occur any time throughout the life of the stand.
Competition and the associated tree mortality in dense stands can leave a more productive stand at maturity than one established at a much lower density, if only the most vigorous trees survive.
Copyright 1999 Province of British Columbia