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Analysis and modelling of interspecies competition during forest secondary succession Bellefleur, Pierre


The Coastal forest of southwestern British Columbia is examined at three levels of interpretation: the Biogeoclimatic Subzone level, the plot level, and the single tree level. These levels correspond to the three major strata of the population: the geographic range, the community, and the individual. The data base consists of 40 years of observations on 730 Permanent Sample Plots describing over 70,000 trees. The highest level of interpretation, the Biogeoclimatic Subzone level, covers several thousand square kilometers of extremely varied topography and climate. The total study area is subdivided into five main low elevation Biogeoclimatic Subzones. The age structure of each subzone is analysed on an average span of 80 years and forest-type succession dynamics is described. The very nature of forest succession from one type to another, with a finite number of possible forest-types over the time horizon, seems admirably suited for a finite-state Markov process. However, the Markov models cannot fit adequately the observations at the subzone level because transition probabilities are not entirely time-homogeneous and because there is a wide range of communities and origins of perturbation within a subzone. The sample plot offers an intermediate level of interpretation and is considered sufficiently homogeneous to represent larger units of forest. The entire forest can be described by the agglomeration of the fundamental units. The growth of a given species is likely to be different in a pure forest than in a mixed one, and between different types of mixed forests. Tree species constitute the pool of biotic variables with the highest biomass and are estimated to have a high biotic impact on each other's growth. Each Biogeoclimatic Subzone is divided into several plot-types which represent fundamental units of forest composition.. The growth of any given species shows, indeed, significant variation from one plot-type to another. The trends in succession at the plot type level coincide closely with those observed at the Biogeoclimatic Subzone level. Thus it is hypothesized that succession at the subzone level is a consequence of variation in species growth rate between plot types, due to site conditions and competition. At the lowest stratum of the population, the growth, mortality, and regeneration of a single tree are investigated. The growth rate of a tree is dependent on its past history, on the climatic and geographic components characterizing a Biogeoclimatic Subzone, and on the other trees growing in its immediate neighborhood. These variables have a very significant effect on whether a tree lives or dies in any time period. The analysis indicates that recently dead stems appear to have a history of sub-standard growth when compared with the population. Moreover, the immediate neighborhood of dead stems corresponds to a specific composition and structure of the vegetation. On the other hand, new stems show large interspecies differences in their preference for forest composition and structure of their immediate surroundings. The habitat composition arid structure leading to the mortality of a stem of one species may constitute a good habitat for the regeneration of a stem of another species. This is viewed as a mechanism which gives rise to plot type succession, which in turn leads to forest-type succession. The levels of the individual, of the community, and of the geographic range display consistent population dynamics. Succession appears to be explained by simple mechanisms involving competition for light and space; it is not necessary to postulate more complex synergistic or antagonistic mechanisms of species interaction.

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