Plant Species Diversity in Old‐Field Successional Ecosystems in Southern Illinois

In old—field ecosystems in the deciduous forest, plant species diversity generally increases with succession and reaches a maximum during the forest stage when structural diversity is highest and at the time when both shade—tolerant and shade—intolerant species are present together. Species diversity may be exceptionally high in succesional communities when there is a high degree of vertical and horizontal heterogeneity. Diversity may be exceptionally low in old—field ecosystems when there is strong dominance by species with alleolopathic chemicals or other effective interference methods. Species distribution curves are geometric during the first few years of succession and gradually change to lognormal as more species are added to the community, the process resulting in a high degree of evenness.