Comparison of temperate forest succession models from different ecoregions, and the role of the early-successional stage

Background/Question/Methods

As one of the first topics specific to the field of ecology, ecological succession has engendered many models. Odum’s Tabular Model summarized various hypotheses. Which species could invade a newly opened area and whether species would be facilitated or inhibited by the presence of other species have been modeled as initial/relay floristics by Egler and in expanded form by Connell and Slatyer. Oliver summarized changes in forest seedling success over time based partly on Egler. . Many quantitative models have been developed for forest succession. The forest is modeled as a collection of patches in which trees die, are replaced and grow according to species-specific equations, often modified by climate or competition. Sometimes species are thought to follow each other according to Markov chain transitions. The primary purpose of the present talk is to examine some of these models to see how well they describe early successional changes. A secondary purpose is to see how well they explain differences in early succession in different ecoregions, most notably between the eastern deciduous and western coniferous forests of North America.

Results/Conclusions

Published forest succession models are inadequate to characterize early succession. The major goal of those models is to predict species composition and ecosystem properties (e.g. biomass) after several decades or centuries. They demonstrate how composition changes with the disturbance regime and with variations in climate or other factors.  Recruitment is usually handled crudely, e.g. by setting a maximum number of saplings and then reducing that number according to competition or other factors. Specific interactions among seedlings and saplings, woody and herbaceous plants or trees and shrubs, although very important in many early succession stages, are ignored in most models. The lack of models for early successional stages affects our ability to explore differences among and within ecoregions. Such differences may be expected based on different climates, soils and species pools. For instance, many forests of western North America experience disturbance regimes dominated by large, intense fires with relatively slow recovery by their relatively few dominant tree species. Many forests of eastern North America experience far less devastating natural disturbances that leave many living stems of many species and have a shorter recovery period (excluding agriculturally-related successions). Because of the importance and uniqueness of early successional stages more efforts to model them are encouraged.