Temporal patterns of vascular plant diversity in southeastern New Hampshire forests

A chronosequence of 22 sites in Durham, NH, was used to study upland successional patterns of species diversity over a span of 14-209 years of forest development and to test the hypothesis that diversity is maximized at mid-succession. Nested quadrat sampling was used to estimate the relative importance values of vascular plant species in the tree, shrub, and herb strata. The successional status of each site was estimated using two dendrochronological indices: oldest tree index and abandonment index. Four indices of species diversity were calculated for each site and stratum: species richness, the Shannon index, Hurlbert's probability of interspecific association, and Pielou's evenness. Within the quadrats of all 22 sites, total richness was 168 species of vascular plants. Richness decreased linearly over the 200-year chronosequence in all strata for both successional indices, from a maximum of 74 species at 14 years to a low of 13 species at 209 years. Evenness showed no evidence of a successional pattern, so the combined indices of diversity (Shannon and Hurlbert) displayed only weak trends. Multiple regression showed that successional age was always a significant predictor of species richness in every stratum (except the shrub stratum when using the abandonment index), and that stem density (i.e., the sample size effect) was not significant in the tree stratum when using the oldest tree index. Age indices held comparable or lower P-values than stem density in most strata. It is likely that richness increased from a low value immediately following farm abandonment to a peak around the age of the youngest sites sampled (over a span of 15-30 years), the rate of increase being governed by the availability of dispersing seeds. The observed decline in richness over the rest of succession was probably caused in part by a decrease in available light. The rate of decline was apparently determined by the rate of competitive elimination of shade-intolerant species. (C) 2003 Elsevier B.V. All rights reserved.