| Patterns and ecological impacts of colonizing woody plants were examined for the relatively poorly studied stage in oldfield succession when woody plant species replace herbaceous plant species as the dominant vegetation. As part of a long-term experiment on oldfield succession and habitat fragmentation in the prairie-forest ecotone of northeastern Kansas, vegetation composition has been sampled since the fall of 1984 using a grid of permanent quadrat stations. Stems of woody plants have been individually tagged and measured since 1994. Using this core data set, three basic hypotheses were addressed.; First, it was hypothesized that woody plant colonization would be influenced by the size of the habitat patch undergoing succession, and the distance to propagule sources. The distribution and abundance of colonizing woody plants was shown to be influenced by these landscape spatial features (habitat patch size and distance to propagule sources) and also by the life history traits (dispersal mode, growth form, and fecundity) of the colonizing species.; Second, it was hypothesized that population growth rates would vary during succession, and might show density-dependence in common species. This was examined by constructing stage-structured transition matrices, for C. drummondii. As expected for a colonizing species, almost all populations of C. drummondii grew, with a finite rate of increase (λ) > 1, except for the most abundant populations (which had λ ≈ 1).; Third, we hypothesized that light competition could be a mechanism leading to the observed density-dependence, and that the developing overstory canopy could also influence the herbaceous plant community. Hemispherical photography was used to characterize the overstory canopy. High heterogeneity in overstory canopy was associated with high plant species richness in the understory community across the site, as forest-affiliated herbaceous plants began to colonize at under dense overstory canopy, and oldfield herbaceous plants began to become locally extinct under closing canopy, but still persisted in woody canopy gaps. The closing overstory canopy also inhibited the establishment of early-successional tree seedlings and saplings. Intermediate canopy closure may be associated with maximum species richness; however, site-level richness was still increasing at the current stage of succession. (Abstract shortened by UMI.)... |
