How landscape context and the local environment affect plant community composition and long-term structural change in a naturally patchy ecosystem.

摘要

Plant communities respond to multiple drivers that operate simultaneously at both local and landscape scales, but the relative importance of these drivers, and the mechanisms by which they influence communities, remain incompletely understood. In this dissertation, I examine the influences of landscape context, soil characteristics, and disturbance on plant communities and plant functional traits in Ozark glades---naturally fragmented dry grasslands. I also examine how local and landscape factors interact with management history to influence woody encroachment patterns, since encroachment represents one of the greatest threats to this ecosystem.;Plant species richness in dolomite glades responds most strongly to soil resource availability, but also increases significantly with glade area and connectivity. Conservative grassland specialist species respond positively to glade area and connectivity, while generalist species do not. Generalists, however, have stronger responses to soil resource availability than specialists. Functional traits predict some species responses to landscape spatial structure and soil resource availability, and phenology mediates these relationships. Spring-blooming species have fewer trait interactions with landscape spatial structure and soil resource availability than summer-blooming species, probably because harsher conditions in hot Ozark summers cause stronger responses to ecological gradients. Summer-blooming species with better persistence and dispersal abilities are more likely to occur in smaller, more isolated glades, and species with traits suggesting more resource-acquisitive life history strategies are more likely to occur in richer soils. Soil also has strong influences on plant communities in sandstone glades, where soil organic matter mediates a shift from lichen-dominance to plant-dominance. Rare, deep soil zones of sandstone glades harbor high plant diversity.;Analysis of landscape imagery shows that median tree cover in glades has increased substantially over the last 75 years---by approximately seven-fold. Broadleaf trees and shrubs tend to encroach near areas that had some woody vegetation in 1939, while cedars prolifically colonize across the landscape. Bimodal frequency distributions of woody cover in glades managed with fire suggest that thresholds and hysteresis characterize encroachment patterns. These results suggest that prescribed fire can prevent new encroachment but that fire does not carry well through densely wooded areas to kill trees and shrubs, probably because trees shade out grassy fuels.