Habitat change in insular grasslands: Woody encroachment alters the population dynamics of a rare ecotonal plant

Inferring habitat requirements of rare plants can be challenging when the few remaining populations occur in sites with divergent successional states. In island-like rock outcrop systems within forest landscapes, edaphic conditions are assumed to modulate successional patterns, but changes to disturbance regimes in the landscape matrix could alter ecotone microenvironments over time. We used demographic surveys and controlled experiments with a dispersal-limited and endangered ecotonal plant, Astragalus bibullatus, to test the hypothesis that woody encroachment from the forest matrix threatens rare plants in globally imperiled limestone cedar glades. Tree canopy cover was more important than edaphic conditions or ground-layer structure for explaining variation in demographic structure. As tree canopy cover increased, stem densities, flowering, and seed production declined. Over three-years, per plant inflorescence production, fruit production, and fruit set were markedly greater in open microhabitats than in edge or closed microhabitats. In contrast, seedling densities peaked in edge and closed microhabitats. Seedling recruitment and seed production were spatially decoupled across a canopy cover gradient, suggesting that shaded sites historically had lower tree canopy cover. Shade reduced growth rates and biomass of seedlings and adults under non-limiting moisture conditions. Although A. bibullatus can persist in degraded ecotones using multiple demographic strategies, growth, flowering and seed production depend on open microhabitats. Our results demonstrate that woody encroachment effects in temperate grasslands extend to island-like rock outcrop systems with unique edaphic conditions. Conservation and recovery programs with rare ecotonal plants should integrate restoration of historical disturbance regimes in the landscape matrix.