Most forest in Eastern North America has regrown following abandonment from agriculture and reflects land-use legacies in composition and structure. The presence of open-grown shade trees in pastures is a structural legacy of agricultural history that may influence recolonization dynamics and create structural heterogeneity in post-agricultural forests. Pasture trees embedded in young forests may create pattern through competitive suppression of establishment; alternatively, they may facilitate recolonization by serving as nuclei for seed deposition and as a source of microhabitat variation in a homogeneous agricultural landscape. I tested the hypothesis that pasture trees influence structure and composition in second-growth forests through mechanisms of competition and facilitation at different points in space. Trees, saplings, tree seedlings, ground-layer vegetation, and environmental variables were sampled and compared at three positions around 13 embedded pasture trees: under the canopy, at the dripline, and in the surrounding forest.
Results/Conclusions
Canopy tree abundance was significantly higher at the dripline than in surrounding forest, and very low under the pasture tree canopy, indicating competitive suppression under the canopy paired with facilitation by seed deposition at the dripline. Tree community composition differed significantly by plot position reflecting shade tolerance and dispersal syndrome. Vertebrate-dispersed species were significantly more abundant at the dripline than other areas, suggesting seed deposition early in succession as a key driver of pattern formation. Abundance of dispersal syndromes in ground-layer plant communities showed significant linkages with pasture trees. Vertebrate-dispersed tree seedlings, ant-dispersed, adhesive, and unassisted herb species were most abundant under embedded pasture trees and at the dripline, indicating continued relevance of pasture trees to forest heterogeneity decades after canopy closure. Evidence of nucleation around pasture trees persists after canopy closure as a form of fossil pattern reflecting the interaction of landscape structure and colonization dynamics.