Altered beta diversity in post-agricultural woodlands: two hypotheses and the role of scale

Agricultural land use commonly leaves a persistent signature on the ecosystems that develop after agricultural abandonment. Th is agricultural legacy limits the biodiversity supported by post-agricultural habitats compared to remnant habitats that have never been used for agriculture. In particular, beta diversity (variation in community composition across space) at both large and small spatial scales can diff er between post-agricultural and remnant habitats, but we do not understand the mechanisms driving these diff erences. We surveyed plant communities at 29 pairs of post-agricultural and remnant longleaf pine woodlands (58 total woodlands) to test for patterns consistent with two hypothesized mechanisms for why post-agricultural ecosystems support altered beta diversity. 1) Post-agricultural sites support diff erent levels of underlying environmental heterogeneity than remnants. 2) Establishment of species associated with remnant habitats into postagricultural woodlands is limited by dispersal and/or environmental conditions. We found no support for the environmental heterogeneity hypothesis and strong support for the idea that species establishment limits reassembling communities. Our results revealed a novel and important nuance to the establishment limitation hypothesis: spatially constrained, but not completely prevented, re-establishment of remnant-associated species in post-agricultural woodlands increased within-site beta diversity, contrary to results at larger among-site (landscape) scales. Our use of a powerful paired-site design permits these insights into how agriculture and abandonment aff ect beta diversity at two spatial scales, highlighting the prominent infl uence of edges even a half century after agricultural abandonment. Th e importance of constrained species establishment during ecosystem recovery, and its scale-dependent eff ect, could provide valuable guidance to enhance the utility of post-agricultural habitats for biodiversity conservation.