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

Abstract

Agricultural land use commonly leaves a persistent signature on the ecosystems that develop after agricultural abandonment. This 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 differ between post‐agricultural and remnant habitats, but we do not understand the mechanisms driving these differences. 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 different levels of underlying environmental heterogeneity than remnants. 2) Establishment of species associated with remnant habitats into post‐ agricultural 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 affect beta diversity at two spatial scales, highlighting the prominent influence of edges even a half century after agricultural abandonment. The importance of constrained species establishment during ecosystem recovery, and its scale‐dependent effect, could provide valuable guidance to enhance the utility of post‐agricultural habitats for biodiversity conservation.