Dispersal limitation and ecological conditions inhibit restoration of post-agricultural plant communities
Alteration of natural ecosystems by humans is the leading cause of biodiversity loss, but active restoration can help re-establish native species and promote biodiversity in degraded habitats. Agricultural lands are a promising area for restoration because they cover a large fraction of the earth’s surface and their abandonment is common. Yet, post-agricultural lands often have distinct plant communities from undisturbed areas even hundreds of years after abandonment. Dispersal limitation and establishment limitation resulting from unsuitable ecological conditions may cause these long-lasting land-use effects, but few experiments have tested this. We studied longleaf pine savannas in South Carolina abandoned from agriculture >50 years ago and that were immediately adjacent to undisturbed remnant populations that could act as natural seed sources for understory species. We tested how restoration may facilitate post-agricultural community recovery by: 1) sowing seeds of 12 species indicative of remnant populations to alleviate dispersal limitation, and 2) thinning trees to mimic canopy density of natural savannas and produce favorable ecological conditions for establishment. We surveyed plant communities within experimental plots two years after initiating experimental treatments.
Our seed addition treatment provided strong evidence for dispersal limitation in post-agricultural landscapes. We observed significant recruitment in 9 of 12 focal species and over 99% of these occurrences were in seed addition plots. Even plots only 10 and 25 m away from potential source populations were still highly seed limited, showing the fine scale of dispersal limitation in preventing community recovery in post-agricultural landscapes. Restoring ecological conditions through canopy thinning had no effect on the establishment of sown species; however, species richness was increased by both seed addition (by 27%) and canopy thinning (by 80%). The highest average species richness was in thinned/seed addition subplots (11 species) and the lowest was in unthinned/non-seed addition subplots (5 species). Together, these results suggest that dispersal limitation is a key factor causing some species to be absent from post-agricultural landscapes and that unsuitable ecological conditions (closed savanna canopy) depresses diversity. Restoring both species composition and species diversity will require active restoration that alleviates dispersal limitation and reinstates appropriate ecological conditions.