Contingent fire effects on granivore removal of exotic woody plant seeds in longleaf pine savannas

Prescribed fire is increasingly used to inhibit woody encroachment into fire-dependent ecosystems, yet its effects on other processes influencing invasion are poorly understood. We investigated how fire influences exotic woody invasion through its effects on granivore activity, and whether these effects depend on the habitat in which seed predation takes place. We quantified seed removal for four species of exotic woody plants (Albizia julibrissin, Elaeagnus umbellata, Melia azedarach and Triadica sebifera) in 17 sites in longleaf pine savanna that varied in time since fire (one or three growing seasons post-fire) in the sandhills region of North Carolina, USA. Within each site, we established paired plots in upland and upland-wetland ecotone communities and presented seeds in depots that allowed either arthropod, or arthropod and small vertebrate access. We found that differences in seed removal with time since fire were contingent on habitat and granivore community. In ecotones, three of four species had higher proportions of seeds removed from plots that were three growing seasons post-fire than plots one growing season post-fire, whereas only T. sebifera showed this pattern in upland habitat. Allowing vertebrate granivores access to seeds enhanced seed removal, and this effect was strongest in ecotone habitat. While granivores removed seeds of all four plant species, removal of E. umbellata was significantly higher than that of the other species, suggesting that granivores exhibited seed selection. These findings suggest that ecotone habitats in this system experience greater seed removal than upland habitats, particularly as time since fire increases, and differences are mainly due to the activity of vertebrate granivores. Such differences in seed removal, together with seed selection, may contribute to variation in exotic woody invasion of longleaf pine savannas.