Herbivory and Seed Banks Will Limit Regeneration and Restoration of an Endangered Subtropical Rainforest

Disturbances are an essential component of forest functionality and composition; however, when communities become disrupted, these disturbances may restrict the growth and productivity of species present. Illawarra subtropical rainforest (ISRF) is a threatened ecological community that is influenced by both native and exotic vertebrate herbivores and exotic plant pressures. The ability of ISRF to regenerate following the removal of large herbivores was assessed in fenced and unfenced sites within fragments of rainforest. To investigate the extent to which herbivory was limiting the restoration of ISRF, five native seedlings of each of eight species were planted at six sites, and growth and herbivore activity was compared between fenced and unfenced sites. We investigated seed bank capacity and composition to determine whether recruitment limitation is also preventing regeneration. Over 46 weeks, seedlings in unfenced sites experienced significantly lower growth, which varied amongst species and sites. High activity of feral deer and swamp wallabies caused significant reductions in seedling heights and the number of leaves. Herbivores reduced the growth of seedlings of Brachychiton acerifolius, Planchonella australis, and Breynia oblongifolia more than other species. Pittosporum multiflorum seedlings were not eaten. Only 297 seeds (10% of all seeds germinating) of 21 native species typical of ISRF communities germinated in the seed bank germination trial in the glasshouse. Most native species were missing, reflecting poor recruitment opportunities from a soil-stored seed bank. Seed banks were dominated by 21 exotic species (2125 seedlings—74% of total), presenting a significant risk to the regeneration of the community. Our results indicated that ISRF communities are unlikely to regenerate naturally and require active planting coupled with effective protective measures from all herbivores to restore fragmented vegetation.