Managing successional trajectories in alien-dominated, novel ecosystems by facilitating seedling regeneration: A case study

Abstract

Managing novel ecosystems that emerge after strong anthropogenic disturbance and the spread of alien species poses complex problems. As a case example, we discuss efforts on the tropical oceanic island Mahé (Seychelles) to promote the regeneration of native trees in tropical secondary forest dominated by the invasive tree Cinnamomum verum (true cinnamon). We monitored the growth of transplanted and self-sown native and alien tree seedlings in both artificially created forest gaps and in the understorey of closed forest. In the understorey, native species survived but grew very slowly, indicating that any recovery of native tree populations would also be very slow. In larger gaps, seedlings of certain invasive species grew particularly fast, suggesting that these species would rapidly dominate. Nutrient addition had only a minor positive effect on seedling growth, especially in the understorey. Although neither closed canopy conditions nor large gaps are conducive to the recovery of native seedlings, our data indicate that intermediate levels of canopy disturbance, i.e. those attained by felling one or a few trees, provide a combination of below- and aboveground resource limitation of seedling growth that may favour native species over invasive species. Based on these findings, we propose a strategy for restoring native vegetation that entails making small gaps in cinnamon-dominated forest and planting them with native tree species. This study illustrates how management strategies can be improved by combining insights gained from past interventions with the results of field and greenhouse experiments.