Background/Question/Methods
The mechanisms that confer resistance to invasion are not well understood in many ecosystems, yet determining these drivers is critical for reducing the impacts of invasive species. Tropical dry forests in Hawaii are some of the most endangered ecosystems in the world, largely due to degradation by invasive C4 grass species, such as African fountain grass (Pennisetum setaceum). Many of these forests have been completely converted to non-native grassland and require intensive management and restoration. Within the context of a forest restoration study, we experimentally investigated three ecosystem properties that may influence resistance to invasion by fountain grass: 1) native plant species richness and abundance, 2) environmental conditions, and 3) disturbance. Using a factorial field experiment on a grass-dominated site, we manipulated environmental conditions (method of grass removal, shade, and water) and evaluated their effects on the survival of outplanted native species. We then stopped applying the environmental treatments and employed a disturbance treatment by removing all species from half of each plot. This treatment allowed us to examine fountain grass recruitment in habitats with similar environmental conditions but different disturbance histories. We recorded outplant survival in 2005, six months following the disturbance treatments. We also recorded outplant survival and the abundance of all species in 2009, four years following the disturbance treatment.
Results/Conclusions
Disturbance had no effect on the presence or abundance of fountain grass. Several results in non-disturbed plots showed that environmental conditions that favored outplant survival also favored re-invasion by fountain grass and other exotic species. First, outplant richness and number of outplants surviving in 2005 were positively correlated with fountain grass abundance in 2009. Second, exotic and native richness were positively correlated. Third, the method of grass removal had similar effects on the number of outplants surviving and the abundance of fountain grass. In disturbed plots, there were no direct effects of environmental conditions on native species richness, abundance, or fountain grass abundance; however, fountain grass abundance was positively correlated with total species richness. These results suggest that there was no direct effect of disturbance on invasion but that conditions that were favorable for native species were also favorable to reinvasion. Fountain grass abundance was positively correlated with the number of outplants that died between 2005 and 2009 but not the number of surviving outplants, further suggesting that once fountain grass invades suitable habitats it may rapidly displace native species.