Experimental assessment of a gateway invader : how yellow bush lupine (Lupinus arboreus) facilitates the loss of native dune vegetation

Although much attention is currently focused on the dynamics of invasive species, we have only a limited understanding of the mechanisms that promote spread of non-natives. In coastal dune habitats of northern California, the non-native yellow bush lupine (Lupinus arboreus) apparently facilitates invasion of other non-native species. It has been suggested that this facilitation results from the lupines' nitrogen-fixing capabilities, such that mortality of individual lupines creates an open, nutrient-rich site suitable for establishment of plant taxa usually intolerant of sterile dune soils. In the stressful dune environment, however, bush lupines may also create facilitating effects by providing shelter from wind and sun exposure. I examined the bush lupine's role in dune habitat and the mechanisms driving associated vegetation shifts using a three-part study. First, to quantify the effects of bush lupine on dune flora I compared vegetation in habitat without lupines to vegetation under living and dead lupines. Second, to characterize the spread and persistence of soil nitrogen after lupine death, I measured soil nitrogen at various distances under experimentally girdled lupines at six-month intervals. Finally, to test the mechanisms by which lupines influence dune vegetation I conducted a fully factorial experiment in which I monitored vegetation changes in plots with windbreaks, shade tents, and/or the addition of nitrogen. Cover of non-native species was significantly elevated under dead lupines, and native species richness was significantly reduced. Soil nitrogen under girdled lupines decreased significantly over one year after lupine death but I was unable to delineate precisely the lateral extent of lupine soil enrichment. In the factorial experiment, nitrogen addition increased both native and non-native cover. Shade also increased native species cover, but decreased non-native cover. No interactions between treatments were found to significantly influence cover. Richness was not significantly affected by any treatment. This study confirms observations made by earlier authors – that bush lupines provide a gateway for non-native species invasion – and determined experimentally the mechanisms by which individual lupines influence vegetation. The initial effect of a lupine is to suppress invasion on the dunes by creating a shaded environment where non-native cover is reduced. However, the influence of lupines on non-native cover changes dramatically after lupine death. When the lupine canopy is lost and the lupine enriched soil is exposed to light, non-native invasion is facilitated. To my knowledge, this study is the first to experimentally assess multiple, alternative explanations for how non-native species facilitate invasions.