Fluctuations in plant resource availability are hypothesized to promote exotic plant invasion by allowing propagules already present in an area a chance to successfully compete for unused resources. To examine the relationship between resource enrichment and exotic species invasion, we used selective logging canopy gaps over a range of sizes (56 m2 to >1500 m2) in a redwood forest (Santa Cruz County, CA) as a surrogate for disturbance intensity and level of pulsed resource enrichment. Measurements of abiotic conditions in gaps ca. 10 yr after logging suggest light is the primary difference in current resource availability, though a pulse of light and nutrients likely occurred at the time of gap formation. Exotic species richness and relative cover increased significantly as gap size increased. In a separate manipulative experiment, we compared understory plant composition between artificially shaded and unshaded plots in 2.5-year-old logging gaps. Shaded plots had a lower proportion of exotic species than did adjacent, unshaded plots, showing that light is a critical resource for exotic species in redwood forest habitats. Taken together, these results support the view that both physical disturbance and increased availability of scarce resources contribute to a community's susceptibility to invasion and suggest a linear relationship between the size of logging gaps and the magnitude of exotic species invasion.
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Vol. 57 • No. 1