Colonization of volcanic deserts from productive patches

Dense vegetation can augment landscape biodiversity, promote recovery of surrounding ecosystems, and facilitate colonization. However, dispersal limits affect vegetation structure early in succession to a degree that is not widely appreciated. We studied two contrasting densely vegetated habitats located on Mount St. Helens to explore their effects on surrounding vegetation. We sampled refugia and adjacent barrens and compared them to a study conducted 5 yr earlier. We sampled isolated wetlands and their surroundings. Species richness, total cover, cover of most dispersal types, the information statistic (H'), and the complement of Simpson's Index (D) were negatively correlated with distance from the refugia up to 32 m. Compared to 1997, the 2002 transects had higher cover, and adjacent samples were more similar to one another. Dominance had shifted from wind-dispersed species to more persistent species with less effective dispersal mechanisms. The pattern of these results demonstrated that refugia were a major, but indirect source of recruitment for many species. The influence of wetlands on surrounding vegetation recovery was even more spatially localized. This study demonstrates that dispersal from surviving and rapidly recovered vegetation has been overemphasized as a mechanism for vegetation recovery. The effects of dense vegetation did not extend beyond 32 m in our studies, and local effects appear to diminish over time.