Long-term effects of Lupinus lepidus on vegetation dynamics at Mount St. Helens

The nitrogen-fixing legume Lupinus lepidus is the most abundant herb on new volcanic surfaces at Mount St. Helen. We compared vegetation structure in 30 Lupinus colonies in three age classes (old, mature, or young based on known years of their establishment) to adjacent sites that were sparsely populated by Lupinus. Our goals were to determine if the age of colonies affected either species composition or vegetation structure and if colonies altered vegetation structure of a site. Species richness increased with site age and colonies had significantly greater richness. Mature colonies had lower cover and frequency of other species, while old colonies had significantly higher cover of other species, particularly mosses. Frequencies in colonies were higher than in adjacent sites. Diversity measures were highest in old colonies and least in mature ones. ANOVA showed that density effects of Lupinus were stronger than age effects on richness, cover, frequency, dominance, and diversity. The floristic similarity between colonies and adjacent sites declined with colony age, and sites without Lupinus became heterogeneous with age. Detrended correspondence analysis demonstrated floristic change as colonies aged and that the differences between colonies and sparse sites increased with age. Regressions of Lupinus frequency with measures of structure showed weak second order relationships that suggested that low densities of Lupinus inhibited other species, while higher densities promoted abundance and diversity. Together these results suggest that Lupinus is a key element that alters floristic successional trajectories and accelerates succession. However, its effects are complicated by the wide ranges of Lupinus density, the sequence of invasion, and by herbivores.