Lupine Colonies ( not yet published ; shorter version ) Long - term effects of

Long-term effects of Lupinus lepidus on vegetation dynamics at Mount St. Helens by Roger del Moral & Lara Rozzell Abstract: The nitrogen-fixing legume Lupinus lepidus is the most abundant herb on recently formed 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 promoted richness. Mature colonies had lower cover and frequency of other species, while old colonies promoted other species, particularly mosses. Frequencies in colonies were higher than in adjacent sites. Diversity measures were highest in old colonies, least in mature ones. Nested ANOVA showed that the density effects of Lupinus were stronger than its age effects. The floristic similarity between colonies and adjacent sites declined with colony age, while sparse sites 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. Linear regressions of Lupinus frequency with measures of structure showed weak second order relationships that suggested that low density of Lupinus inhibited other species, while higher densities promoted abundance and diversity. Together, these results suggest that Lupinus alters floristic successional trajectories, promotes successional development, and affects adjacent vegetation. However, these effects are complicated the wide fluctuations inherent in Lupinus, the sequence of invasion, and by herbivores. Lupinus remains the key element among several which promote the development of a vegetation mosaic. Lupinus will continue to affect future vegetation long after conifers exclude it due to its effects on soil. LUPINE COLONIES are dense sites dominated by Lupinus lepidus, the most studied species on Mount St. Helens. Some patches were established in 1981, others only within the last few years. Between 2001 and 2003, we sampled 30 patches and found interesting contrasts between young, mature and old patches. In each patch, a belt of 20 alternatiing1-m quadrats was established. A comparable transect was established outside the patch parallel to the first one. Species cover and frequency were determined for each quadrat based on 25 sub-quadrats. Data are reported for frequency, which does not give undue weight to less abundant species. Our objectives are to determine if vegetation structure in Lupinus colonies of different ages differs significantly and to determine if floristic contrasts between colonies and adjacent sparse populations change with colony age.