Community reorganization in forest understories buried by volcanic tephra

Disturbance is a key factor contributing to community organization. Deposition of tephra (aerially transported volcanic ejecta) is a widespread disturbance of global relevance, but its effects on ecosystems have received limited attention. We studied forest understory community change for 30 yr following tephra deposition from the 1980 eruptions of Mount St. Helens (Washington State). Four old-growth, subalpine conifer stands had a wide range of initial damage and patterns of community re-development. We measured understory diversity, structure, and species composition and calculated relationships of plant cover with environment and cover of other plants. Overall, those communities that were altered greatly by tephra tended to converge with time on their pre-eruption characteristics; however, substantial divergence occurred in some situations. For example, moss cover failed to reach pre-eruption levels in all stands, whereas importance and diversity of woody plants sometimes greatly exceeded pre-eruption values. Plant-environment relationships that were significant before the eruption disappeared and did not re-develop. Smaller plants were more affected by environment than larger ones. Relationships before the eruption and also 30 yr after the eruption were primarily with other plant species, whereas relationships just after the eruption were with tephra depth and factors that modified its impact. Understory plant importance was usually lower beneath a tree canopy than in gaps, but there was little sign of interference from understory growth forms. Post-eruption soil disturbance usually increased understory plant importance, while woody debris sometimes decreased herb and tree seedling cover. Tephra deposition, which did not immediately kill canopy trees, differed from the disturbances usually studied (e.g., fire, windthrow, bark beetles). Even so, these lessons from our study should be widely applicable: Similar species may respond differently; minor, early environmental differences may induce major, long-term community change; successional trajectories may diverge from the pre-disturbance community; and secondary disturbances may modify successional trajectories.