The most widespread form of disturbance resulting from volcanic eruptions is the deposition of tephra, which has major effects on vegetation. In May 1980, the eruption of Mount St. Helens deposited a range of tephra depths in similar old-growth forests, creating a disturbance gradient. We used this gradient – a natural experiment – to evaluate the effects on forest understory plants of this common but understudied disturbance. During summer 1980, we established permanent, 1-m2 plots in four forests (two with 4.5 cm and two with >12 cm tephra). At each tephra depth, one stand was herb-rich and the other herb-poor; the herb-rich stands were mostly snow covered at the time of the eruption, in contrast to the herb-poor stands. At each site, we used 100 natural plots and an additional 50 plots that we cleared of tephra, which served as our indicator of pre-disturbance vegetation. We recorded plant species cover and density periodically from 1980 to 2016. Here we use ordinations and a variety of other analytic techniques to assess the extent to which vegetation has converged on its pre-disturbance state over 36 years, and how temporal changes vary among sites, growth forms and species.
Initial effects of the tephra deposit differed greatly among growth forms, tephra depths, and initial conditions. Bryophytes were almost eliminated, herbaceous plants mostly survived 4.5 cm tephra but not deep tephra, and woody plant survival was strongly reduced where tephra fell on snow. After 36 years, major changes during succession had allowed most vegetation attributes at most sites to approach their pre-disturbance status. However, substantial and significant differences remained for some growth forms at some sites. Total bryophyte cover was still reduced, with slow recovery of species that were dominant before the disturbance. Shrubs had largely rebounded from the pronounced decrease caused by tephra falling on snow, but cover of one common huckleberry species was still reduced relative to pre-disturbance values. Although total herb cover approached the pre-disturbance state, species differed greatly in response; some had elevated cover while others remained reduced. Thus, composition of the herb layer has been altered, and trajectories of change indicate that cover is converging on pre-disturbance values for some species but not others. Overall, the effects of this tephra disturbance on the understories of these old-growth forests were pronounced, are still apparent 36 years later, and may represent persistent alterations in species composition.