Long-term responses of forest-floor bryophytes buried by tephra in the 1980 eruption of Mount St. Helens

Abstract

Responses in bryophyte communities following volcanic disturbance are not well understood. The eruption of Mount St. Helens on 18 May 1980 deposited large amounts of tephra (aerially transported volcanic ejecta) on nearby forests in southwestern Washington and presented an opportunity to examine bryophyte succession, with a focus on mosses. We examined changes in moss community over 36 years following this disturbance, using permanent plots located in old-growth conifer forests. We used an experimental design where we removed the tephra from some of the plots shortly after the eruption. Initial dramatic decreases in total bryophyte cover, moss species richness, and diversity in plots with intact tephra were followed by increases by 2016. Community profiles also shifted through time but were somewhat site-specific. Moss community change was related to changes in vascular plant species and was driven by changes in a few key moss species with distinct growth strategies. Bryophyte change through time was most pronounced in the plots buried by tephra, but differed among sites. Overall, total bryophyte cover had recovered, reaching our estimate of pre-disturbance levels at all sites after 36 years, but trajectories of change differed among sites, demonstrating the importance of idiosyncratic site factors and the dynamics of the vascular plant species.