A little disturbance goes a long way: 33-year understory successional responses to a thin tephra deposit

Large volcanic eruptions can alter forest plant communities through a variety of mechanisms, including direct destruction of forests and changes to forest soils through tephra (aerially transported volcanic ejecta) deposits. While many studies have examined succession following direct destruction of forests, impacts to plant communities through tephra effects are less obvious, especially where the tephra depth is less than plant height. We used a 33-year experiment in an old growth forest that received shallow tephra deposition in the 1980 eruption of Mount St. Helens (WA, USA), to examine plant communities. We determined if community differences between plots with and without tephra: (1) were detectable, and (2) changed over time. We found that plant communities differed significantly between plots with and without tephra after 33 years. Further, differences were stronger after 33 years than at two years following the eruption. Species richness increased over time in both plots with and without tephra, but live cover was largely stable after two years. Nevertheless, communities shifted in different directions over time, where the changes in species composition and abundance immediately following tephra deposition were inconsistent with net changes that occurred over 30 years afterwards. These results suggest that widespread and apparently minor deposits of tephra, usually interpreted to be of transient importance if any, may induce long-term modifications of understory plant communities.