Tephra deposition is the most common and widespread volcanic agent of ecosystem disturbance. Accordingly, determining the predictable relationships between plant growth form and responses to tephra deposition is important for understanding ecosystem response. Nevertheless, tephra impacts can be difficult to generalize due to differences in 1) tephra depth, 2) tephra particle size distribution and consistency, 3) tephra chemistry, 4) presence of snowpack and rainfall, and 5) tree canopy resistance to (and effects on) deposition. Here, we examine studies of plant responses to tephra deposition in forests around the globe. We first evaluate recent eruptions with respect to the above five disturbance properties, and, second, we classify dominant plant responses in each system based on a plant trait model developed for responses to the Mount St. Helens eruption in 1980. Briefly, we consider plant phenology (deciduous or evergreen), location of perennating organs (Raunkiaer life form), adventitious rooting, presence of vegetative spread (stoloniferous, rhizomatous, or none), geometry of meristematic growth/penetrating ability (clustered versus open growth habit), and relative growth rate within systems (r versus K-selection). We compare ecosystem responses to disturbance in each system based on these factors in combination.
Volcanic eruptions differ in their quantity, characteristics, and extent of tephra deposition. Furthermore, plant community composition among volcanic regions is extremely variable. Nevertheless, some generalities are possible. We find that tephra depth strongly affects plant response, with important thresholds near 2 cm (measurable community change), 10-25 cm (loss of some plant groups), and >100 cm (significant tree death). While tephra can also provide limiting plant nutrients, nutrient concentrations in tephra are often low, and prohibitively thick deposits may be required to deliver significant beneficial macro- and micronutrients. Factors such as snowpack, phenology, and plant growth form have tremendous impacts in systems where snow accumulates in the winter. Even small amounts of tephra can kill many forest canopy epiphytes. Nevertheless, lichen and bryophyte survival can be high along tree boles, which can serve as refugia. Ecosystem recovery is aided by combinations of plant traits that allow penetration of deposits and migration of perennating organs, especially in deposits between 10 cm-100 cm deep. Finally, edaphic effects may significantly alter the seedbed environment for regenerating species. While mechanical and chemical effects of tephra are often short-lived, tephra deposition represents an important selective event for plants, resulting in long-term community and ecosystem change.