Secondary disturbance following a deposit of volcanic tephra: a 30-year record from old-growth forest understory

Abstract

Forest disturbance is usually described by effects on trees, and small disturbances to forest understory are seldom studied. Nevertheless, effective analyses of succession need to consider both stand-replacing and subsequent “secondary” disturbances in both the canopy and understory. We estimated the characteristics of 13 types of secondary disturbances in old-growth forest understory, as well as change in the canopy cover, after the deposition of tephra (aerially transported volcanic ejecta) in 1980 from Mount St. Helens, Washington. We sampled 100 plots (1 m2) at each of four sites for vegetation change and types of disturbance at 10 different times from 1980 to 2010; we sampled the tree canopy above each plot in 1980 and 2016. The number of canopy gaps increased by 23% and mean gap dimension increased by 68% over 36 years, mostly from the loss of Abies amabilis (Douglas ex Loudon) Douglas ex Forbes. Secondary disturbance in understory affected 1.4% of stand area per year. The areas affected by soil disturbance and effects of woody litter were similar. Erosion, greater in deep than in shallow tephra, peaked in 1981, whereas most litter-caused disturbances increased after 2000. Less frequent litter-based disturbances covered greater area. Our results differ from conclusions about nonvolcanic understory disturbances. Secondary disturbances are variable, need more study, and are likely to affect many other systems.