Resilience and alternative equilibria in a mire plant community after experimental disturbance by volcanic ash

Abstract

Following disturbance events vegetation can either be resilient and return to its original state, or there can be shifts in vegetation composition and abundance patterns that may indicate alternative equilibiria. We conducted a long-term field experiment that simulated impact by aerially transported volcanic ejecta (tephra) in a Sphagnum-dominated plant community in order to test the effects of this type of large-scale disturbance. Sixty plots of 1.4 x 1.4 m were established at Sarobetsu mire in northern Hokkaido, Japan and subjected to seven treatments (including the control) with natural tephra or ground glass shards differing in grain size, layer thickness and season of application. Water chemistry and vegetation were surveyed before tephra application and during five and eight years after the perturbation, respectively. Leaching of ions from fine-grained glass shards caused a sustained increase of soil water pH and electric conductivity. Under coarser materials water quality differed little from the control, but a short-lived peak of potassium suggested that mechanisms like nutrient release from decomposing plant material may influence water chemistry after volcanic disturbance. The perturbation initially reduced the cover of the dominant functional group (Sphagnum mosses) in all treatments; vascular plants were less affected. All species were able to recover by growing through the tephra, and open tephra surfaces were colonized by ubiquitous cryptogams, but not by spermatophytes. In contrast to the overall resilient behaviour of the vegetation, in some plots that received natural tephra an alternative state with high cover of the dwarf shrub Myrica gale developed. The patterns indicated that physical and chemical properties of the tephra determined the initial effects on plants, but that stochastic processes contributed to subsequent succession. These are likely to have effects on ecosystem functioning, e.g. hydrological processes and carbon sequestration.