Testing conceptual models of early plant succession across a disturbance gradient

Studies of succession have a long history in ecology, but rigorous tests of general, unifying principles are rare. One barrier to these tests of theory is the paucity of longitudinal studies that span the broad gradients of disturbance severity that characterize large, infrequent disturbances. The cataclysmic eruption of Mount St. Helens (Washington, USA) in 1980 produced a heterogeneous landscape of disturbance conditions, including primary to secondary successional habitats, affording a unique opportunity to explore how rates and patterns of community change relate to disturbance severity, post-eruption site conditions and time. In this novel synthesis, we combined data from three long-term (c. 30-year) studies to compare rates and patterns of community change across three 'zones' representing a gradient of disturbance severity: primary successional blast zone, secondary successional tree blowdown/standing snag zone and secondary successional intact forest canopy/tephra deposit zone. Consistent with theory, rates of change in most community metrics (species composition, species richness, species gain/loss and rank abundance) decreased with time across the disturbance gradient. Surprisingly, rates of change were often greatest at intermediate-severity disturbance and similarly low at high- and low-severity disturbance. There was little evidence of compositional convergence among or within zones, counter to theory. Within zones, rates of change did not differ among 'site types' defined by pre- or post-eruption site characteristics (disturbance history, legacy effects or substrate characteristics). Synthesis. The hump-shaped relationships with disturbance severity runs counter to the theory predicting that community change will be slower during primary than during secondary succession. The similarly low rates of change after high- and low-severity disturbance reflect differing sets of controls: seed limitation and abiotic stress in the blast zone vs. vegetative re-emergence and low light in the tephra zone. Sites subjected to intermediate-severity disturbance were the most dynamic, supporting species with a greater diversity of regenerative traits and seral roles (ruderal, forest and non-forest). Succession in this post-eruption landscape reflects the complex, multifaceted nature of volcanic disturbance (including physical force, heating and burial) and the variety of ways in which biological systems can respond to these disturbance effects. Our results underscore the value of comparative studies of long-term, ecological processes for testing the assumptions and predictions of successional theory.