Compound disturbance in a managed landscape: Ecological effects of catastrophic blowdown, salvage-logging, and wildfire in a subalpine forest.

摘要

Compound disturbances occur in rapid sequence and override ecosystem recovery processes, often resulting in an ecosystem regime shift. This study evaluates the potential compounding effect of salvage-logging and wildfire following wind disturbance on seedling regeneration, understory composition, and ecosystem processes in a subalpine forest that sustained a catastrophic blowdown in 1997, was partially salvage-logged in 1999, and burned in 2002. From 2000 to 2002, I measured seedling density, composition, and growth; understory vegetation cover and composition; and leaf litter decomposition and nitrogen cycling rates in blown down, salvage-logged, and undisturbed forest areas. Results showed that despite massive structural disruption of the overstory, tight biotic control was maintained over ecosystem processes in the blowdown. Rates of nutrient cycling and leaf litter decomposition did not differ from intact areas. Recovery mechanisms included accelerated growth of understory seedlings and expansion of understory vegetation cover. Downed wood moderated light and temperature conditions in a manner that favored vegetation growth.; In contrast, mechanized salvage-logging resulted in elevated soil temperatures, soil compaction and erosion, and reduced rates of net and gross nitrogen cycling. Seedling density and understory vegetation cover were also reduced; new seedling establishment was minimal. Conditions in salvage-logged areas were similar to south-facing clearcuts, where failures in natural regeneration are common, suggesting that if new seedling establishment remains low, reestablishment of forest cover in salvage-logged areas will be delayed. Furthermore, sites with extremely low seedling densities may transition from a spruce-fir dominated ecosystem to a subalpine meadow. Thus, salvage-logging following windthrow is a compound disturbance that disrupts recovery mechanisms, converting a biologically intact ecosystem into a modified state, where a shift in ecosystem regime is possible.; Following the 2002 fires in the study area, I expanded the study to evaluate the effects of prior blowdown and salvage-logging disturbances on post-fire regeneration dynamics. Initial results suggest that fire tends to erase the effects of previous disturbances on soil properties and processes. However, new seedling establishment was only observed in previously undisturbed burned areas, and not in burned blowdown or burned salvage-logged areas, suggesting that with time, the effects of pre-fire disturbances may be more pronounced.