Ecosystems are increasingly exposed to multiple disturbances as human land management practices fold into natural disturbance regimes and climate change presents system-scale stresses. Large-scale disturbance interactions and the legacies that might set initial conditions for ecosystem recovery are not well understood. We are studying the roles of historical contingencies and ecological resilience in subalpine ecosystem dynamics. We are establishing a long-term study to explore how multiple, rapidly sequenced disturbances impact ecosystem and successional processes, and whether thresholds exist that significantly alter landscape heterogeneity and dynamics. Our study area in northwestern Colorado’s Routt National Forest experienced a large blowdown in 1997, salvage logging in 1999-2001, and a stand-replacing fire in 2002. From 2000-2002, we sampled understory diversity, conifer regeneration, soil properties and nitrogen cycling in intact, blowdown and logged blowdown landscapes. In the first two years following the wildfire of 2002, we measured nitrogen availability, vegetation cover and seedling density in the burned landscapes with the intact, blowdown and logging histories. Four to six years later we inventoried seedling density, understory diversity and cover, soil properties (including temperature, moisture, carbon and nitrogen content, pH, texture) and soil available nitrogen in each disturbance combination.
Results/Conclusions
Despite massive structural disruption of the overstory, tight biotic control was maintained over ecosystem processes following the blowdown. Rates of nitrogen cycling did not differ from intact areas, and recovery mechanisms included accelerated growth of understory seedlings and expansion of understory cover. Salvage-logging resulted in elevated soil temperatures, soil compaction and erosion, and reduced rates of nitrogen cycling. Initially, wildfire in 2002 appeared to erase the effects of previous disturbances on soil properties and processes, however a pattern in new seedling establishment emerged (seedlings observed only in forests with no recent pre-fire disturbance) that suggested a role of disturbance history. After six years, post-fire succession was significantly less in sites where blowdown, but not logging, occurred prior to the fire, suggesting that fuels present in the blowdown contributed to fire severity by creating higher temperatures for extended time periods. Nitrogen availability and total nitrogen were significantly decreased and total carbon increased in areas that were logged prior to the fire, indicating an influence of pre-fire surface soil conditions. This study suggests that interactions among disturbances impact post-disturbance biogeochemical and succession processes, and legacies of historical disturbance contribute to future structure and functioning of subalpine landscapes.
