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Disturbance Dynamics in Subalpine Forest
Ecological theory suggests that succession becomes increasingly unpredictable as disturbance intensity increases. If disturbances occur in rapid succession, regeneration patterns and mechanisms are much less predictable than after a single disturbance and novel successional pathways are possible. Increasingly, ecosystems are exposed to multiple disturbances as human land management practices fold into natural disturbance regimes, perhaps magnifying system-scale stresses. The largest known blowdown ever recorded in the Rocky Mountain region occurred in Colorado’s Routt National Forest in 1997. Two years later, the U.S. Forest Service began salvage-logging operations in selected areas. In 2002, the second largest fire during a record fire season in Colorado consumed a large portion of the area. Our research seeks to understand the interactive effects of rapidly sequenced disturbances on regeneration and biogeochemical dynamics of subalpine forest ecosystems.
Research by Dr. Cristina Rumbaitis-del Rio in the Routt National Forest demonstrated that subalpine forests experiencing wind disturbance retain tight biotic control on regeneration processes. However, salvage-logging following the blowdown resulted in a highly modified ecosystem that was compositionally and functionally different from unlogged blowdown areas. Salvage-logging in wind-disturbed areas reduced net nitrogen mineralization rates in soil, eroded and compacted the organic soil horizon, and significantly reduced understory vegetation cover and tree seedling density. Other research has shown that regeneration may fail under these conditions, resulting in a shift from coniferous forest vegetation to subalpine grassland communities, which can preclude conifer establishment for a century or more. We are establishing a longer-term study of forest resilience and regeneration in this region. Work in the first two years following the 2002 fire suggests that, initially, the effects of a severe fire tend to “erase” the effects of previous disturbances on soil properties, and nitrogen cycling. However, with time, the effects of disturbances that occurred prior to the wildfire may become more pronounced. [E.g., first-year seedlings were only observed in the control-burned areas with no prior (recent) disturbance.] Continued work in this study area will follow biogeochemistry and regeneration dynamics to further establish if post-fire conditions are qualitatively or quantitatively different as a result of pre-fire disturbance histories. We will consider the spatial and temporal scales of influence for the primary variables controlling natural seedling regeneration. Our work will be based on an integration of field observations, remote sensing, geospatial analysis, and forest growth modeling. We seek to identify critical thresholds important to successful forest recovery and to produce management tools for strategic planning in response to large-scale disturbances.