COS 93-2 - Compounding disturbances and their impact on regeneration of subalpine tree species

Thursday, August 11, 2011: 8:20 AM
9AB, Austin Convention Center
Brian Justin Buma, Natural Science, University of Alaska Southeast, Juneau, AK and Carol A. Wessman, Cires, University of Colorado-Boulder, Boulder, CO

Multiple, interacting disturbances are a topic of intense, current interest in ecology.  Landscape-scale disturbances strongly influence structure and function of large areas and therefore impact multiple scales and systems.  While individual disturbances have been well studied, interactions between multiple events (“compounded perturbations”) have the potential to cause ecological surprises.  Of particular concern is the potential ability of compounding disturbances to overwhelm species resilience mechanisms, potentially resulting in dramatic changes to ecosystem character.  We investigated the interactive effects of three major disturbances (wind, logging, and fire) on the dominant tree species of the Rocky Mountain subalpine zone; because this ecosystem can exist in multiple stable states, the resilience of tree species is crucial to long-term ecosystem composition post-disturbance.  A 1997 blowdown created a mosaic of initial disturbance severities (from 0-100% of the canopy blown down); salvage logging from 1998-2001 occurred in some areas.  Finally, in 2002, a large wildfire burned portions of the blowdown areas, logged areas, and undisturbed forest.  This combination created a rare opportunity to investigate disturbance interactions along a gradient of combined severities.  In 2010, tree regeneration and abiotic factors were surveyed in severely burned sites which differed in the initial disturbance severity (density of downed trees) and the presence of salvage logging.  Data was analyzed using non-parametric methods and CART.


Results indicate that post-fire lodgepole pine was heavily influenced by the severity of the blowdown; however the presence of salvage logging increased lodgepole seedling densities.  Spruce densities were insensitive to prior disturbance of any kind; aspen densities were higher in logged areas, but otherwise not related to previous disturbance severity.  In terms of general functional responses, it appears that fire-dependent resilience mechanisms (cone serotiny in lodgepole pine) were overcome by the interaction of blowdown and fire, likely driven by an increase in fuel loading.  Seed dispersal regeneration strategies were not affected by multiple disturbances, although disturbance interactions which cause larger spatial extents of disturbed areas may impact dispersal-based resilience strategies.  Under scenarios of increased disturbance, it is conceivable that a lack of resilience to compound events will be found in species with complex or specific disturbance-response relationships as opposed to invader and avoider-type species.  In this study area, the legacies of the interaction will likely involve an increase in spatial heterogeneity, decreased resilience to future fires, and large areas of newly formed subalpine grasslands and aspen stands.

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