Interactions among natural disturbances in subalpine forests in northwestern Colorado

I studied the interactions among fire, spruce beetle (Dendroctonus rufipennis), and wind disturbances in subalpine forests in northwestern Colorado. The central aim of this research was to understand how fire history affects forest susceptibility to subsequent disturbances by spruce beetle and blowdown, how the occurrence of beetle outbreak affects stand susceptibility to subsequent fire, and how these forests respond to blowdown.; I reconstructed the spatial disturbance history in a two c. 4,500 hectare areas in northwestern Colorado. The Flat Tops study area was affected by fire and beetle disturbances and the Mount Zirkel study area was affected by fire and wind disturbances. Disturbance history was reconstructed by first identifying distinct patches in the landscape on aerial photographs, and then in the field by determining the disturbance history of each patch by dating ages of post-fire cohorts, fire scars, dates of mortality of dead trees, and releases on remnant trees. A geographic information system (GIS) was used to examine the relationship between large disturbances in these areas.; The fire regime of the Flat Tops study area was characterized by large, infrequent, severe fires. The study area was also affected by a severe spruce beetle outbreak in the 1940s and a subsequent low-severity fire. Stands affected by stand-initiating fire in the late 19th century were less affected by the beetle outbreak than older stands, but stands which initiated after fire in 1796 and c. 1700 and old-growth stands did not show differential susceptibility to beetle outbreak.; The work in this dissertation shows that the ecological effects of even very high severity disturbances are mediated by the legacies of prior disturbance by fire. The present study also shows that beetle outbreaks do not always follow blowdowns and that the legacy of blowdowns can influence forest processes for decades following the blowdown event. The patterns and processes of these subalpine forests were strongly influenced by the interactions among fire, insect, and wind disturbances. Natural disturbances do not act in isolation from each other, but rather synergistically affect the structure and function of ecosystems. In systems where multiple disturbances operate in the landscape, it is necessary to study the interactions among those disturbances in order to more completely understand the dynamics of those systems. (Abstract shortened by UMI.)...