THE ROLE OF DISTURBANCE, TOPOGRAPHY, AND FOREST STRUCTURE IN THE DEVELOPMENT OF A MONTANE FOREST LANDSCAPE

Human set fires beginning in the mid 1800s and repeated insect outbrea ks of western spruce budworm (Choristoneura occidentalis Free.) and Do uglas-fir bark beetle (Dendroctonus pseudotsugae Hopk.) during the pas t 50 years have resulted in a dramatic change in the montane (<ca. 290 0 m) forest landscape of the Colorado Front Range. Here, I examine the historical and spatial relationship between these disturbance agents and topography using stand structure and dendroecological data from 38 contiguous stands. These data suggest that aspect and relief are impo rtant factors determining the spatial and temporal patterns of disturb ance, succession, and rates of stand development. The rate of postfire stand development and hence, subsequent stand susceptibility to insec t outbreaks appears to be related to aspect. North-facing stands exper ience rapid postfire development and greater susceptibility to insect attack due to higher host tree densities, larger mean tree size, and a more uniform distribution of host trees over larger contiguous areas. Postfire stand recovery on south facing slopes appears to be slower a nd stand susceptibility to insect attack is less due to lower host den sities, smaller mean tree size, and a less uniform distribution of hos t trees over smaller areas. Relief, independent of aspect, enhances th e structural diversity of the forest landscape by promoting irregular burn patterns and intensities, thus creating a fire-induced mosaic of different aged stands. As these different aged stands continue to grow older, they reach a stage of development susceptible to insect outbre aks at different times. As a result, insect-induced changes in the str uctural characteristics of the current landscape emulate fire-induced landscape patterns that developed largely due to human activities begi nning in the 1860s.