THE INFLUENCE OF BARK BEETLE-INDUCED TREE MORTALITY ON GEOMORPHIC PROCESSES OF SAN JUAN FOREST NEAR OURAY, CO

Degradation of the natural environment has resulted from bark-beetle destruction at various geographic locations around Earth. Bark-beetle investigations result in wide-spread tree mortality, which has caused the loss of vegetation cover and led to increased kindling resulting in increased fire hazard potential. In addition, the loss of trees also has resulted in increased rates of slope erosion.

Presently, widespread tree mortality is occurring in the San Juan Mountains of southwestern Colorado. Methods of remote sensing for classifying mountain pine beetle-induced tree mortality, have been developed, and we applied these to study spruce (Dendroctonus rufipennis) and fir-engraver beetle (Scolytus ventralis) outbreaks, which have been occurring in the Ouray, Colorado area over the last 20 years. Aerial photography was used to produce a tree-scale map of gray-stage beetle kills across the surrounding forest. Landsat time-series change detection algorithm, LandTrendr, was used to identify periods of stability and change in tree stands.

Combining fieldwork, remote sensing, and GIS, we modeled the rate of mortality for trees in the area. We used this rate of mortality to examined the relationship between tree mortality and rates of erosion. Values of water uptake and interception are known for single species of pine and fur trees. Knowing these rates for individual trees, we can calculate the amount of “free” water available as surface flow and resulting erosion as the infestation increased. Sample plots were used to establish changes in surface compaction resulting in overland flow and surface-wash erosion for different aspects and different elevations. Soil type, geology, slope angle and length, aspect, and species canopy cover served as the framework to develop a dynamic model of the area. Derivative maps were created to model erosion rates as well as canopy cover. Comparison of USGS discharge records for gage-station locations were used to determine increased rates of runoff prior to and during the bark-beetle infestation. This study will help to highlight the important link between bark-beetle outbreaks and the amount of resulting erosion in alpine areas.