Compound disturbance effects on vegetation recovery from spruce beetle outbreak and high-severity wildfire

Authors: Amanda Carlson*, Colorado State University, Jason Sibold, Colorado State University , Tim Assal, USGS
Topics: Biogeography, Spatial Analysis & Modeling, Mountain Environments
Keywords: forests, wildfire, bark beetles, disturbance, ecology
Session Type: Paper
Day: 4/12/2018
Start / End Time: 5:20 PM / 7:00 PM
Room: Bayside B, Sheraton, 4th Floor
Presentation File: No File Uploaded


Severe spruce beetle outbreaks are raising concerns over increased wildfire danger in Rocky Mountain subalpine forests. Although many recent studies have found no conclusive link between beetle outbreaks and increased fire size or canopy mortality, few studies have addressed whether these combined disturbances produce compounded effects on short-term vegetation recovery. To assess this, we examined the spatial relationship between pre-fire spruce beetle outbreak severity and vegetation recovery in the West Fork Complex fire, southwest Colorado, USA. Vegetation recovery was assessed using the Landsat-derived Normalized Difference Vegetation Index (NDVI) two years after fire. Beetle outbreak severity was estimated using vegetation index differences (dVIs) derived from pre-outbreak and post-outbreak Landsat images, capturing the magnitude of beetle-caused canopy change. Of the dVIs we tested, we found the change in Normalized Difference in Moisture Index (dNDMI) to be well-correlated with field measurements of outbreak severity (R2 = 0.66). Using dNDMI as a proxy for outbreak severity, we found a significant negative correlation between outbreak severity and post-recovery NDVI in sequential autoregression models which also accounted for pre-disturbance NDVI, topography, and weather conditions at the time of burning. The correlation was stronger for locations with outbreak ≥ 5 years old at the time of fire than for locations with outbreak > 5 years old. These results indicate that vegetation recovery processes may be negatively impacted by severe spruce beetle outbreaks occurring within several years of stand-replacing wildfire. Beetle-caused tree mortality may result in increased surface fuel loads, which can increase fire impacts to soils.

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