One if by stem, two if by leaf: Bark beetle and defoliator impacts on tree mortality and surface fuels in Pacific Northwest forests

Background/Question/Methods

In an era defined by global environmental change, it is important to quantify the impacts and interactions of natural disturbances such as insects and wildfires.  Across western North America, two prevalent native insects are mountain pine beetle (MPB; Dendroctonus ponderosae; a bark beetle) and western spruce budworm (WSB; Choristoneura occidentalis; a defoliator).  The goal of this study was to characterize the effects of these bark beetles and defoliators on tree mortality and surface fuels, leveraging a combination of field and remote sensing observations (satellite and aerial surveys).  To this end, we stratified the Pacific Northwest study region by forest type, insect agent (MPB vs. WSB), and time since insect disturbance.  We compared multi-date field measurements at federal inventory plots (current vegetation survey network; n = 12,000) with Landsat TM/ETM+ spectral trajectories since 1985 and with cumulative mortality estimates from forest health aerial detection surveys since 1980.  We also integrated the remote sensing and field datasets to develop insect, fuel, and fire maps across Oregon and Washington forests from 1980 to 2010.

Results/Conclusions

Despite high variation across the study region, spatiotemporal patterns were evident in the ground-based and remote sensing records of insect disturbance.  MPB outbreaks occurred in two phases—the first in central Oregon during the 1980s and the second dispersed throughout the Eastern Cascades during the 2000s.  Reflecting differences in their habitat susceptibility and epidemiology, WSB outbreaks began in the 1970s in eastern Oregon and appeared to spread west during the 1980s and north into Washington during the 1990s and 2000s.  Whereas MPB outbreaks exhibited highly variable temporal signatures (both short- and long-duration tree mortality), WSB outbreaks consistently were associated with long-duration tree mortality followed by recovery.  For both insects, cumulative overstory tree mortality ranged widely and was generally partial to moderate (well less than 100% mortality) at the forest stand scale.  In terms of surface fuels, down coarse woody detritus estimates were associated more strongly with Landsat spectral indices than aerial observations.  In contrast, fine woody detritus and forest floor depth were not well correlated with either remote sensing dataset.  Given the possible increase of insect and fire activity in western forests, the accurate assessment and monitoring of these disturbances will be crucial for sustainable ecosystem management.