New Survey Tools Cast Doubt on Old Assumptions

Here in the Northwest, researchers and forest managers are hoping to understand what it will take to make our region’s forests more resilient in the face of climate change and disturbances from pests, including the notorious mountain pine beetle. Two recent studies attempt to better define and quantify what this resilience might look like.

The first study, published in Frontiers in Ecology and Evolution, challenges the conventional wisdom that removing small trees increases the resiliency of forests to wildfire. This belief is based on the assumption that historically, forests in the American West consisted of mostly large trees. However, following an extensive analysis of historic land surveys, the paper’s authors, William Baker and Mark Williams, came to the opposite conclusion, that, in fact, small trees constituted the majority of trees in the historic forests of the American West.

The authors further surmise that these historic forests included a variety of tree species and sizes, and that this diversity provided for a high level of resilience to disturbance. (Small trees are more resistant to bark beetles, for instance.) In a related analysis, they determined that from 1999 to 2012 insects affected more than five times the forest area than moderate- to high-severity wild fires, meaning insects are the major threat to these forests, not fires. The authors suggest that removing small trees in an effort to “restore” forests and make them less susceptible to disturbance — especially wild fires — probably does neither.

In a related paper published in Forest Ecology and Management, Garrett Meigs and colleagues propose a novel method to better quantify the impacts of insect pests on forests in the American West.

Focusing on forest disturbance from mountain pine beetle and western spruce budworm, the authors’ method combines complementary forest-health aerial detection surveys (using both light fixed-wing aircraft and helicopters) with Landsat satellite time series (satellite photos taken at regular time periods) and forest inventory data (collected via “boots on the ground” sampling). The result was an “insect atlas,” allowing for comparisons across time, space and types of insects, something that has not been previously possible.

Meigs and colleagues note an interesting finding: The western spruce budworm outbreaks have exceeded the more well-known mountain pine beetle outbreaks in both extent and total tree mortality at both the regional and ecoregional scales. (An ecoregion is a large geographic area with a distinct assemblage of species, natural communities, and environmental conditions.)

Our Editorial Opinion: The Meigs approach has important advantages over previous approaches based on aerial detection surveys (ADS) only. For instance, it captures fine-scale detail within ADS polygons, which results in reduced estimates of insect extent. Given projected increases in fire and insect outbreaks in western forests, this new method has several important management implications, providing a key context for strategic planning, ecosystem monitoring and silvicultural prescriptions, and by identifying “hotspots” where pest insects have had synergistic impacts.

Citations: Baker, W.L., & M.A. Williams (2015) Bet-hedging dry-forest resilience to climate-change threats in the western USA based on historical forest structure, Frontiers in Ecology and Evolution, 2(88). doi: 10.3389/fevo.2014.00088

Meigs, G.W., R.E. Kennedy, A.N. Gray, & M.J. Gregory (2015) Spatiotemporal dynamics of recent mountain pine beetle and western spruce budworm outbreaks across the Pacific Northwest Region, Forest Ecology and Management 339, 71–86. doi: 10.1016/j.foreco.2014.11.030 0378-1127

Photo Caption: “Mountain pine beetle caused mortality in ponderosa and lodgepole pine.” Photo Credit: US Forest Service- US Department of Agriculture. 

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