Do insect outbreaks reduce the severity of subsequent forest fires?

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  • Understanding the causes and consequences of rapid environmental change is an essential scientific frontier, particularly given the threat of climate-and land use-induced changes in disturbance regimes. In western North America, recent widespread insect outbreaks and wildfires have sparked acute concerns about potential insect-fire interactions. Although previous research shows that insect activity typically does not increase wildfire likelihood, key uncertainties remain regarding insect effects on wildfire severity (i.e., ecological impact). Recent assessments indicate that outbreak severity and burn severity are not strongly associated, but these studies have been limited to specific insect or fire events. Here, we present a regional census of large wildfire severity following outbreaks of two prevalent bark beetle and defoliator species, mountain pine beetle (Dendroctonus ponderosae) and western spruce budworm (Choristoneura freemani), across the US Pacific Northwest. Wefirst quantify insect effects on burn severity with spatial modeling at the fire event scale and then evaluate how these effects vary across the full population of insect-fire events (n = 81 spanning 1987-2011). In contrast to common assumptions of positive feedbacks, we find that insects generally reduce the severity of subsequent wildfires. Specific effects vary with insect type and timing, but both insects decrease the abundance of live vegetation susceptible to wildfire at multiple time lags. By dampening subsequent burn severity, native insects could buffer rather than exacerbate fire regime changes expected due to land use and climate change. In light of these findings, we recommend a precautionary approach when designing and implementing forest management policies intended to reduce wildfire hazard and increase resilience to global change.
  • Keywords: fire ecology, remote sensing, regime change, forest health, defoliator, bark beetle, disturbance interactions
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  • Meigs, G. W., Zald, H. S., Campbell, J. L., Keeton, W. S., & Kennedy, R. E. (2016). Do insect outbreaks reduce the severity of subsequent forest fires?. Environmental Research Letters, 11(4), 045008. doi:10.1088/1748-9326/11/4/045008
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  • 11
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  • 4
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  • This research was supported in part by NASA Headquarters under the NASA Earth and Space Science Fellowship Program-Grant NNX11AM01H, the USDA Forest Service, Forest Health Monitoring (FHM) Evaluation Monitoring-Grant WC-F-09-2, and the USDA McIntire-Stennis Forest Research Program. The development of the LandTrendr-based disturbance maps reported in this paper was supported by the USDA Forest Service Northwest Forest Plan Effectiveness Monitoring Program, the North American Carbon Program through grants from NASA's Terrestrial Ecology, Carbon Cycle Science, and Applied Sciences Programs, the NASA New Investigator Program, the Office of Science (BER) of the US Department of Energy, and the following Inventory and Monitoring networks of the National Park Service: Southwest Alaska, Sierra Nevada, Northern Colorado Plateau, and Southern Colorado Plateau.
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