Photo by Phil van Mantgem, USGS.
It’s been ten years since Andy Westerling and his colleagues published their landmark paper on climate change and fire. In that paper, they showed that increased fire activity in the western United States was related more to a changing climate than to past forest management or fire suppression—a drastically different narrative than had been understood previously.
Their analysis showed that large wildfire activity increased “suddenly and markedly” in the mid-1980s, with more frequent large wildfires that burned for longer periods, and a wildfire season that extended well beyond the historical window, thanks to increased spring and summer temperatures and earlier spring snowmelt. They were able to separate climate change and forest management by including forests with a wide range of fire regimes, including some with infrequent, high-severity regimes and others adapted to more frequent, low-severity fires. They found the same patterns in place across these diverse forest types, with the most pronounced changes in forests where the last century of land use hadn’t exerted much influence on forest structure and fuels, and fire suppression hadn’t derailed natural fire regimes in major ways.
The publication of this paper was a watershed moment for the fire world: if climate change trumps forest management, what does that say for forest restoration and fuels management efforts aimed at righting the wrongs of the past and reducing future fire risk? However intriguing the paper’s findings, they were also somewhat discouraging.
Since then, there has been a wealth of additional research on climate change and fire, and the story has become even more complex. Whereas the Westerling paper looked primarily at fire activity (i.e., large fire frequency, fire duration and the length of the fire season), other work has since focused on climate change and fire severity, trying to better understand the effects of warming trends on spatial patterns of fire severity, fire behavior and individual tree mortality.
Along these lines, another gem of a paper was published in 2013 in Ecology Letters. That work, by USGS research ecologist Phil van Mantgem and his colleagues, showed that warming trends may be increasing forest fire severity (i.e., trees killed) independent of the intensity of the fires (i.e., how hot they burn). Their work demonstrated that chronic stresses in western forests, including drought and climate change, may make individual trees more vulnerable to fire, even when fires align well with historical fire regimes. Again, not an encouraging message for folks on the ground, especially those working to reintroduce fire to their local forests.
But hold the tears—there is good news coming from some of the same scientists!
Van Mantgem and others are just about to publish an article in Fire Ecology that has a more hopeful spin for forest and fire managers. This new paper presents research they completed during the 2014 drought, when the authors compared tree mortality in prescribed burned and unburned low-elevation mixed-conifer forests in the Sierra Nevada of California. Results showed that stands with a history of prescribed burning were less dense and had a lower incidence of recently dead trees. After accounting for differences in individual tree size, they showed that trees in stands with a history of prescribed burning appeared to have lower probabilities of drought-induced mortality. Relative to white fir and incense cedar, sugar pine and ponderosa pine were particularly sensitive to a history of prescribed burning, with high probabilities of mortality in unburned stands. The authors note that further work needs to be done to test if these effects held during the continued drought in 2015, and to determine if these results apply to other species, forest types and management treatments. However, these findings do complement research on the role of mechanical treatments in boosting forest resiliency (e.g., D’Amato et al. 2013, Thomas and Waring 2014), and they offer much-needed encouragement in what can seem an overwhelming time of change.
I won’t be surprised if some of you had a guttural reaction to the opening paragraphs of this blog—it’s difficult to imagine a scenario where forest management isn’t the primary driver of fire activity. However, it’s important to remember that the Westerling paper was assessing broad trends in fire size and duration and the length of fire season, not fuels treatment effectiveness or forest resiliency, and the paper acknowledges that in many forests, climate is layered on top of important management-related effects. Ultimately, we know that local expertise and adaptive management—informed by long-term monitoring and research like that presented above—are critical for pursuing the moving target that is fire adaptation.
Further reading:
D’Amato, A.W., J.B. Bradford, S. Fraver and B.J. Palik. 2013. Effects of thinning on drought vulnerability and climate response in north temperate forest ecosystems. Ecological Applications, 23: 1735-1742.
Thomas, Z. and K.M. Waring. 2014. Enhancing resiliency and restoring ecological attributes in second-growth ponderosa pine stands in northern New Mexico, USA. Forest Science, 60.
van Mantgem, P.J., Nesmith, J.C.B., Keifer, M., Knapp, E. E., Flint, A. and L. Flint. 2013. Climatic stress increases forest fire severity across the western United States. Ecology Letters, 16(9): 1151-1156.
van Mantgem, P.J., A. Caprio, N.L. Stephenson and A. Das. In press. Does prescribed fire promote forest resistance to drought in low elevations forests of the Sierra Nevada, California, USA? Fire Ecology, 12(1).
Westerling, A.L., H.G. Hidalgo, D.R. Cayan and T.W. Swetnam. 2006. Warming and earlier spring increase western US forest wildfire activity. Science, 313(5789): 940-943.
Editor’s Note: This is the first of a standing monthly fire science blog post. Have a topic you want addressed? Email Lenya with ideas or comment here!
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