Prescribed Fires Help Forests Survive Droughts

Fire exclusion (i.e., trying to eliminate fires from the landscape) was, for much of the 20th century, the prevailing policy in the management of forests in the Sierra Nevada. Much has been written on the effects of this management strategy on creating landscapes vulnerable to destructive fires, but there's been little research on how fire exclusion might affect the resilience of forests to other potentially damaging events. A recent paper by van Mantgem and colleagues in forests helps to fill that gap by examining how fire may affect the ability of forests to withstand droughts.

Mechanical thinning and prescribed fires are common strategies for reducing fuel load and they may also benefit the remaining trees by reducing the competition for sunlight, water, and nutrients. The authors of this paper were interested to know whether the removal of small trees by prescribed fires, alone, could confer advantages to the remaining trees, without the added benefits derived from mechanical thinning. To answer this question, they compiled data from forests in the Sequoia and Kings Canyon National Parks (CA) that have been managed, since the 1960's, to experience fires from prescribed burns as well as from lightning strikes. Their approach consisted of identifying trees (fir and pine) killed by the 2012-2014 drought and comparing tree mortality between plots where fire was allowed and plots where fire was excluded.

Study sites in Kings Canyon and Sequoia National Parks. The red dots show the location of the burned plots and the blue dots show the location of the unburned plots. The burned and unburned sites were chosen in proximity to each other to control for other factors such as topography, precipitation, etc.

After analyzing their data, the authors found that the burned plots had fewer and larger trees; although this was expected, it validated of one of their key assumptions - that prescribed fire leads to less competition.

Unburned plots have a higher stem density (i.e., more trees) than unburned plots. Figure 2a from van Mantgem et al.

Importantly, they also found that more trees had died during the drought in the unburned plots than in the burned plots. However, this effect differed according to tree size, whereby the smallest trees were affected the most.

The bar plot below shows that there were more drought-killed trees in the unburned plots (white bars) relative to the burned plots (black bars) for the small and medium-size trees. The difference in mortality for the large trees was not statistically significant. Figure 3 from the paper.

Because there were fewer trees in the burned plots, the authors suggested that reduced competition for resources was a contributing factor in reducing mortality for the smaller trees. While this is an encouraging result, the authors highlighted the lack of a significant effect for the larger trees which, presumably, are the ones that we're most interested in protecting. Although the authors didn't explicitly explore this idea, one might hypothesize that the larger trees are not affected by the thinning of smaller trees by fire because they have deeper roots that can draw water from underlying bedrock. An interesting paper by Dr. Daniella Rempe (UT Austin) has shown how trees in northern California rely on water stored in weathered bedrock to get through the summer dry season. If larger trees have deeper roots that can tap into weathered bedrock more efficiently than smaller trees, perhaps the large trees and the small trees aren't actually competing for water - this might explain why the removal of small trees by fire didn't help the larger trees to a significant degree.

Trees send their roots down to access water in weathered bedrock. Figure 1 from Rempe and Dietrich (2018).

Finally, although van Mantgem and his colleagues conclude that prescribed fire appears to increase the resiliency of forests to drought, they caution that it's a complicated issue. For example, prescribed fires may also damage trees and increase their vulnerability to bark beetle attack. Nevertheless, the results from this study are intriguing and shed light on a complex system in a way that can only help future management decisions.