Water-stressed plants promote wildfire spread

Knowledge of plant sensitivity to dry conditions can improve wildfire forecasting

As much the west becomes hotter and drier, large wildfires such as the 2007 Zaca Fire, which burned nearly 250,000 acres in Santa Barbara, are not uncommon. Now, the question is how to utilize plant characteristics and local ecology to better predict, manage, and prepare for such blazes.

JOHN NEWMAN/US FOREST SERVICE

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New research from the journal Nature reveals that the ability of local plants to withstand an increasingly arid environment greatly influences the effect of climate on wildfire, necessitating the creation of new models to accurately forecast wildfire risk.

In the Western United States, wildfire is ubiquitous, representing an omnipotent force with profound impacts on human communities and environmental processes alike. Although fire plays a natural role in regulating many ecosystems, anthropogenic climate change and urban expansion have exacerbated wildfire spread and intensity in recent years.

In 2021 alone, fire engulfed nearly 3 million acres in California, a magnitude of area greater than burned a decade prior. As wildfire occurrence continues to transcend seasonal boundaries, fire ecologists and emergency managers have shifted their attention from fighting fires to predicting patterns in wildfire vulnerability. That way, they can identify human communities and wildlands most likely to experience damage and take appropriate action ahead of time.

The term “wildfire vulnerability” encompasses the impact of climate on a fire’s burn area. Years of research on wildfire dynamics suggest a strong positive relationship between vapor pressure deficit (VPD), a proxy for how dry the air is, and the amount of area burned. That is, the less moisture in the atmosphere—or conversely, the higher the VPD—the more likely fire will scorch large portions of the landscape. This destructive relationship is of particular interest as hot and dry conditions become commonplace in much of the Western U.S.

While the relationship between climate and wildfire is well-documented, not all regions respond to similar changes in VPD, or aridity, in the same way. This disparity prompted Stanford researcher Krishna Rao and an international collaboration of scientists to search for the ecological missing link connecting aridity and fire spread.

Rather than explore large-scale atmospheric processes, the team looked for answers closer to home, hypothesizing that the distribution of vegetation and its sensitivity to water limitations drive wildfire vulnerability to a greater degree than previously thought.

Just as wildfire is not equally prevalent across areas with the same climate, heat and drought do not uniformly impact all vegetation types. Rather, some exhibit higher plant-water sensitivity (PWS) than others, meaning they are less equipped to retain moisture when water in the atmosphere is scarce. Plant physiology, including root depth, and soil properties, are in large part responsible for such differences.

To test the interaction between PWS and wildfire vulnerability, Rao and colleagues relied on remote sensing to detect vegetation moisture content across the Western U.S. The researchers then translated plant moisture into PWS and correlated the measurements with wildfire vulnerability from 2001 to 2020.

True to their premonitions, the study authors observed that for the same increase in VPD, the area burned increased more in plots where vegetation moisture displayed higher sensitivity to water limitations. In other words, more land will burn in water-stressed environments where plants can’t cope with so little moisture, and are therefore more flammable, compared to similar environments populated by robust vegetation.

As PWS increases (x-axis), so does wildfire vulnerability (y-axis). “Low hazard” characterizes areas where PWS is relatively low and change in burn area per change in VPD is constant. “High hazard” locations occur where high PWS interacts with VPD to significantly influence the amount of land burned.

RAO ET AL./PLANT WATER SENSITIVITY REGULATES WILDFIRE VULNERABILITY

This strong relationship between PWS and wildfire vulnerability held true across spatially disparate locations, especially in forests and shrub-dominated ecosystems, independent of previous precipitation events. Such results advance the idea that local processes like plant and soil interactions modulate the impact of aridity on wildfire spread, meaning examining climate and regional processes will not be enough to predict wildfire risk.

In their research, Rao and colleagues stumbled across two other unsettling, interconnected trends, the first being that the vegetation most sensitive to water limitations is disproportionately located where aridity is increasing fastest. The team refers to this co-occurrence of high PWS and high VPD as a “double-hazard,” amplifying the impact of climate change on wildfire in ecologically significant regions such as the southern Sierra Nevada.

Additionally, these double-hazard areas happen to be where the wildland-urban interface (WUI)—human settlements adjacent to pristine wilderness—is experiencing the greatest population growth. WUI expansion into these sensitive ecosystems increases human exposure to wildfire not only because of more anthropogenic ignitions but also because local plants are simply unable to tolerate a changing climate.

The study results strongly suggest that ecological factors are critical wildfire drivers. Yet, large-scale analyses of fire ignition and spread seldom examine the impact of plant physiology. Consequently, much of the west is likely at higher risk than reflected in current fire modeling.

Given the adverse implications of unnaturally large and frequent fire events for ecosystem health, human life, and property, scientists need an accurate understanding of all factors dictating wildfire vulnerability. Further study and incorporation of local ecology, including PWS, will be an essential step forward in improving wildfire forecasting and risk characterization, benefiting both humans and the environment.

Academic Citations

Rao, K., Williams, A. P., Diffenbaugh, N. S., Yebra, M., & Konings, A. G. (2022). Plant-water sensitivity regulates wildfire vulnerability. Nature Ecology & Evolution, 6(3), 332–339. https://doi.org/10.1038/s41559-021-01654-2