Plant-Water Sensitivity Regulates Wildfire Vulnerability

Wildfire burned area in the western United States has increased four-fold in the last four decades, driven largely by increased vapor pressure deficit (VPD). The sensitivity of burned area to VPD can vary significantly among ecosystems, with plant responses to water limitation affecting vegetation moisture and thus fire ignition and spread. However, the influence of the buffering capacity of vegetation moisture during water limitation has been mostly unstudied because of a lack of large-scale vegetation moisture data. Using continental-scale maps of vegetation moisture derived from microwave remote sensing and machine learning, we quantify the influence of plant-waters sensitivity to water limitation (plant-water sensitivity) on increase in burned area. We demonstrate that the slope between annual burned area and VPD is strongly linked to plant-water sensitivity (R2=0.71, p<0.0001). For the same rise in VPD, burned area increased by more than twice as much in ecosystems with high plant-water sensitivity compared to those with low plant-water sensitivity. This indicates that plant-water sensitivity exhibits a strong control on VPD-driven increases in burned area. The spatial distribution of plant-water sensitivity exacerbated two additional factors controlling human wildfire risk. First, between 1980 and 2020, VPD rose fastest in regions with high plant-water sensitivity, thus compounding wildfire hazard. Second, between 1990 and 2010, population grew faster in regions of the wildland-urban interface with very high plant-water sensitivity than in less vulnerable regions. For example, between 1990 and 2010, the population in the highest-hazard regions grew by 160%, a rate of increase that is ~50% greater than that of the wildland-urban interface overall. This previously-unrecognized increase in wildfire risk has resulted in the addition of ~1.5 million people (equivalent to the combined population of San Francisco and Seattle) in the highest-hazard regions in the wildland-urban interface. Furthermore, if recent trends in VPD and demographic shifts continue, human wildfire risk in the western US can be expected to increase in the future.