Plant-water sensitivity regulates wildfire vulnerability

Abstract

Extreme wildfires extensively impact human health and the environment. Increasing vapour pressure deficit (VPD) has led to a chronic increase in wildfire area in the western United States, yet some regions have been more affected than others. Here we show that for the same increase in VPD, burned area increases more in regions where vegetation moisture shows greater sensitivity to water limitation (plant-water sensitivity; R² = 0.71). This has led to rapid increases in human exposure to wildfire risk, both because the population living in areas with high plant-water sensitivity grew 50% faster during 1990-2010 than in other wildland-urban interfaces and because VPD has risen most rapidly in these vulnerable areas. As plant-water sensitivity is strongly linked to wildfire vulnerability, accounting for ecophysiological controls should improve wildfire forecasts. If recent trends in VPD and demographic shifts continue, human wildfire risk will probably continue to increase.

Fig. 1. PWS and its link to wildfire vulnerability.

a, Sensitivity of burned area to VPD as a function of PWS. Points indicate data for 15 equal-vegetated area bins of PWS (Supplementary Fig. 1). Whiskers indicate 1 standard error in the estimate of slope. Pink bars at the bottom represent wildfire hazard due to PWS. b, The PWS calculation is illustrated for a sample pixel originating from the first (last) PWS bin and shown on the left (right). In each case, for visual simplicity, only data for the time lag leading to the highest slope between LFMC anomaly and climate-derived moisture balance anomaly are shown, even though PWS is calculated as the sum of all slopes (see Methods). c, Annual burned area versus mean annual VPD (April–March) for all pixels in the first (last) PWS bin is shown on the left (right). In all panels, the first (last) PWS bin is indicated by blue (yellow); thick lines indicate linear regression best fits, with grey bands indicating 95% confidence intervals. For the geographic locations of data presented in b and c, see Supplementary Fig. 2.

Fig. 2. Variable importance of plant and soil hydraulic traits to predict PWS.

Variable importance is estimated from average reduction in node impurity in random forests (see Methods). K_s denotes saturated soil hydraulic conductivity, n denotes the shape parameter of soil water retention curves, ψ₅₀ denotes xylem water potential at 50% loss in xylem conductivity, and g₁ denotes stomatal conductance slope parameter from ref. , which is inversely proportional to the square root of water use efficiency. For description and data source of traits, see Supplementary Table 1.

Fig. 3. Large VPD trends and high PWS co-occur in the western US.

a, Joint density of VPD trend from 1980–2020 and PWS. Darker colours indicate higher density. Box of double-hazard regions represents areas where wildfire hazard is high due to the co-occurrence of high PWS and VPD rise. b, Boxplot showing PWS distribution for four VPD trend bins. Box length indicates the interquartile range, the bisector indicates the median and whiskers extend to 1.5 times the interquartile range. The number of pixels in the four ascending VPD trend bins are 24,935, 52,591, 96,791 and 103,729, respectively. ε denotes the range in which VPD trends belong. c,d PWS map (c) and VPD trend map (d) with double-hazard region contour overlaid in pink. White patches in c indicate that PWS is unavailable due to insufficient data (see Methods). Black lines indicate state boundaries.

Fig. 4. The WUI population in high wildfire hazard regions has risen at the fastest pace and experienced the most increase in percentage burned area per unit rise in VPD.

a, WUI populations in 1990 and 2010 in each hazard zone (hazard zones shown in Fig. 1a). b, Percent change in burned area (relative to 2001) per unit rise in VPD versus percent change in WUI population from 1990–2010 for each hazard zone.

Extended Data Fig. 1. Conceptual diagram showing the effect of plant-water sensitivity (PWS) on burned area.

The DFMC denotes dead fuel moisture content. It represents climate-derived moisture balance (see Methods). The LFMC denotes live fuel moisture content.