Wildfire burned area has increased four-fold in the last four decades in the western US, in part due to increasing atmospheric aridity from global warming. The sensitivity of burned area to atmospheric aridity can vary significantly among ecosystems. Plant responses to atmospheric drought can affect live fuel moisture content (LFMC; mass of plant water per unit dry biomass) and thus fire spread, but this influence has been studied relatively little at ecosystem-scale, likely because of a lack of data on LFMC at large scales. We developed wall-to-wall maps of LFMC for the western US at 15-day intervals from 2016 - 2020. To do so, we combined microwave remote sensing observations from Sentinel-1 synthetic aperture radar (which are sensitive to water content) and optical remote sensing observations from Landsat (which are sensitive to biomass) in a recurrent neural network. The maps were cross-validated using ground measurements from the United States Forest Service National Fuel Moisture Database (R2 = 0.63). Using these LFMC maps, we demonstrate that plant physiological factors controlling the buffering capacity of plant water against atmospheric aridity (termed plant-water sensitivity or PWS) mediates the sensitivity of burned area to vapor pressure deficit (VPD; an indicator of atmospheric aridity). The temporal slope between annual burned area and mean fire-season VPD was strongly linked to PWS (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 PWS compared to those with low PWS. The spatial distribution of PWS exacerbated two additional factors controlling human wildfire risk. First, between 1980 and 2020, VPD rose the fastest in regions with high PWS, thus compounding wildfire risk. Second, between 2001 and 2016, the population residing in regions within the wildland-urban interface (WUI) with very high PWS (which are more vulnerable to wildfire) grew more quickly than in less vulnerable regions. The WUI consists of the area where houses are in or near wildland vegetation and is known to have increased wildfire risk. In the most drought-vulnerable regions of the WUI, the population increased by two million people (equivalent to the combined population of San Francisco and Seattle). Because the impact of atmospheric aridity on vegetation moisture is strongly linked to fire vulnerability, accounting for relationships between climate and vegetation moisture is likely to improve wildfire risk forecasts.
