Atmospheric dryness drives boreal aboveground biomass dynamics through cascading effects on tree growth and fire

Atmospheric dryness, quantified as vapor pressure deficit (VPD), is rising with climate warming, yet its joint impacts on tree growth, fire activity, and biomass dynamics remain understudied at broad spatial scales. Here, we investigate how interannual variability in VPD relates jointly to tree growth, fire activity, and aboveground biomass dynamics across the western North American boreal–arctic transition encompassed by NASA’s ABoVE study domain. We assembled long-term time series of spring and summer daytime VPD (1951–2022), tree-ring based basal area increments (846 trees across 199 national forest inventory plots; 1950–2009), annual area burned (1950–2020), and Landsat-derived aboveground biomass increments (1985–2014). Pairwise relationships were quantified with Pearson correlations accounting for serial persistence; a structural-equation-model diagram summarizes significant linkages. Rising VPD promoted basal area increment reductions and increased annual area burned, whereas aboveground biomass increased with higher basal area increments and declined with increasing annual area burned. Structural equation modeling revealed that VPD does not act directly on biomass stocks; instead, it influences biomass accumulation through cascading effects—by suppressing tree growth and amplifying fire activity, which together govern long-term carbon storage. Our findings caution that remote sensing alone may fail to capture the sensitivity of biomass accumulation to atmospheric dryness, as physiological stress and disturbance interactions often leave subtle or lagged signatures in satellite-derived metrics. The results illustrate the need to move beyond isolated treatment of fire and growth in ecosystem models. Incorporating these dual pathways into fire behavior and carbon budget models is essential for anticipating boreal forest trajectories under continuing warming.