A forest is not a pan of water: Temperature and vapor-pressure deficit as potent drivers of regional forest drought stress and tree mortality

Background/Question/Methods

As climate changes, drought may reduce tree productivity and survival across many forest ecosystems; however, the relative influence of specific climate parameters on forest decline is poorly understood. We derive a forest drought-stress index (FDSI) for the southwestern United States using a comprehensive set of regional tree-ring records representing the three most common southwestern conifer species for AD 1000–2007. We compare the FDSI to observed records of climate, satellite-derived vegetation greenness, forest-inventory measurements, bark-beetle attack, and wildfires to infer relationships between forest drought stress and some important mechanisms of regional tree mortality. We use an ensemble of climate projections to estimate the future trend in regional forest drought stress and place it in the context of the past 1,000 years of forest-stress variability.

Results/Conclusions

The FDSI is approximately equally influenced by warm-season vapor-pressure deficit (largely controlled by temperature) and cold-season precipitation, together explaining an astounding 82% of southwestern FDSI variability. Correspondence between FDSI and measures of forest productivity, tree mortality, bark-beetle outbreak, and wildfire validate the FDSI as a holistic indicator of forest vigor. If vapor-pressure deficit continues increasing as projected by climate models, then mean annual forest drought-stress levels by the 2050s will exceed those of the most severe droughts in the past 1,000 years. Collectively, the results foreshadow twenty-first-century changes in forest structures and compositions, with transition of forests in the southwestern United States, and perhaps water-limited forests globally, towards distributions unfamiliar to modern civilization.