As the 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 tree-ring data set representing AD 1000-2007. The FDSI is approximately equally influenced by the warm-season vapour-pressure deficit (largely controlled by temperature) and cold-season precipitation, together explaining 82% of the FDSI variability. Correspondence between the FDSI and measures of forest productivity, mortality, bark-beetle outbreak and wildfire validate the FDSI as a holistic forest-vigour indicator. If the vapour-pressure deficit continues increasing as projected by climate models, the mean forest drought-stress by the 2050s will exceed that 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.
Bibliographical noteFunding Information:
The work was supported by LANL-LDRD and DOE-BER. We acknowledge contributors to the International Tree-Ring Databank and funding by the NSF (grant 0823090) for tree-ring data. We thank contributors of fire-scar data to the FACS database, accessed with assistance from E. Bigio. Unpublished fire-scar data donated by C. Aoki, P. Brown, E. Heyerdahl, P. Iniguez, M. Kaib and R. Wu. J. Paschke provided access to USFS FHTET data. M. Brown provided access to GIMMS AVHRR NDVI data. Dynamically downscaled model climate data came from NARCCAP, funded by NSF, DOE, NOAA and EPA. We appreciate constructive comments from P. Brown, K. Cavanaugh, M. Crimmins, P. Fulé, S. Garrity, J. Grahame, D. Gutzler, J. Hicke, X. Jiang, S. Leavitt, M. Massenkoff, A. Meddens, J. Michaelsen, C. Millar, B. Osborn, H. Powers, T. Rahn, N. Stephenson, C. Still, C. Tague and C. Xu.