In the last several decades in the southwestern United States, there has been a strong shift toward drier conditions in spring and summer. This shift can be described as an increase in the vapor pressure deficit (VPD), the difference between how much water the atmosphere contains and the maximum amount it can hold. Elevated VPD is linked to wildfire, drought and high temperature extremes, three important hazards in the region. There are limited data on VPD for the last ~50 years, and in arid and semi-arid regions, the observations disagree with model simulations, highlighting the need for longer-term VPD reconstructions. This project will use oxygen isotope measurements in tree rings to reconstruct VPD for the last 500 years, and compare the reconstruction to model simulations to assess how well the reconstruction and simulations agree, characterize the variability in VPD through time and identify the mechanisms that drive VPD changes. The Broader Impacts include opportunity for undergraduate student participation in the research. The goals of this project are to develop reconstructions of vapor pressure deficit (VPD) using tree-ring oxygen isotopes in the southwestern United States for the last 500 years. The project will update and extend existing tree-ring width and wood density data from archived and new tree-ring samples to reconstruct temperature and hydroclimate, and measure oxygen isotopes on those samples to reconstruct VPD. The VPD reconstruction will be compared