Drought conditions have been projected to increase globally as atmospheric CO2 (ca) and vapor pressure deficit (VPD) increase. Experimental and modelling studies have suggested that forest responses to drought may be bolstered by increased ca, but it remains unclear how prevalent this response is in natural forest ecosystems. In the Southwestern United States (SWUS), the North American Monsoon (NAM) provides large inputs of moisture to ecosystems during the growing season, interrupting maxima in temperature and VPD and providing forests with a growing-season reprieve from stress. In this talk, I will discuss a framework we developed using intrinsic water use efficiency and evaporative water use efficiency to evaluate how a dominant species’ drought response within the Western US, Pinus ponderosa, has been affected by increased ca and the NAM during a multi-decadal megadrought (MD). Our results suggest that the NAM, and its interactions with VPD and ca, contributes to meaningful discrepancies in drought stress between populations that reliably receive moisture from it and those which do not. These discrepancies have implications for forest health and function across the Western US during future droughts, likely enhanced by anthropogenic driven climate change.