The arrival of the North American Monsoon System (NAMS) terminates a 
presummer hyperarid period in the southwestern United States (U.S.), 
providing summer moisture that is favorable for forest growth. Montane 
forests in this region rely on winter snowpack to drive much of their 
growth; the extent to which they use NAMS moisture is uncertain. We 
addressed this by studying stable carbon and oxygen isotopes in 
earlywood and latewood from 11 sites along a latitudinal gradient 
extending from Arizona and
New Mexico to Utah. This study provides the first regional perspective 
on the relative roles of winter versus summer precipitation as an 
ecophysiological resource. Here we present evidence that Ponderosa pine 
uses NAMS moisture differentially across this gradient. 13C/12C ratios 
suggest that photosynthetic water use
efficiency during latewood formation is more sensitive to summer 
precipitation at the northern than at the southern sites. This is likely 
due to the fact that NAMS moisture provides sufficiently favorable 
conditions for tree photosynthesis and growth during most years in the 
southern sites, whereas the northern sites
experience larger summer moisture variability, which in some years is 
limiting growth. Cellulose δ18O and δ13C values revealed that 
photoassimilates in the southern sites were produced under higher vapor 
pressure deficit conditions during spring compared to summer, 
demonstrating a previously underappreciated effect
of seasonal differences in atmospheric humidity on tree ring isotope 
ratios. Our findings suggest that future changes in NAMS will 
potentially alter productivity and photosynthetic water use dynamics 
differentially along latitudinal gradients in southwestern U.S. montane 
forests.