In the southwestern U.S. the North American Monsoon (NAM) delivers summer precipitation from July – September and the rest is provided as snowmelt in early spring. Both periods serve as an important water source for plants, but due to the heterogeneous nature of the NAM, not all locations within the region receive precipitation, leading to differences in soil moisture. Understanding how spatially heterogeneous soil water storage controls plant water use is needed, especially with the increasing use of stable isotopes within tree ring cellulose to study the relationship between climate and water cycling. Thus, we examined the importance of NAM precipitation as a water source in different populations of Pinus ponderosa growing in the northern boundary of the NAM. We asked: 1) How variable is water use of NAM rains among seven populations of Pinus ponderosa? and 2) How does Pex modulate the degree in which VPD and source water are recorded in wood cellulose? We measured xylem water isotopes in trees from seven sites across southern Utah and northern Arizona during the 2018 and 2021 growing season. We also measured the δ¹⁸O of cellulose for early wood and late wood in the 2018 annual ring. We found that in both years trees switched from using a snowmelt dominated water source to a monsoon dominated water source, but the degree to which depended on antecedent moisture conditions from snowmelt. Using our understanding of changes in tree source water, we were able to detangle the effects of VPD and source water in the δ¹⁸O signal found in cellulose and determined that aridity plays a role on the source/sink dynamics controlling the influence of VPD on δ¹⁸O of cellulose. By understanding how the isotopic signal of xylem water changes throughout the growing season, we can better recognize how different hydroclimates play a role in our interpretation of the δ¹⁸O of cellulose.