In the south-western U.S., the North American Monsoon (NAM) delivers half of the annual precipitation from July – September and serves as an important water source for plants. The NAM is most intense in Southern Arizona and New Mexico, and while it can extend into northern Arizona and southern Utah, the northern boundary of the NAM remains unresolved. Further, winter precipitation can provide an important water source for forested areas within this region. The amount and timing of winter precipitation can influence how long trees remain active during the hyper-arid months of May and June. In this study, we examined the importance of the NAM precipitation as a water source in different populations of Pinus ponderosa growing at the northern boundary of the NAM. We asked: 1) What populations of Pinus ponderosa rely on NAM precipitation? and 2) Does the amount of winter precipitation affect the utilization of NAM precipitation? In this study, we measured xylem water isotopes from eight sites across southern Utah and Arizona before and after the 2018 NAM to understand tree water use within the region. We used PRISM estimates of precipitation, temperature, and vapor pressure deficit for the water year to characterize the different precipitation inputs.
Results/Conclusions
We found differences in NAM water use among the eight Pinus ponderosa populations and that this varied depending on the location of the tree populations within the NAM region. The more southern sites had a distinct separation between water use before and after the monsoon, while the more northern sites had less of a distinct change in source water. We also found sites located in southern Utah received more winter precipitation than sites in northern Arizona. We hypothesis that differences in NAM water use among the different populations is influenced by the different amount of winter precipitation each site received. During years with a wet winter, trees actually utilize more summer precipitation because the fine roots remain active for longer and can take advantage of the summer rains. However, during years with a dry winter, trees will use deeper water, and therefore use less summer precipitation. By understanding how the isotopic signal of xylem water changes after the monsoon, during late season cellulose formation, we can better constrain how source water is incorporated into wood cellulose.