Abstract
The North American Monsoon delivers warm season precipitation to much of the southwestern United States, yet the importance of this water source for forested ecosystems in the region is not well understood. While it is widely accepted that trees in southwestern forests use winter precipitation for earlywood production, the extent to which summer (monsoon season) precipitation supports latewood production is unclear. We used tree ring records, local climate data, and stable isotope analyses (δ18O) of water and cellulose to examine the importance of monsoon precipitation for latewood production in mature ponderosa pine (Pinus ponderosa Dougl.) in northern Arizona. Our analyses identified monsoon season vapor pressure deficit (VPD) and Palmer Drought Severity Index (PDSI) as significant effects on latewood growth, together explaining 39% of latewood ring width variation. Stem water and cellulose δ18O analyses suggest that monsoon precipitation was not directly used for latewood growth. Our findings suggest that mature ponderosa pines in this region utilize winter precipitation for growth throughout the entire year. The influence of monsoon precipitation on growth is indirect and mediated by its effect on atmospheric moisture stress (VPD). Together, summer VPD and antecedent soil moisture conditions have a strong influence on latewood growth.
Highlights
The North American Monsoon delivers warm season precipitation to much of the southwestern United States, yet the importance of this water source for forested ecosystems in the region is not well understood
Our study strongly suggests that mature ponderosa pines in northern Arizona do not use monsoon monsoon precipitation to produce latewood
PDSItrees characterizes long-term drought and at the monthly scale is influenced by the water balance of used monsoon water, we found evidence that latewood growth increased in response to lower previous monsoon is a reflects more direct quantify higher the immediate short-term and effects of vapor pressure deficit (VPD) months, during this season
Summary
The North American Monsoon delivers warm season precipitation to much of the southwestern United States, yet the importance of this water source for forested ecosystems in the region is not well understood. American Monsoon System has had major shifts in the past [16,17] and a substantial northward shift in monsoon precipitation distribution is expected in the future [18,19,20]. This shift could reduce summer precipitation in areas that receive a large fraction of annual precipitation during the summer monsoon season. Understanding the ecological implications of these potential changes in the timing and form of precipitation can be improved by studies of seasonal sources of moisture to dominant vegetation
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