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Variation in the Bimodal Precipitation Regime of Southern Nevada, USA Measured in Tree-Ring Isotopes
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The North American Monsoon System (NAMS) is a seasonal climatic phenomenon vital to ecosystems and human population centers in the southwestern USA and northwestern Mexico. Inter-annual variation of the NAMS bimodal precipitation regime is more predictable in its core region of northwestern Mexico, Arizona, and New Mexico, USA than at its northwest boundary. The NAMS onset and strength is projected to shift during the 21st century because of climate change, yet there is high uncertainty of how and when that might happen. We combined extensive field measurements of micrometeorology, stable isotopes, and tree rings from an old-growth ponderosa pine (Pinus ponderosa) stand in southern Nevada, USA. We tested if variation in the bimodal precipitation regime could be measured in the stable isotopes of tree rings. Hourly measurements from a meteorological station adjacent to the ponderosa pine stand were associated with regional seasonal patterns of integrated water vapor transport from 2011-2017. Biweekly measurements of stable isotopes in precipitation and xylem water were related to stable isotopes in the α-cellulose of tree rings to interpret subseasonal climatic patterns for the 2015 and 2016 growing season. Finally, a novel index calculated as the difference between the δ13C of α-cellulose in the latewood and earlywood (δ13CDIFF = latewood – earlywood) portions of a tree ring was highly influenced by the fraction (fSUMMER) of summer precipitation (June-September) to previous cool-season precipitation (October – April). The δ13CDIFF is an index that measures the water-stress of a tree throughout the growing season and could assist with dendroclimatic reconstructions near the boundary of the NAMS.