P2C2: Past Extension of the North American Monsoon System (NAMS) into the Great Basin Reconstructed from Cell-to-Ecosystem Dendrochronology

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Long-term annual to decadal variability of water supply in the Great Basin region of North America is not well understood. Predictions of water supply and groundwater recharge under future warming scenarios can be improved by providing more accurate representations of past climatic changes derived from multi-century-long tree-ring records. Latewood-width and false-ring chronologies have been linked to summertime precipitation, and therefore can provide a record of warm-season moisture over several centuries. This project will exploit a unique potential for reconstructing past variability of the North American Monsoon System (NAMS), one of the key mechanisms for warm-season moisture delivery to the southern Great Basin. New ring-width chronologies combined with anatomical and stable isotopic analyses of conifer species in southern Nevada will be used to reconstruct the northern extension of the NAMS. Cellular-level dendroclimatology is an evolving area of research, and detailed studies of wood formation will complement existing projects in the NAMS region. The project will help clarify the connection between summertime circulation patterns and the El Niño Southern Oscillation (ENSO), as well as the contribution of seasonal moisture regimes during prolonged drought episodes. Expected research results will refine predictions of future differences in water availability between the American Southwest, the Great Basin/Mojave Deserts, and the Four-Corners Region.This research is motivated by the pressing need to quantify the environmental controls of wood formation and the mechanisms underlying dendroclimatic proxies. The project, a collaboration between the University of Nevada, Reno, and the Université du Québec à Chicoutimi, builds on state-of-the-art instrumental transects recently established across two mountain ranges in the Great Basin. Specific research goals are: (1) to mechanistically determine the climatic drivers for earlywood/latewood and false-ring formation in populations of ponderosa pine located at the northwestern limit of the NAMS; and (2) to reconstruct summer monsoon influences on tree growth in this NAMS boundary region as an index of NAMS extension, and its interannual variability, over the past few centuries. Field sampling will take place in the Sheep Range, where the "Nevada Climate-ecohydrological Assessment Network" (NevCAN) provides measurements of atmospheric, pedologic, and vegetational variables along a valley-to-mountain-top transect. Detailed calibration of ponderosa pine tree-ring records will be achieved by combining wood anatomical measurements with stable isotopic analyses of oxygen and carbon from tree rings. Stable oxygen isotopes will also be measured from soil water, atmospheric water vapor, and precipitation to better calibrate the tree ring isotope measurements. Funding supports training of a postdoctoral researcher and a graduate student. The research results will be integrated into graduate and undergraduate level courses at the University of Nevada, Reno, a campus whose student population is constantly increasing in number and diversity.