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University of Nevada, Reno Variation in the Bimodal Precipitation Regime

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University of Nevada, Reno Variation in the Bimodal Precipitation Regime University of Nevada, Reno Variation in the Bimodal Precipitation Regime of Southern Nevada, USA Measured in Tree-Ring Isotopes A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Ecology, Evolution, and Conservation Biology by Charles Myers Truettner Dr. Franco Biondi – Major Advisor Dr. Adam Z. Csank – Co-advisor August, 2020 THE GRADUATE SCHOOL We recommend that the dissertation prepared under our supervision by Charles Myers Truettner entitled Variation in the Bimodal Precipitation Regime of Southern Nevada, USA Measured in Tree-Ring Isotopes be accepted in partial fulfillment of the requirements for the degree of Doctor of Philosophy Franco Biondi, Ph.D. Advisor Adam Z. Csank, Ph.D. Co-advisor Michael D. Dettinger, Ph.D. Committee Member Peter J. Weisberg, Ph.D. Committee Member Simon R. Poulson, Ph.D. Graduate School Representative David W. Zeh, Ph.D., Dean Graduate School August 2020 i Abstract 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 13 (δ CDIFF = latewood – earlywood) portions of a tree ring was highly influenced by the fraction (fSUMMER) of summer precipitation (June-September) to previous cool-season 13 precipitation (October-April). The δ CDIFF 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. ii Acknowledgments I am in gratitude for the enthusiasm and guidance from my dissertation committee: Dr. Franco Biondi, Dr. Adam Csank, Dr. Simon Poulson, Dr. Michael Dettinger, and Dr. Peter Weisberg. Dr. Emanuele Ziaco, Dr. Scott Strachan, and Nicholas Miley were influential scientifically and emotionally during my experience at University of Nevada, Reno. Alyssa Mineau and Daniel McCready assisted in the laboratory. Finally, I thank all the professors and graduate students in the EECB, Geography, and Hydrology Graduate Programs for their support in this interdisciplinary project. iii Table of Contents List of Tables…………………………………………..…………………………………….…v List of Figures………………………………………………………...…..……………………vi General Introduction……………………………...…………………...………..….……….1 References……………………………………………………………….……………3 Chapter 1: Seasonal Analysis of the 2011–2017 North American Monsoon near its Northwest Boundary……………………………………………………..……....…….……5 Abstract………………………………………………………………………..…….…5 Introduction………………………………………………………………..…….……..6 Data and Methods………………………………………………………..…….……..8 Results……………………………………………………………………….……….11 Discussion……………………………………………………………………………16 Conclusions……………………………………………………………………….….22 Acknowledgments………………………………………………………….………..23 References………………………………………………………………….………..23 Figures………………………………………………………………………….…….32 Tables………………………………………………………………………….……..38 Chapter 2: Subseasonal Ponderosa Pine Radial Growth Response to Warm- Season Precipitation amid Drought in Southern Nevada………………..…….……39 Summary……………………………………………………………………………..39 Introduction…………………………………………………………………………..40 Methods………………………………………………………………………………44 Results………………………………………………………………………………..53 Discussion……………………………………………………………………………56 Conclusion……………………………………………………………………………60 Acknowledgments…………………………………………………………...………61 References…………………………………………………………………...………61 Figures………………………………………………………………………………..71 Tables…………………………………………………………………………………76 iv Chapter 3: Bimodal Precipitation Variation Measured in Intra-Annual Tree-Ring Isotope Chronologies from Southern Nevada, USA ……………………....…….…...77 Abstract………………………………..…………………………….……….….…….77 Introduction…………………………………...…………………………..…………..78 Methods…………………………………………………………………….…………81 Results………………………………………………………….……………….…….84 Discussion……………………………………………………………………....…….88 Conclusion…………………………………………………………………….………90 References……………………………………………………………………………91 Figures………………………………………………………………………………...98 Appendix A…………………………………………………………………….……………104 Appendix B……………………………………………………………………….…………105 Appendix C…………………………………………………………………………….……107 Appendix D……………………………………………………………………………….…111 Appendix E………………………………………………………………………….………112 Appendix F……………………………………………………………………….…………113 Appendix G………………………………………………………………………..………..115 Appendix H…………………………………………………………………….……………118 Appendix I………………………………………………………………………...…………119 v List of Tables Chapter 1 Table 1. Summary of seasonal and water year precipitation totals (PPT) at our study site in the Sheep Range of southern Nevada. Chapter 2 Table 1. Diameter at Breast Height (DBH), height, and final date of lignification for tree- ring growth quartiles for the 2015 and 2016 tree rings from the twelve ponderosa pines sampled vi List of Figures Chapter 1 Figure 1. Interpolated pseudo-color map of Pearson’s linear correlation between July– September total precipitation at our mountain site (solid white circle) and at each 800-m PRISM grid cell in the surrounding region during 1895–2015. All correlations were statistically significant (p-value < 0.01), with lower values usually corresponding to lower elevations, such as the Las Vegas valley (black circle). Figure 2. Time series plots of daily environmental variables measured at our study site. (a) Average dewpoint temperature and total precipitation; a 9.4 °C threshold of dewpoint temperature was used to identify the arrival of North American Monsoon (NAM) precipitation. (b) Volumetric water content at two soil depths (2–17 cm and 17–32 cm) and days when snow was present (see text for details). Figure 3. Precipitation percentages at our study site for cool (1 October–31 March), early warm (1 April to the day prior to 9.4 °C dewpoint threshold), and late warm (day of 9.4 °C dewpoint threshold to 30 September) seasons of the 2012–2017 water years. Figure 4. Average diurnal cycles of weather and soil variables by season. The variability of each hourly value is shown by vertical bars corresponding to two standard errors above and below the mean. Meteorological variables include: (a) total precipitation, (b) dewpoint temperature, (c) air temperature, (d) vapor-pressure deficit, (e) and solar radiation. Soil variables are (f) volumetric water content (VWC) at 2–17 cm soil depth, and (g) VWC at 17–32 cm soil depth. Figure 5. Total weekly precipitation plotted against average weekly dewpoint temperature for early and late warm season at our study site. The larger precipitation events occurred during the late warm season as dewpoint temperature increased. Figure 6. Seasonal averages of integrated water vapor transport (IVT) for the 2011– 2017 water years. Pseudo-color shading with overlaid vectors (arrows) was used to indicate the amount of moisture and the direction of transport. Cool-season IVT, and resulting precipitation was large in 2011, 2016, and 2017, as well as for the 2015 late warm season. Chapter 2 Figure 1. Location of the study site (2320 m) in the Sheep Range of southern Nevada, U.S.A. 35 km north-northwest of Las Vegas. The study site is included in the Nevada Climate-Ecohydrological Assessment Network (NevCAN) located in the Desert National Wildlife Refuge. Figure 2. Microscopic image of the 2015 and 2016 tree rings from a sampled ponderosa pine at the NevCAN Montane site highlighting the variation in wood anatomy including lumen diameter (LD) and cell-wall thickness (CWT). Tree rings were sliced into quartiles (Q12015 - Q42016) and classified into subseasons depending on the tree-ring phenodate. Subseasons include: 2015 Monsoon Season (MS2015), 2015 Post-Monsoon Season (PM2015), 2016 Early Warm Season (EWS2016), 2016 Monsoon Season (MS2016), and vii 2016 Post-Monsoon Season (PM2016). A false ring is present in MS2015 and separates the subseasons. Figure 3. The δ18O VSMOW (‰) and δ2H VSMOW (‰) measurements for precipitation, xylem water (large and small trees), and soil water (10 cm depth and 20 cm depth) collected during the 2015 and 2016 growing season at the NevCAN Montane Site in the Sheep Range of southern Nevada. Precipitation was classified into five subseasons (A) and were used to plot the local meteoric water line (δ2H = 8.27*δ18O + 9.7) compared to the global meteoric water line from Rozanski et al. (1993) (δ2H = 8.13*δ18O + 10.8). The δ18O VSMOW (‰) of precipitation, stem water, and soil water δ18O VSMOW (‰) are plotted with daily precipitation for the study time period
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