THE EFFECTS OF PRECIPITATION VARIABILITY ON C4 PHOTOSYNTHESIS, NET PRIMARY PRODUCTION AND SOIL RESPIRATION IN A CHIHUAHUAN DESERT GRASSLAND by MICHELL L. THOMEY A.A., Liberal Studies, Fullerton College, 1994 B.S., Biology, California State University, Fullerton, 1998 M.S., Biology, California State University, Fullerton, 2003 DISSERTATION Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy Biology The University of New Mexico Albuquerque, New Mexico July, 2012 ii ACKNOWLEDGEMENTS I want to especially acknowledge my family, Jerry, Bette, Shon, Annette for all your support and encouragement over the years; and Josiah for the joy. I want to thank my graduate committee, Scott, Will, Bob and Jesse. Each one of you truly represents my research interests. I have learned so much from you. Scott thanks for, supporting my research, teaching me to be a better writer, pushing me beyond my self-imposed limits, and giving me a chance to pursue my dream. Will, I remember reading about your research during my Master’s degree and being so intimidated when I first started working with you. Over the years, you have helped me with my research and to navigate my way through this business with common sense. Thank you. Heather Paulsen and Cheryl Martin. You always worked everything out, always had a solution, and here I am. Thank you. Sevilleta LTER Staff – You often stepped in and helped out even though it was likely beyond the job description. Your support of me and my research has been invaluable. “The global C cycle is changing, so we have a long list of tantalizing, relevant questions to puzzle, amuse and bemuse us. And, like never before, we have a ready audience, waiting for our findings and insights. That may make us a little nervous, but at the same time it adds urgency and spice to the science. It may be, a century from now, when our work is done and the current turbulence of the C cycle has been calmed, that our successors will look back and say: it must have been an exciting time for scientists, back then, in the golden age of C cycle science.” - H.H. Janzen iii THE EFFECTS OF PRECIPITATION VARIABILITY ON C4 PHOTOSYNTHESIS, NET PRIMARY PRODUCTION AND SOIL RESPIRATION IN A CHIHUAHUAN DESERT GRASSLAND by Michell L. Thomey A.A., Liberal Studies, Fullerton College, 1994 B.S., Biology, California State University, Fullerton, 1998 M.S., Biology, California State University, Fullerton, 2003 Ph.D., Biology, University of New Mexico, 2012 ABSTRACT Although the Earth’s climate system has always been inherently variable, the magnitude and rate of anthropogenic climate change is subjecting ecosystems and the populations that they contain to novel environmental conditions. Because water is the most limiting resource, arid-semiarid ecosystems are likely to be highly responsive to future climate variability. The goal of my dissertation is to understand how precipitation variability affects primary productivity and soil respiration in desert grassland ecosystems. Initially, I reviewed the literature to understand how climate change affects net ecosystem exchange (NEE) across the warm deserts of North America (Chapter 2). Next, I examined the effects of precipitation frequency and intensity on soil moisture (θ), leaf-level photosynthesis (Anet), predawn leaf water potential (Ψpd), aboveground net primary productivity (ANPP), and soil respiration (Rs) (Chapter 3). Last, I studied how iv large (10 mm) and extreme (30 mm) rainfall events with extended dry periods affected the ecophysiological response of two co-occurring dominant perennial C4 grasses, Bouteloua eriopoda and B. gracilis across an arid-semiarid ecotone (Chapter 4). v TABLE OF CONTENTS Chapter 1. Introduction ……………………………………………… 1 Chapter 2. Implications of Climate Change and Future Management of the North American Warm Deserts ……………………………………………... 3 Abstract ……………………………………………………….. 4 Introduction …………………………………………………… 5 The Deserts of North America ………………………………... 6 Carbon cycling in the North American warm deserts: Precipitation …………………………………………... 10 CO2 …………………………………………………..... 13 Warming ……………………………………………..... 16 Environmental hurdle #1: Disturbance ………………………... 17 Environmental hurdle #2: Invasive species ………………….... 19 Carbon Sequestration and land management ………………….. 21 Figure ………………………………………………………….. 22 Figure caption ………………………………………………….. 23 Chapter 3. Effect of Precipitation Variability on Net Primary Production and Soil Respiration in a Chihuahuan Desert Grassland …………….... 25 Abstract ………………………………………………………… 26 Introduction …………………………………………………….. 27 Materials and Methods …………………………………………. 29 Study site ……………………………………………….. 29 Experimental Design …………………………………… 29 Gas Exchange Measurements ………………………….. 30 vi Leaf Water Potential …………………………………… 31 Aboveground Net Primary Productivity (ANPP) ……………………… 32 Soil Moisture and Soil Respiration …………………….. 32 Data Analysis …………………………………………... 33 Results ………………………………………………………….. 34 Pulse-response after experimental rain events …………. 35 Seasonal responses ……………………………………... 36 Discussion ……………………………………………………… 39 Acknowledgements ……………………………………….......... 47 Tables …………………………………………………………... 48 Figures …………………………………………………….......... 49 Figure captions …………………………………………………. 57 Chapter 4. Effects of Monsoon Precipitation Variability on the Ecophysiological Response of Two Dominant C4 Grasses Across an Arid-Semiarid Ecotone …………………………………………………………………. 61 Abstract …………………………………………………………. 62 Introduction …………………………………………………….. 63 Materials and Methods …………………………………………. 65 Study area ………………………………………………. 65 Study sites ……………………………………………… 66 Experimental design …………………………………… 67 Sensor and field measurements ………………………... 68 Data analysis …………………………………………... 70 Results …………………………………………………………. 71 Shelter effects ……………………………………......... 71 vii Soil water dynamics …………………………………… 71 Monsoon rainfall variability: intraspecific plant response ........................................................................ 72 Monsoon rainfall variability: interspecific plant response ……………………………………………….. 73 Discussion …………………………………………………….. 74 Monsoon rainfall variability: Anet and soil water dynamics …………………………………………........ 74 Monsoon rainfall variability: intraspecific plant response. 75 Monsoon rainfall variability: interspecific plant response …………………………………………......... 76 Acknowledgements …………………………………………………... 80 Tables ……………………………………………………................... 81 Figures ……………………………………………………………….. 84 Figure captions ………………………………………………………. 88 Chapter 5. Conclusion ……………………………………………….. 90 References …………………………………………………………… 93 Chapter 1 …………………………………………………….. 93 Chapter 2 …………………………………………………….. 94 Chapter 3 …………………………………………………….. 105 Chapter 4 …………………………………………………….. 110 Chapter 5 …………………………………………………….. 116 viii Chapter 1 Introduction Although Earth’s climate system has always been inherently variable (Vitousek 1992), the magnitude and rate of change associated with anthropogenic climate change is subjecting ecosystems and the populations that they contain to novel environmental conditions (Seastedt et al. 2008). At present, carbon dioxide (CO2) concentrations are increasing roughly 2 parts per million (ppm) annually and mean global temperatures are projected to increase 1.1oC – 6.4oC by the end of this century (IPCC 2007). Because primary production and soil respiration across terrestrial ecosystems can have positive or negative feedbacks on atmospheric CO2 concentrations (Huxman et al. 2004; Bardgett et al. 2008), it is essential to understand how species, communities and ecosystems will likely respond to progressive climate change. In North America, desert grasslands occur from Mexico north into Arizona, New Mexico and Texas (Allred 2005). While the North American Prairie has been subjected to land-use degradation, extended drought and overgrazing in desert grasslands has exacerbated the decline in herbaceous cover which is often replaced by woody species (Schlesinger et al. 1990; Van Auken 2000; Peters and Gibbens, 2006). Because water is the most limiting resource, arid ecosystems are also likely to be highly responsive to future climate variability (Diffenbaugh et al. 2008). Understanding how climate change will impact desert grasslands is important because these communities: 1) provide habitat for plants, animals and soil organisms that often live near their physiological limits (Lauenroth and Milchunas 1992), 2) provide soil stability decreasing dust production (Archer and Predick 2008; Field et al. 2010; Ravi et al. 2010) and 3) sustain grazing lands that support livestock production. 1 Arid-semiarid ecosystems are defined by highly variable seasonal and inter- annual precipitation, potential evapotranspiration that exceeds precipitation, and “pulsed” precipitation events that drive biotic activity until available water is depleted (Noy-Meir 1973; Reynolds et al. 2004; Collins et al. 2008). In arid and semiarid ecosystems the magnitude and frequency of pulsed events trigger a hierarchy of biotic activity (Schwinning and Sala 2004). For example, Chen et al. (2009) note that in the Mongolian steppe, pulses ≤5 mm stimulated heterotrophic respiration while pulses > 25 mm resulted in carbon-sequestration. Understanding how the spatial and temporal patterns in precipitation affect primary productivity and soil respiration is important because soil water availability is a primary regulator of carbon exchange (Huxman et al. 2004). The goal of my dissertation is to understand how increased precipitation variability affects