Collaborative Proposal: Forest carbon-water interactions in relation to the North American Monsoon climate system

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Regional climate affects the forest growth and water use in the semi-arid western US. These montane forests depend on water from winter snow and summer rain. This water is provided by different climate systems. The North American Monsoon climate system occurs each year across the southwestern US. It delivers summer rain through near-daily thunderstorms that move northward from the Gulf of California. Winter snow comes from longer-term storms that originate above the Pacific Ocean off the coast of California and move eastward. Water from these different climate systems have different chemistry and timing. Using tree rings this research project will provide insights into how forest growth and water use has responded to past climate variation and how variation in one part of the season (e.g., winter or summer) influences forest growth and water use in the following seasons. This region has experienced reduced snow packs, warmer winter temperatures and an increase in droughts. Therefore, it is important to understand how the change in water availability is affecting the growth of these forests. An understanding of how drought controls forest growth is crucial to not only predicting natural resource production, but also predicting and controlling forest fire frequencies. The research will contribute to an exhibit entitled: 'The Fingerprints of Forest Responses to Climate Variation', in The Laboratory of Tree Ring Research and at the Biosphere 2 campus at the University of Arizona. The research will include the training of two post-doctoral fellows and numerous undergraduate students, with an emphasis on recruitment from the large Hispanic and Native American student populations at the University of Arizona. This research will explore whether variation in the amount of summer or winter precipitation over the past fifty years, in any given year, is negatively correlated with the amount of precipitation during the previous winter or the following summer, respectively, and whether those correlated influences have affected forest wood production and water-cycling in predictable ways. Researchers will examine how trees mobilize stored sugar resources to sustain wood production and water cycling during years with extremely low winter or summer rainfall, and the degree to which the use of stored sugars differentially buffers the effects of extreme winter or summer drought. This project will design a new generation of mathematical models to better predict how variation in winter versus summer precipitation influences forest wood production and water use, and whether there are limits to sustaining wood production following multi-year droughts. The research will utilize advanced techniques in the analysis of stable isotopes in the cellulose extracted from individual tree-rings. A major part of the studies will focus on tree rings produced during the 1960's and 1970's, when atomic bomb testing produced an atmospheric spike of radioactive 14C that was subsequently incorporated into trees. The 14C provides a detectable tracer that facilitates determination of the age of stored sugars in trees and the utilization of those tagged sugars in the production of tree rings specifically, and timber generally. New theory will be developed to provide a foundation for understanding and predicting how trees incorporate stable isotopes of carbon and oxygen from the atmosphere into their wood. All data will be deposited in the International Tree Ring Data Bank, which is administered by the US Department of Commerce, National Oceanic and Atmospheric Administration, and which has been used in numerous multi-national collaborations on understanding how climate controls the health and vigor of forest ecosystems.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.