The focus of this research is to understand the coupling between two important biogeochemical cycles: the carbon and water cycle. In the subalpine forest ecosystem, which has been hypothesized as a major carbon sink in the Western U.S., tree growth and soil microorganism activity are highly controlled by the availability of moisture. This project aims to understand how different precipitation types (winter snow versus summer rains) affect carbon uptake in this ecosystem. By analyzing the carbon isotope ratio of recently fixed sugars, measurements of tree water use efficiency (WUE) will be calculated throughout the year. A model will then be developed to calculate carbon assimilation from measurements of WUE and transpiration. Oxygen and hydrogen isotopes will be used to trace water sources used by trees, and the dominant water source will be related to forest carbon gain in order to investigate the influences of precipitation type on forest carbon uptake.
