This research uses a series of physiological models and empirical measurements to evaluate biogeochemical controls over coupled carbon-water cycles in forest systems from the individual plant to the stand scale. Leaf, litter, and soil organic matter were analyzed for stable isotope ratios of oxygen and carbon to examine causal links between plant to ecosystem scale productivity and water balance. A series of latitudinal and altitudinal transects established reflecting a chronology of soil development in the California Sierra Nevada was used to study the effects of climatic and edaphic factors on the expression and preservation of plant isotopic signals.
This research suggests that soil physical and chemical properties are more relevant in predicting water use efficiency than climate. We suggest these soil based metrics as a way to improve the understanding of possible stress in Sierra Nevada forests. We find that environmental conditions such as precipitation and temperature do not accurately predict water use efficiency across the western slope of the Sierra Nevada. Instead, factors of soil development, such as soil water capacity, define the efficiency of high altitudinal forests at the extremes of the coniferous zone while at mid elevations the nutrient availability predicts efficiency. This study has characterized the main drivers of water use efficiency across dominant Sierra Nevada forests, yielding an assessment of controls over productivity and stress via water use efficiency.