Abstract
Water and carbon exchange between the land and atmosphere plays a crucial role in the functioning of ecosystems around the globe. In order to predict how ecosystems will respond to climate change, it is important to understand how they respond to current variation in environmental drivers. Eddy covariance (EC) is a technique that is used extensively to measure net fluxes of carbon dioxide and water vapour. These flux measurements are often partitioned in order to obtain values of the constituent fluxes of photosynthesis, respiration, transpiration, and direct evaporation, which are used to verify Earth-system models. The methods most commonly used to partition carbon and water vapour fluxes are based on assumptions that do not hold under a variety of conditions. In the case of carbon dioxide, the light inhibition of leaf respiration is not taken into account, which results in photosynthesis and respiration fluxes being underestimated in the early part of growing seasons. In the case of water vapour, the most common assumption is that of evaporation being negligible after a few days without rainfall, which can lead to an overestimation of transpiration. In this work I utilise techniques that avoid these assumptions in order to investigate the seasonality of leaf respiration and the response of water use efficiency (WUE)—defined as the ratio of photosynthesis to transpiration—to environmental controls, at a forested site in central Virginia.
I quantify leaf respiration at the ecosystem scale by fitting light response curves to net photosynthesis flux values obtained using the flux variance similarity partitioning method, and find it to be larger in the first half of the growing season than the second, while the sensitivity of leaf respiration to light remains constant throughout the growing season. I measure WUE entirely independently of EC using solar induced chlorophyll fluorescence (SIF), sap flow, and a thermal camera, and compare the results to values derived from EC. There is poor agreement between the two methods, but this work stands as a proof of concept that can be improved upon in future research. I also investigate the relationships of leaf respiration and WUE with the environmental controls of temperature and water availability. Unlike temperature, I find soil water content has no effect on leaf respiration or the sensitivity of leaf respiration to light. When calculated using SIF and sap flow, WUE decreases in magnitude as soil water content increases, but I find no relationship when using WUE values derived from EC measurements. The effect of vapour deficit on WUE differs depending on the measurement method.
