High Resolution Estimates of Gross Primary Productivity Derived From Tree-Ring Stable Isotopes and Sap-Flux Measurements

High-resolution estimates of Gross Primary Productivity (GPP) provide valuable information regarding carbon cycling in complex forest ecosystems. In the southwestern United States where the North American Monsoon contributes to a bimodal precipitation regime, understanding how alterations to these modes of precipitation influences GPP should be a priority. Eddy-Covariance (EC) flux towers are a powerful tool that excel at characterizing ecosystem-scale carbon-water interactions, but for fine-scale fluxes, they lack the necessary resolution. GPP varies on a tree-by-tree basis and better understanding the variability may elucidate how it will impact the terrestrial carbon budget in the future. We took an ecophysiology approach to understanding fine scale variability of GPP, derived from plant water-use efficiency (WUE) and GPP relations from stable isotopes in tree-ring cellulose. Using the Farquhar model, WUE is defined as the amount of carbon fixed (GPP) relative to the amount of water lost through transpiration (E, or WUE = GPP/E). We used sap flow sensors to measure daily transpiration values. To estimate WUE, we measured the phloem sugar δ13C values and the δ13C of cellulose from tree rings. We then estimated whole tree GPP as the product of the WUE and E. Overall, the individual trees exhibit a pattern of GPP that corroborates estimates obtained from an EC flux tower at the same site. Using our estimates of whole-tree GPP we then compared the variability of GPP across the landscape by examining two groups of trees. The first group was characterized as being exposed near the top of the ridge while the second group was slightly further downslope. Our method consistently showed that the exposed group experienced higher water stress and were subsequently less productive than their counterparts. These findings suggest that the approach developed here represents a viable method to quantitatively examine fine-scale carbon-water relations.