The effects of elevated CO2 and nitrogen fertilization on stomatal conductance estimated from 11 years of scaled sap flux measurements at Duke FACE.

In this study, we employ a network of thermal dissipation probes (TDPs) monitoring sap flux density to estimate leaf-specific transpiration (EL) and stomatal conductance (GS) in Pinus taeda (L.) and Liquidambar styraciflua L. exposed to +200 ppm atmospheric CO2 levels (eCO2) and nitrogen fertilization. Scaling half-hourly measurements from hundreds of sensors over 11 years, we found that P. taeda in eCO2 intermittently (49% of monthly values) decreased stomatal conductance (GS) relative to the control, with a mean reduction of 13% in both total EL and mean daytime GS. This intermittent response was related to changes in a hydraulic allometry index (AH), defined as sapwood area per unit leaf area per unit canopy height, which decreased a mean of 15% with eCO2 over the course of the study, due mostly to a mean 19% increase in leaf area (AL). In contrast, L. styraciflua showed a consistent (76% of monthly values) reduction in GS with eCO2 with a total reduction of 32% EL, 31% GS and 23% AH (due to increased AL per sapwood area). For L. styraciflua, like P. taeda, the relationship between AH and GS at reference conditions suggested a decrease in GS across the range of AH. Our findings suggest an indirect structural effect of eCO2 on GS in P. taeda and a direct leaf level effect in L. styraciflua. In the initial year of fertilization, P. taeda in both CO2 treatments, as well as L. styraciflua in eCO2, exhibited higher GS with NF than expected from shifts in AH, suggesting a transient direct effect on GS. Whether treatment effects on mean leaf-specific GS are direct or indirect, this paper highlights that long-term treatment effects on GS are generally reflected in AH as well. [ABSTRACT FROM AUTHOR]