Fertilization intensifies drought stress: Water use and stomatal conductance of Pinus taeda in a midrotation fertilization and throughfall reduction experiment

Background/Question/Methods
Mid-rotation fertilization is a widely used method to increase productivity of intensively managed southern pine forests, however the impacts of such management on stand water use are poorly understood. We examined the effects of experimentally altered nutrient and water availability on stem volume, leaf area, transpiration per unit ground area (Ec) and canopy conductance per unit leaf area (Gs) of an operationally managed pine plantation during its 8th and 9th growing seasons. Treatments consisted of a factorial combination of throughfall reduction (30% reduction in throughfall versus ambient precipitation) and fertilization (a complete suite of essential nutrients versus ambient soil conditions) beginning in April 2012. A network of 80 sap flux sensors was installed across treatments and analyzed using a hierarchical Bayesian state-space framework to infer trends in Gs and Ec, as well as responses to soil moisture and vapor pressure deficit.

Results/Conclusions
Despite unusually high rainfall during the study period and a lack of leaf area index (LAI) response, both Ec and Gs decreased in response to fertilization and throughfall reduction. Fertilization increased stem volume increment (21 %) in 2013. Treatment differences were greatest in the growing season of 2013, when Ec was on average 19%, 13% and 29% lower in the throughfall reduction (D), fertilization (F) and combined treatment (FD) than the control (C), respectively. The responses to volumetric soil water content (VWC) indicate that lower Ec in F resulted from a decrease in Gs at high VWC, while those in D were due to Gs responses to low VWC from throughfall reduction. Decreases observed in FD resulted from a combination of these two factors. These results are consistent with fertilization-induced decreases in root hydraulic conductivity or allocation to fine root biomass, which then imposes water limitation comparable to that caused by the throughfall exclusion. If such a response were to persist beyond the initial years of fertilization and past canopy closure, the physiological drought experienced by fertilized forests would be greater than by nutrient-limited forests under same hydrologic conditions, potentially increasing their vulnerability to hydraulic failure.