Disentangling the impacts of rising temperature and vapor pressure deficit on tree hydraulic functions and carbon dynamics

Leonie Schönbeck and Charlotte Grossiord, School of Architecture, Civil and Environmental Engineering ENAC, École Polytechnique Fédérale de Lausanne EPFL, Lausanne, Switzerland, Leonie Schönbeck and Charlotte Grossiord, Community Ecology, Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Lausanne, Switzerland, Philipp Schuler and Marco M. Lehmann, Forest Dynamics, Swiss Federal Institute for Forest Snow and Landscape Research WSL, Birmensdorf, Zürich, Switzerland

Background/Question/Methods
Despite the increasing importance of temperature and VPD as drivers of plant functioning in many terrestrial biomes, few studies have isolated the physiological effects of rising evaporative demand vs. temperature on plants, limiting our ability to anticipate future impacts on terrestrial ecosystems. Here, we aimed at detecting individual and combined effects of high temperature and VPD on plant water and carbon relations. We expected high evaporative demand to affect stomatal behavior and high temperature to increase biochemical processes like photosynthesis, as long as foliar temperatures did not exceed an optimum temperature for net carbon gain. We exposed three-year-old potted saplings of F. Sylvatica, Q. Pubescence, and Q. Ilex to six treatments during one growing season. Three VPD levels of 0.7, 1.3, and 1.9 kPa were applied in three climate chambers at 25°C, and in three chambers at 30°C. The soil was well-watered to field capacity at all times. We measured hydraulic traits such as stomatal conductance (gs), and its sensitivity to VPD changes, leaf water potential (Ψleaf), cuticular conductance (gmin), turgor loss point (ΠTLP), percentage loss of xylem conductivity (PLC), as well as carbon related traits such as biomass, assimilation rate (A) and non-structural carbohydrates (NSC).
Results/Conclusions
High VPD caused a reduction in gs in most species, although this pattern was most visible at 25°C. At 30°C, the saplings may have kept homeostatic gs, even in high VPD, to increase evaporation for leaf cooling. In F. Sylvatica, the homeostatic gs led to significantly more negative midday Ψleaf and higher PLC. These results indicate that even in the absence of soil drought, atmospheric evaporative demand may exert hydraulic stress on sensitive species. Other parameters, such as cuticular conductance, were not affected by any treatments during the one growing season, indicating that possible acclimation of leaf structural characteristics such as the protective cuticular layer may occur with the construction of new leaf tissues or under longer stress exposure. Carbon related traits such as A and NSC were not or less affected by temperature and VPD treatments, indicating that mainly plant hydraulics is affected by changes in atmospheric evaporative demand. A combination of high VPD, inducing stomatal closure, and high temperatures, requiring open stomata for leaf cooling, might pose an acclimation trade-off for plants, highlighting the importance of disentangling those two important drivers for improved climate-vegetation predictions.