Evaluating the effect of plant water availability on inner alpine coniferous trees based on sap flow measurements

Climate simulations anticipate an increase in mean summer temperature with synchronous decrease in summer precipitation during the course of the current century in Central Europe. As a consequence, transpiration of forest trees and stands may be altered along with soil water availability. In this study, the effect of reduced plant water availability to conifers was investigated into an open Pinus sylvestris forest (Erico-Pinetum typicum; P. sylvestris 60 %, Picea abies 20 %; and Larix decidua 20 %) within the inner alpine dry valley of the Inn River in Tyrol, Austria. For reducing plant water availability, we installed a transparent roof construction above the forest floor to prevent precipitation to reach the soil. The roofed area covered 240 m² and included 10 trees. A respective number of 11 trees served as controls in the absence of any manipulation. Roofing significantly reduced plant water availability as indicated in lower predawn needle water potentials. Sap flow density (Qs) was 63, 47, and 24 % lower in roofed P. sylvestris, P. abies, and L. decidua trees, respectively, as compared to control trees. Our findings suggest that P. sylvestris and P. abies behaves “isohydric” as they close their stomata relatively early under conditions of reduced plant water availability and thus stabilize their water relations, whereas L. decidua behaves “anisohydric” and maintains high transpiration rates.