Tree species interactions alter aridity impacts on foliar and woody traits in Fagus sylvatica L.

Margaux Didion-Gency, Arthur Gessler and Pierre Vollenweider, Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Birmensdorf, Switzerland, Arthur Gessler and Nina Buchmann, Eidgenössische Technische Hochschule Zürich ETH, Zürich, Switzerland, Christoph Bachofen and Charlotte Grossiord, Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Lausanne, Switzerland, Christoph Bachofen and Charlotte Grossiord, School of Architecture, Civil and Environmental Engineering ENAC, École Polytechnique Fédérale de Lausanne EPFL, Lausanne, Switzerland, Xavier Morin, CEFE, Centre National de la Recherche Scientifique CNRS, Montpellier, France, Eduardo Vicente, CEAM Foundation, University of Alicante, Alicante, Spain

Background/Question/Methods
Despite increasing reports of severe impacts of drought on forest ecosystems, community-level processes, which could modulate climate impacts, are rarely accounted for. While numerous studies indicate a positive effect of tree species composition on a wide range of ecosystem functions and services, little is known about how interactions influence intraspecific responses to drought. We quantified the intraspecific variation in leaf and woody physiological, morphological, and anatomical traits in mature common beech trees along an aridity gradient in French Alps in response to two tree species composition (monospecific vs. mixed stands) with silver fir and downy oak.
Results/Conclusions
Higher foliar carbon isotopic composition suggests that trees in mixed stands were more drought-stressed than in monospecific stands at the driest sites. In wetter areas, less négatives predawn leaf water potential, higher chlorophyll content, and larger but fewer xylem vessels suggest that trees in mixed stands were more efficient than monospecific ones. Our findings suggest that mixed stands between beech and oak increase the degree of drought experienced by beech in dry environments because of increased competition. In contrast, in milder climates, such interactions with fir trees become beneficial and could increase productivity.