DOI: 10.5061/dryad.gtht76hn1
title: Interactive effects of tree species mixture and climate on foliar and woody trait variation in a widely distributed deciduous tree

description: Despite increasing reports of severe drought and heat impacts on forest
 ecosystems, community-level processes, which could potentially modulate
 tree responses to climatic stress, are rarely accounted for. While
 numerous studies indicate a positive effect of species diversity on a wide
 range of ecosystem functions and services, little is known about how
 species interactions influence tree responses to climatic variability. We
 quantified the intraspecific variation in 16 leaf and wood physiological,
 morphological, and anatomical traits in mature beech trees (Fagus
 sylvatica L.) at six sites located along a climatic gradient in the French
 Alps. At each site, we studied pure beech and mixed stands with silver fir
 (Abies alba Mill.) or downy oak (Quercus pubescens Willd.). We tested how
 functional traits differed between the two species mixtures (pure vs.
 mixed stands) within each site and along the climatic gradient. We found
 significant changes in many traits along the climatic gradient as
 conditions progressively got drier and warmer. Independent of the mixture,
 reduced leaf-level CO2 assimilation, stomatal size, and thicker leaf
 cuticles, consistent with a more conservative resource use strategy, were
 found. At the drier sites, higher foliar stable carbon isotopic
 composition (d13C), thicker mesophyll tissues, and lower specific leaf
 area (SLA) in pure stands suggests that beech had more acquisitive traits
 there compared to mixed stands. At the wetter sites, trees in beech-silver
 fir mixtures had higher chlorophyll concentration, lower d13C, larger
 xylem vessels, and higher SLA, suggesting a more acquisitive resource use
 strategy in mixed stands than in pure stands. Our work revealed
 that species interactions are significant modulators of functional traits,
 and that they can be just as important drivers of intraspecific trait
 variation as climatic conditions. We show that downy oak mixtures lead to
 an adaptive drought response by common beech in dry environments. In
 contrast, in milder climates, interactions with silver fir seem to
 increase beech’ resource acquisition and productivity. These findings
 highlight a strong context-dependency and imply that incorporating local
 interspecific interactions in research on climate impacts could improve
 our understanding and predictions of forest dynamics.
description: Sixteen
 physiological, morphological, and anatomical traits related to water and
 carbon transport, use and uptake were measured on 120 selected beech trees
 in the middle of the growing season between mid-July and mid-August 2019.
 The sampling started in the southern sites, proceeding then northwards to
 account for differences in the growing season length and start (i.e.,
 shorter in the northern sites) and ensure that measurements were conducted
 in the middle of the growing season at all sites. Samples were harvested
 in non-rainy conditions by tree climbers at all the sites, except LU where
 the sampling was performed using a telescopic pole pruner. No processed of
 the data were done before using.