Contrasting patterns of tree species mixture effects on wood δ13C along an environmental gradient
G. de Streel , C. Collet , I. Barbeito , K. Bielak , A. Bravo‑Oviedo , G. Brazaitis , L. Coll , L. Drössler , D. Forrester , M. Heym , M. Löf , Maciej Pach , H. Pretzsch , R. Ruiz‑Peinado , Jerzy Skrzyszewski , J. Stankevičiūtė , M. Svoboda , K. Verheyen , T. Zlatanov , D. Bonal , Q. Ponette
AbstractEstablishing mixed-species stands is frequently proposed as a strategy to adapt forests to the increasing risk of water scarcity, yet contrasted results have been reported regarding mixing efects on tree drought exposure. To investigate the drivers behind the spatial and temporal variation in water-related mixing efects, we analysed the δ13C variation in 22-year tree ring chronologies for beech and pine trees sampled from 17 pure and mixed pine–beech stands across a large gradient of environmental conditions throughout Europe. In the pure stands, average δ13C values were lower for beech (−27.9‰ to −22.2‰) than for pine (−26.0‰ to −21.1‰), irrespective of site conditions. Decreasing SPEI values (calculated over June to September) were associated with an increase in δ13C for both species, but their efect was infuenced by stand basal area for pine and site water availability for beech. Mixing did not change the temporal constancy of δ13C nor the tree reaction to a drought event, for any of the species. While the mixing efect (Δ δ13C=δ13C pure stands−δ13C mixed stands) was on average positive for beech and non-signifcant for pine across the whole gradient, this efect strongly difered between sites. For both species, mixing was not signifcant at extremely dry sites and positive at dry sites; on moderately wet sites, mixing was positive for beech and negative for pine; at sites with permanent water supply, no general patterns emerge for any of the species. The pattern of mixing efect along the gradient of water availability was not linear but showed threshold points, highlighting the need to investigate such relation for other combinations of tree species.
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