Establishing 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 effects on tree drought exposure. To investigate the drivers behind the spatial and temporal variation in water-related mixing effects, 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 effect was influenced 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 effect (Δ δ13C = δ13C pure stands − δ13C mixed stands) was on average positive for beech and non-significant for pine across the whole gradient, this effect strongly differed between sites. For both species, mixing was not significant 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 effect 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.

中文翻译：

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树种混合对木材 δ13C 影响的对比模式沿环境梯度

建立混合物种林分经常被提议作为使森林适应日益增加的缺水风险的策略，但关于混合对树木干旱暴露的影响的报告结果相反。为了研究与水相关的混合效应的空间和时间变化背后的驱动因素，我们分析了 22 年树木年轮年表的 δ13C 变化，这些变化是从 17 个纯和混合松树 - 山毛榉林中采样的山毛榉和松树跨越大环境梯度整个欧洲的情况。在纯林分中，无论立地条件如何，山毛榉的平均 δ13C 值（-27.9‰ 至 -22.2‰）都低于松木（-26.0‰ 至 -21.1‰）。降低 SPEI 值（在 6 月至 9 月期间计算）与两个物种的 δ13C 增加有关，但它们的影响受到松树的基础面积和山毛榉的现场可用水量的影响。对于任何物种，混合并没有改变 δ13C 的时间稳定性，也没有改变树木对干旱事件的反应。虽然混合效应（Δ δ13C = δ13C 纯林分 - δ13C 混合林分）在整个梯度上平均对山毛榉为正，对松树不显着，但这种影响在不同地点之间存在很大差异。对于这两个物种，在极端干燥的地方混合不显着，在干燥的地方混合是积极的；在适度潮湿的地方，混合对山毛榉呈阳性，对松树呈阴性；在有永久供水的地点，没有任何物种出现一般模式。沿可用水量梯度的混合效应模式不是线性的，而是显示阈值点，