Abstract：
To increase the resilience of forests to drought and other hazards, foresters are increasingly planting mixed stands. This requires knowledge about the drought response of tree species in pure and mixed-culture neighborhoods. In addition, drought frequently interacts with continued atmospheric nitrogen (N) deposition. To disentangle these factors for European beech, Norway spruce and Douglas fir, we conducted a replicated 3-factorial sapling growth experiment with three moisture levels, (high, medium, and low), two N levels (high and ambient), and pure and mixed-culture neighborhoods. We measured biomass, stomatal conductance (GS), shoot water potential (at predawn: ΨPD, midday, and turgor loss point: ΨTLP), branch xylem embolism resistance (Ψ50) and minimum epidermal conductance (Gmin). The three species differed most with respect to Gmin (10-fold higher in beech than in the conifers), hydroscape area (larger in beech), and the time elapsed to reach stomatal closure (TΨGS90) and ΨTLP (TTLP; shorter in beech), while Ψ50 and ΨTLP were remarkably similar. Neighborhood (pure vs mixed-culture) influenced biomass production, water status and hydraulic traits, notably GS (higher in Douglas fir, but lower in spruce and beech, in mixtures than pure culture), hydraulic safety margin (smaller for beech in mixtures), and TΨGS90 and TTLP (shorter for spruce in mixture). High N generally increased GS, but no consistent N effects on leaf water status and hydraulic traits were detected, suggesting that neighbor identity had a larger effect on plant water relations than N availability. We conclude that both tree neighborhood and N availability modulate the drought response of beech, spruce, and Douglas fir. Species mixing can alleviate the drought stress of some species, but often by disadvantaging other species. Thus, our study suggests that stabilizing and building resilience of production forests against a drier and warmer climate may depend primarily on the right species choice; species mixing can support the agenda.
摘要：
提高森林抵御干旱和其他灾害的能力,林农越来越多地种植混合林。这需要了解纯种和混合文化社区中树种的干旱响应。此外,干旱经常与持续的大气氮（N）沉积相互作用。为了解开欧洲山毛榉的这些因素，挪威云杉和道格拉斯冷杉，我们在三个湿度水平下进行了重复的三因子树苗生长实验，(高,中等和低)，两个N级（高和环境）以及纯文化和混合文化社区。我们测量了生物量，气孔导度（GS），射击水势(黎明前:ΦPD,中午,和膨大损失点：kWTLP），分支木质部栓塞抗性(VL50)，和最小表皮电导（Gmin）。这三个物种在Gmin方面差异最大（山毛榉比针叶树高10倍），水景面积（山毛榉较大），以及达到气孔关闭的时间（TVLGS90）和VLTLP（TTLP；山毛榉中较短），而W50和WTLP非常相似。邻里（纯vs.混合培养)影响了生物量生产，水分状况和水力特征，特别是GS(道格拉斯冷杉较高，但在云杉和山毛榉中较低，在混合物中比纯培养物)，水力安全裕度(混合物中山毛榉较小)，和TkWGS90和TTLP（混合物中的云杉较短）。高N通常会增加GS，但未检测到氮素对叶片水分状况和水力性状的一致影响，表明邻居身份对植物水分关系的影响大于氮的可用性。我们得出的结论是，树木邻域和氮素可用性都可以调节山毛榉的干旱响应，云杉和道格拉斯冷杉.物种混合可以缓解某些物种的干旱胁迫，但通常是对其他物种不利。因此，我们的研究表明，稳定和建立生产森林对干燥和温暖气候的复原力可能主要取决于正确的物种选择;物种混合可以支持议程。
