1. Biodiversity effects on productivity and other ecosystem functions are strongly dependent on climate and resource availability. Based on the stress‐gradient hypothesis, under conditions of greater abiotic stress, diversity effects on plant performance are intensified due to the increased relative importance of positive plant interactions. However, whether this hypothesis is consistently applicable in forest systems remains unclear. A field trial was established to test the stress‐gradient hypothesis and examine diversity effects on above‐ground biomass production of young trees in mixtures exposed to different water availability.
2. Six native tree species of northern temperate forests (Acer saccharum, Betula papyrifera, Larix laricina, Picea glauca, Pinus strobus and Quercus rubra) were planted as monocultures and as mixtures of two, four and six species. For five growing seasons, four replicates of each community were exposed to conditions of either low‐ or high‐water availability created by rainfall exclusion and weekly irrigation, respectively. Growth‐years 4 and 5 were significantly different when the climatic water balance of the growing seasons was compared. We tested the effects of functional diversity on: (a) total growth of mixtures under low‐ and high‐water availability, and (b) annual growth in years 4 (higher water availability, 2017) and 5 (lower water availability, 2018).
3. Annual growth of most species in both years was greater under high‐ versus low‐water availability. Functional diversity had a significant positive effect on total biomass production and annual growth, and this effect was more strongly expressed under high‐water availability. Functional diversity effects on annual growth did not differ between years 4 and 5 regardless of their climatic water balance. Functional and species identity were key to understanding productivity responses to mixture and treatment effects.
4. Synthesis. Contrary to the stress‐gradient hypothesis, the positive effects of functional diversity on productivity were enhanced by high‐water availability and were independent of seasonal water balance.

中文翻译：

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幼树群落中的过高产量不支持压力梯度假说，并因功能多样性和较高的水利用率而受到青睐。

1. 生物多样性对生产力和其他生态系统功能的影响在很大程度上取决于气候和资源的可获得性。基于逆境梯度假说，在非生物逆境更大的条件下，由于积极的植物相互作用的相对重要性增加，因此对植物生长的多样性影响增强。但是，尚不清楚该假设是否在森林系统中始终适用。建立了一个田间试验，以测试压力梯度假说并检验多样性对暴露于不同水量的混合物中幼树地上生物量生产的影响。
2. 北方温带森林的六种本土树种（枫宏枫，桦桦，落叶松，青海云杉，松属刺和栎属。）是作为单一栽培种以及两种，四​​种和六种的混合物种植的。在五个生长季节中，每个社区的四个副本都分别受到降雨排除和每周灌溉造成的低或高水利用率的条件。比较生长季节的气候水平衡，第4年和第5年的生长显着不同。我们测试了功能多样性对以下方面的影响：（a）低水和高水可利用量下混合物的总增长，以及（b）第4年（2017年可利用的水资源量较高）和第5年（2018年可利用的水资源量较低）的年增长率。
3. 在高水量和低水量的情况下，这两个年份中大多数物种的年增长率都更高。功能多样性对生物质总产量和年均增长产生了显着的积极影响，而在高水可利用量下，这种影响表现得更为强烈。功能多样性对年均增长的影响在第4年和第5年之间没有差异，无论其气候水平衡如何。功能和物种同一性是了解生产力对混合物和处理效果的响应的关键。
4. 综合。与压力梯度假说相反，功能多样性对生产力的积极影响因高水利用率而增强，并且不受季节水平衡的影响。