1. Tree-species mixture effects (e.g. complementarity and facilitation) have been found to increase individual-tree productivity, lessen mortality and improve recruitment in forests worldwide. By promoting more efficient and complete resource use, mixture effects may also lessen individual-tree-level water stress, thus improving drought resistance. We investigated the influence of mixture effects on tree productivity, mortality and recruitment across broad compositional and moisture gradients in high-elevation Interior West US mixed-conifer communities, where Populus tremuloides (trembling aspen) is the major contributor to functional diversity. Our goal was to provide a more complete scientific foundation for managing these drought-prone, fire-excluded systems under an uncertain climate.
2. We used landscape-scale national forest inventory data to examine mixture effects on P. tremuloides and the major associated conifer species, Pseudotsuga menziesii, Pinus contorta, Abies lasiocarpa and Picea engelmannii. Using generalized linear mixed modelling, we isolated the influences of P. tremuloides relative density and climate on tree-level (stems ≥ 12.7 cm DBH) growth, mortality and stand-level recruitment (presence/absence of new trees). Cold-season precipitation (PPT) and warm-season vapour pressure deficit (VPD) served to represent soil moisture supply and demand, respectively.
3. Populus tremuloides growth declined as interspecific density increased. In contrast, Pinus contorta and A. lasiocarpa growth increased with P. tremuloides density. For all species except A. lasiocarpa and P. menziesii, growth increased under higher PPT and VPD. Populus tremuloides mortality increased under high VPD but not with interspecific relative density. We found limited evidence that A. lasiocarpa mortality decreased as P. tremuloides density increased. Populus tremuloides recruitment declined steeply above 25% interspecific relative density. We found a decline in conifer recruitment odds as P. tremuloides density increased, ranging from strong in P. contorta to insubstantial in P. engelmannii.
4. Synthesis. Our findings have implications for sustaining mixed-conifer communities impacted by climate change and historical fire exclusion. Mixtures of P. tremuloides and conifers may improve conifer growth while adversely impacting P. tremuloides growth relative to pure stands. Higher conifer productivity combined with lower P. tremuloides recruitment at high conifer relative density may accelerate succession.

中文翻译：

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物种混合效应和气候影响美国西部内陆杨树针叶树群落的生长、补充和死亡率

1. 已发现树种混合效应（例如互补性和促进作用）可以提高单棵树的生产力，降低死亡率并改善全世界森林的补充。通过促进更有效和更完整的资源利用，混合效应还可以减轻个体树木水平的水分压力，从而提高抗旱性。我们研究了混合效应对美国西部高海拔混合针叶树群落的广泛组成和水分梯度中树木生产力、死亡率和补充的影响，其中杨树（颤杨）是功能多样性的主要贡献者。我们的目标是为在不确定的气候下管理这些易干旱、无火灾的系统提供更完整的科学基础。
2. 我们使用景观尺度的国家森林清单数据来检查混合对P. tremuloides和主要相关的针叶树种Pseudotsuga menziesii、Pinus contorta、Abies lasiocarpa和Picea engelmannii 的影响。使用广义线性混合模型，我们隔离了P. tremuloides相对密度和气候对树级（茎 ≥ 12.7 cm DBH）生长、死亡率和林分级补充（新树的存在/不存在）的影响。冷季降水 (PPT) 和暖季蒸汽压差 (VPD) 分别代表土壤水分供需。
3. 随着种间密度的增加，杨树的生长下降。相比之下，Pinus contorta和A. lasiocarpa 的生长随着P. tremuloides密度的增加而增加。对于除A. lasiocarpa和P. menziesii之外的所有物种，在较高的 PPT 和 VPD 下生长均增加。山杨死亡率在高 VPD 下增加，但在种间相对密度下没有增加。我们发现有限的证据表明A. lasiocarpa死亡率随着P. tremuloides密度的增加而降低。山杨种间相对密度超过 25% 时，招募急剧下降。我们发现随着P. tremuloides密度的增加，针叶树的招募几率下降，从P. contorta 的强到P. engelmannii 的不明显。
4. 合成。我们的发现对维持受气候变化和历史火灾排除影响的混合针叶树群落具有重要意义。P. tremuloides和针叶树的混合物可能会改善针叶树的生长，同时对P. tremuloides相对于纯林的生长产生不利影响。在高针叶树相对密度下，较高的针叶树生产力与较低的P. tremuloides募集相结合可能会加速演替。