Data from: The effect of competition on responses to drought and interannual climate variability of a dominant conifer tree of western North America

To accurately predict how ecosystems will respond to climate change – and how management actions can influence such responses – scientists and managers need a better understanding of how and when biotic interactions modify climate–growth relationships. However, current research has largely ignored the role of competition in modulating climate–growth relationships of mature trees. In this study, we assessed the effect of competition on tree responses to drought and interannual climate variability as well as linkages between climate sensitivity and morphological characteristics of the stem wood.
We sampled 10 sites in north-eastern Washington, USA, and used dendroecology to quantify responses of Douglas-fir (Pseudotsuga menziesii) to drought and climate variability. Tree-ring series were converted to basal area increment series, and the effects of competition on climate–growth relationships were assessed at the tree and site levels using a combination of correlation analyses and linear mixed-effects models.
Competition did not affect tree responses to extreme drought. When soil moisture was below average, tree growth was tightly coupled to climate variability for all trees, regardless of their competitive status. However, in wet years, competition had a pronounced, positive effect on climate sensitivity of growth.
Trees with more competition from neighbours exhibited a significantly higher proportion of sapwood area in latewood (a morphological trait associated with greater drought resistance).
Synthesis. Our results suggest that a tree's ability to cope with environmental variability is driven not just by the proximate effects of neighbours on resource availability, but also by phenotypic plasticity and long-term adaptations to competitive stress (such as changes in carbon allocation). Findings have important implications for individual-tree and stand-level growth models and may help managers better understand how their activities will modify tree responses to climate change.

为精准预测生态系统对气候变化的响应——以及管理措施如何影响此类响应——科学家与管理者需要更深入理解生物交互如何、在何时调控气候-生长关系。然而当前研究在很大程度上忽略了竞争在调控成熟树木气候-生长关系中的作用。本研究评估了竞争对树木响应干旱及年际气候变异性的影响，同时探究了气候敏感性与茎干木材形态特征之间的关联。

我们在美国华盛顿州东北部选取了10个样地，采用树木年代生态学（dendroecology）方法量化花旗松（Douglas-fir, Pseudotsuga menziesii）对干旱与气候变异性的响应。将树轮序列转换为断面积增量（basal area increment）序列，并结合相关分析与线性混合效应模型，分别在树木个体与样地两个尺度上评估竞争对气候-生长关系的影响。

竞争并未显著影响树木对极端干旱的响应。当土壤含水量低于平均水平时，无论树木的竞争处境如何，其生长均与气候变异性紧密相关。但在湿润年份，竞争对生长的气候敏感性具有显著的正向调控作用。

受邻近个体竞争压力更大的树木，其晚材边材面积（sapwood area）占比显著更高——这是一种与更强抗旱性相关的形态性状。

研究综合。本研究结果表明，树木应对环境变异性的能力不仅取决于邻近个体对资源可获得性的直接影响，还受到表型可塑性以及对竞争胁迫的长期适应性（如碳分配格局的改变）的驱动。本研究结果对单木与林分尺度的生长模型具有重要指导意义，同时可帮助管理者更好地理解其管理活动将如何改变树木对气候变化的响应。