Data from: Density-dependent vulnerability of forest ecosystems to drought

Climate models predict increasing drought intensity and frequency for many regions, which may have negative consequences for tree recruitment, growth, and mortality, as well as forest ecosystem services. Furthermore, practical strategies for minimizing vulnerability to drought are limited. Tree population density, a metric of tree abundance in a given area, is a primary driver of competitive intensity among trees, which influences tree growth and mortality. Manipulating tree population density may be a mechanism for moderating drought-induced stress and growth reductions, although the relationship between tree population density and tree drought vulnerability remains poorly quantified, especially across climatic gradients.
In this study, we examined three long-term forest ecosystem experiments in two widely-distributed North American pine species, ponderosa pine Pinus ponderosa (Lawson & C. Lawson) and red pine Pinus resinosa (Aiton), to better elucidate the relationship between tree population density, growth, and drought. These experiments span a broad latitude and aridity range and include tree population density treatments that have been purposefully maintained for several decades. We investigated how tree population density influenced resistance (growth during drought) and resilience (growth after drought compared to pre-drought growth) of stand-level growth during and after documented drought events.
Our results show that relative tree population density was negatively related to drought resistance and resilience, indicating that trees growing at lower densities were less vulnerable to drought. This result was apparent in all three forest ecosystems, and was consistent across species, stand age, and drought intensity.
Synthesis and applications. Our results highlighted that managing pine forest ecosystems at low tree population density represents a promising adaptive strategy for reducing the adverse impacts of drought on forest growth in coming decades. Nonetheless, the broader applicability of our findings to other types of forest ecosystems merits additional investigation.

气候模型预测，全球多数区域的干旱强度与发生频率均呈上升趋势，这可能对树木更新、生长、死亡以及森林生态系统服务功能造成负面影响。此外，当前用于降低森林干旱脆弱性的实用策略仍较为匮乏。树木种群密度（tree population density）是衡量特定区域内树木丰度的指标，也是驱动树木间竞争强度的核心因素，进而影响树木生长与死亡过程。尽管树木种群密度与树木干旱脆弱性之间的关联仍未得到充分量化——尤其是在不同气候梯度下——但调控树木种群密度或可成为缓解干旱胁迫及生长抑制的可行手段。
本研究针对北美广泛分布的两种松树——美国黄松（*Pinus ponderosa* Lawson & C. Lawson）与北美红松（*Pinus resinosa* Aiton）开展的三项长期森林生态系统实验进行分析，旨在进一步阐明树木种群密度、生长与干旱之间的关联。这些实验覆盖了广泛的纬度与干旱梯度范围，且包含了已人工维持数十年的树木种群密度调控处理组。本研究探究了在已记录的干旱事件发生期间及之后，树木种群密度如何影响林分生长的干旱抗性（干旱期间的生长状况）与恢复力（干旱后生长相较干旱前的恢复情况）。
研究结果表明，相对树木种群密度与干旱抗性及恢复力呈负相关关系，即生长于较低种群密度下的树木，其干旱脆弱性更低。该结论在全部三项森林生态系统实验中均成立，且不受物种、林分年龄以及干旱强度的影响。
综合与应用
本研究结果表明，通过调控维持较低的树木种群密度来经营松林生态系统，是未来数十年缓解干旱对森林生长负面影响的极具潜力的适应性策略。不过，本研究结论对其他类型森林生态系统的普适性仍有待进一步探究。