Data from: Disentangling competitive versus climatic drivers of tropical forest mortality

1. Tropical forest mortality is controlled by biotic and abiotic processes, but how these processes interact to determine forest structure is not well understood. Using long-term demography data from permanent forest plots at the Paracou Tropical Forest Research Station in French Guiana, we analyzed the relative influence of competition and climate on tree mortality. We found that self-thinning is evident at the stand level, and is associated with clumped mortality at smaller scales (< 2 m) and regular spacing of living trees at intermediate (2.5-7.5 m) scales. A competition index based on spatial clustering of dead trees was used to build predictive mortality models, which also accounted for climate interactions.
2. The model that most closely fitted observations included both the competition index and climatic variables, with climate-only and competition-only models performing worse than the full model. There was strong evidence for size-specific mortality, with highest mortality for small and very large trees, as well as sensitivity of trees to drought, especially when temperatures were high, and when soils were water saturated. The effect of the competition index was more complex than expected a priori: a higher CI index was associated with lower mortality odds, which we hypothesize is caused by gap-phase dynamics, but there was also evidence for competition-induced mortality at very high index values.
3. The strong signature of competition as a control over mortality at the stand and individual scales confirms that it plays a very important role in determining tropical forest structure. The complexity of the competition-mortality relationship and its interaction with climate indicates that a thorough consideration of the scale of analysis is needed when inferring the role of competition in tropical forests, but demonstrates that climate-only mortality models can be significantly improved by including competition effects, even when ignoring species-specific effects.
4. Synthesis Empirical models such as the one developed here can help constrain and improve process-based vegetation models, serving both as a benchmark and as a means to disentangle mortality processes. Tropical ecosystem dynamic models would benefit greatly from explicitly considering the role of competition in stand development and self-thinning while modeling demography, as well as its interaction with climate.

1. 热带森林树木死亡受生物与非生物过程共同调控，但目前对这些过程如何协同作用以决定森林结构的认知仍不够充分。本研究依托法属圭亚那帕拉库热带森林研究站（Paracou Tropical Forest Research Station）的永久森林样地长期种群动态（demography）监测数据，分析了竞争与气候对树木死亡的相对影响。研究发现，林分（stand）水平上存在显著的自疏（self-thinning）现象，该现象与小尺度（<2米）下的聚集性死亡、中等尺度（2.5-7.5米）下存活树木的规则分布模式相关。我们采用基于枯树空间聚集性的竞争指数（competition index, CI）构建了树木死亡预测模型，同时纳入了气候交互效应。
2. 拟合效果最优的模型同时包含竞争指数与气候变量，仅考虑气候或仅考虑竞争的单因素模型表现均劣于全变量模型。研究证实存在显著的体型特异性死亡规律：小型和超大型树木的死亡率最高；同时树木对干旱胁迫表现出显著敏感性，尤其在高温以及土壤水分饱和的环境下。竞争指数的效应比先验预期更为复杂：较高的竞争指数与较低的死亡发生概率相关，我们推测这一现象源于林隙动态（gap-phase dynamics）过程；但在竞争指数极高的情况下，也存在竞争诱导的树木死亡现象。
3. 竞争作为林分与个体尺度上调控树木死亡的关键因子的显著信号，证实其在塑造热带森林结构中发挥着至关重要的作用。竞争-死亡关系的复杂性及其与气候的交互作用表明，在推断竞争在热带森林中的生态作用时，需充分考虑分析尺度；同时本研究表明，即便忽略物种特异性效应，仅基于气候的死亡预测模型也可通过纳入竞争效应得到显著优化。
4. 研究总结 本研究开发的经验模型可为基于过程的植被模型提供约束与改进路径，既可作为模型验证的基准，也可用于解析树木死亡的调控过程。热带生态系统动态模型在模拟种群动态时，若明确纳入竞争在林分发育与自疏中的作用，及其与气候的交互效应，将大幅提升模型性能。