Abstract Competition among trees is an important driver of community structure and dynamics in tropical forests. Neighboring trees may impact an individual tree’s growth rate and probability of mortality, but large‐scale geographic and environmental variation in these competitive effects has yet to be evaluated across the tropical forest biome. We quantified effects of competition on tree‐level basal area growth and mortality for trees ≥10‐cm diameter across 151 ~1‐ha plots in mature tropical forests in Amazonia and tropical Africa by developing nonlinear models that accounted for wood density, tree size, and neighborhood crowding. Using these models, we assessed how water availability (i.e., climatic water deficit) and soil fertility influenced the predicted plot‐level strength of competition (i.e., the extent to which growth is reduced, or mortality is increased, by competition across all individual trees). On both continents, tree basal area growth decreased with wood density and increased with tree size. Growth decreased with neighborhood crowding, which suggests that competition is important. Tree mortality decreased with wood density and generally increased with tree size, but was apparently unaffected by neighborhood crowding. Across plots, variation in the plot‐level strength of competition was most strongly related to plot basal area (i.e., the sum of the basal area of all trees in a plot), with greater reductions in growth occurring in forests with high basal area, but in Amazonia, the strength of competition also varied with plot‐level wood density. In Amazonia, the strength of competition increased with water availability because of the greater basal area of wetter forests, but was only weakly related to soil fertility. In Africa, competition was weakly related to soil fertility and invariant across the shorter water availability gradient. Overall, our results suggest that competition influences the structure and dynamics of tropical forests primarily through effects on individual tree growth rather than mortality and that the strength of competition largely depends on environment‐mediated variation in basal area.

中文翻译：

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竞争影响亚马逊和热带非洲环境梯度中的树木生长，但不影响死亡率

摘要 树木之间的竞争是热带森林群落结构和动态的重要驱动因素。相邻的树木可能会影响单个树木的生长速度和死亡概率，但这些竞争效应的大规模地理和环境变化尚未在整个热带森林生物群落中进行评估。我们通过开发非线性模型，将木材密度、树木大小、和邻里拥挤。使用这些模型，我们评估了可用水量（即气候水分亏缺）和土壤肥力如何影响预测的地块级竞争强度（即生长减少的程度，或死亡率增加，通过所有个体树木的竞争）。在两大洲，树木基面积的增长随着木材密度的增加而减少，而随着树木大小的增加而增加。增长随着社区拥挤而下降，这表明竞争很重要。树木死亡率随木材密度而降低，一般随树木大小而增加，但显然不受邻里拥挤的影响。在样地之间，样地级竞争强度的变化与样地基面积（即样地中所有树木的基面积总和）最密切相关，在基面积高的森林中，生长的减少幅度更大，但在亚马逊地区，竞争强度也因地块级别的木材密度而异。在亚马逊，由于湿润森林的基础面积更大，竞争强度随着水的供应而增加，但与土壤肥力的相关性微弱。在非洲，竞争与土壤肥力的相关性较弱，并且在较短的可用水梯度中保持不变。总的来说，我们的研究结果表明，竞争主要通过影响个体树木的生长而不是死亡率来影响热带森林的结构和动态，而且竞争的强度在很大程度上取决于环境介导的基底面积变化。