Widespread higher-order interactions enhance the latitudinal tree diversity gradient

In this study, we assembled multiple-census data from 31 large, permanent forest plots spanning tropical to boreal terrestrial biomes. With this dataset, we aimed to (i) assess whether HOIs among trees are prevalent in global forests, (ii) examine how pairwise interactions and HOIs vary with latitude (Q2); and (iii) test how latitudinal changes in HOIs contribute to the latitudinal tree diversity gradient (Q3). To achieve these goals, we first estimated both the pairwise interactions and HOIs demographic growth (for 1367 tree species-plot combinations) and survival models (for 1226 tree species-plot combinations), respectively. Three types of growth and survival models were built: (i) null models with no biotic interactions, (ii) models including only pairwise interactions, and (iii) models including both pairwise interactions and HOIs. We then compared AIC value to determine if HOIs-inclusive models (third type) best fit growth and survival of trees across species and forest plots (Q1). Finally, we related the estimated pairwise interactions and HOIs to latitudes (Q2), and compared changes in the structural stability across forest plots with and without HOIs (Q3).本研究收集了涵盖31处大型永久森林样地的多期普查数据集，这些样地覆盖从热带到寒带的陆地生物群系。基于该数据集，本研究旨在达成三个核心目标：(i) 评估树木间的高阶相互作用（Higher-order Interactions, HOIs）是否在全球森林中普遍存在；(ii) 探究成对相互作用与高阶相互作用如何随纬度变化（Q2）；(iii) 检验高阶相互作用的纬度变化如何驱动树木多样性的纬度梯度（Q3）。为实现上述目标，我们首先分别构建了对应1367个树木物种-样地组合的成对相互作用与高阶相互作用种群生长模型，以及对应1226个树木物种-样地组合的存活模型。本研究共构建三类生长与存活模型：(i) 无生物相互作用的零模型，(ii) 仅包含成对相互作用的模型，(iii) 同时包含成对相互作用与高阶相互作用的模型。随后我们通过比较赤池信息准则（Akaike Information Criterion, AIC）值，判断包含高阶相互作用的模型（第三类模型）是否最适配全球各物种与森林样地内树木的生长与存活情况（Q1）。最终，我们将估算得到的成对相互作用与高阶相互作用与纬度进行关联分析（Q2），并对比了存在与不存在高阶相互作用的森林样地的结构稳定性变化（Q3）。