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).