Biotic interactions among temperate rainforest trees are non-additive and environmentally dependent

1. Species performance in the realised niche is jointly shaped by both abiotic and biotic processes. Moreover, interactions between and within abiotic and biotic processes generate non-additivities, resulting in density dependence that varies in strength or even direction across environments. If ignored, these non-additivities can lead to inaccurate predictions of species responses to changes in environment and community composition. 2. There are increasing empirical efforts to test the constancy of pairwise biotic interactions along environmental and compositional gradients, but rarely along both. We address this gap using nationwide forest inventory data that span broad ambient temperature and moisture gradients throughout New Zealand. 3. We analysed tree diameter growth of six focal tree species as a function of neighbour densities and climate, while accounting for potential abiotic and biotic non-additivities arising from neighbour × climate and neighbour × neighbour statistical interactions, respectively. We kept the large number of parameters manageable using Bayesian shrinkage priors and interpretable using average predictive comparisons. 4. We found that the most complex model—featuring biotic interactions that changed with climate and higher-order interactions with intermediary species—had the highest predictive accuracy of tree diameter growth. Compared to climate, biotic interactions typically had stronger effects on tree diameter growth, especially when they were subjected to non-additivities from local climate and the density of a third species. Most non-additivities tended to weakly exacerbate pairwise competition, whereas the few strong non-additivities tended to alleviate pairwise competition or even produce pairwise facilitation. 5. Synthesis: Our study highlights the importance of the interplay between abiotic and biotic processes when predicting how biotic interactions may structure communities under global change. When quantifying the relative importance of biotic and abiotic processes on species performance, we show that the conclusion varies depending on whether we are looking at direct or indirect effects. With accumulating evidence of non-additive biotic interactions, the next crucial step is to uncover their underlying mechanisms.