Light competition affects how tree growth and survival respond to climate

Abstract Competition between individuals is a key process that drives tree growth and survival in forests. Ecological theories predict that the effect of competition should be weaker in stressful environments. However, quantitative studies have failed to reach a consensus on the direction of the interaction between climate and competition. In this study, we demonstrate that this interaction appears clearly when we explicitly focus on light competition. We analysed the effect of light competition on tree growth and survival along both temperature and aridity gradients for the 33 major European tree species. We collected forest inventories from nine European countries, encompassing over 1 million trees from Spain to Scandinavia. We used species‐specific crown allometric equations to connect this extensive database to the SamsaraLight ray tracing model and to calculate a tree‐based light competition index from the light intercepted by the tree crown. Within a given species' climatic niche, the effect of light competition on tree growth and survival decreased towards both the dry and cold margins, supporting the stress gradient hypothesis. Climate mainly affected tree growth in light, with slower growth in drier or colder conditions. In contrast, for survival, climate mainly affected trees in shade, with better survival in the dry or cold stress margins. Among species, the mean sensitivity of tree growth and survival to light competition decreased with increasing mean aridity niche and shade tolerance of the species. Synthesis. Our study emphasises the importance of considering species‐specific interactions between light competition and climate on tree growth and survival. The impact of climate change on an individual tree is likely to depend on its light competition status within the forest stand, as well as its species‐specific climatic niche and shade tolerance.