Climatic change only stimulated growth for trees under weak competition in central boreal forests

Global change ecologists have often used trees under weak competition (e.g. dominant/codominant trees) to examine relationships between climatic change and tree growth. Scaling up these results to a forest relies on the assumption that the climatic change-tree growth relationship is not affected by tree-level competition. Using permanent sample plot data from the central Canadian boreal region where warming did not result in water deficit, we tested the above-mentioned assumption by looking at whether the relationship between climatic change and tree growth varied with tree-level competition, which was quantified using a modified Hegyi competition index. We found that tree growth increased over time for trees under weak competition, but decreased for those under strong competition. The divergent temporal trends among trees under different levels of competition led to a non-significant change in growth for our study plots. Growth increased with regional warming, atmospheric [CO2] and water availability for trees under weak competition, but not for those under strong competition. Synthesis. Our results suggest that upscaling the growth responses of dominant/codominant trees to climate change to a forest or a region can lead to biased estimates. Tree-level competition should be taken into account when expressing climatic change and tree growth relationships.