Tree species abundance changes at the edges of their climatic distribution: An interplay between climate change, plant traits and forest management

Abstract Climate change is anticipated to have an increased impact on tree populations located at the edges of their climatic tolerances. However, there is still uncertainty about how the interaction between climate change and functional traits drives changes in tree species abundance at climate edges, especially in the context of the abandonment of the traditional forest activity. We used data from ~445,000 monitored tree stems from 68 species in Spain to (1) quantify tree species abundance changes at the edges of their climatic distributions (cold and wet vs. warm and dry) over the last 25 years and (2) determine the impact of climate change, functional traits and forest densification (used here as an indicator of forest regrowth following anthropogenic disturbance) on these abundance changes. We developed a null model to test whether tree species abundance changes at the climate edges were higher or lower than expected, considering random stem gains and losses along the entire climate gradient. Across the study area, we found an average increase in tree species abundance over time. Our findings also show a higher than random average increase in tree species abundance at the cold and wet edge, but a random increase at the warm and dry edge. Tree species abundance changes along the entire climate gradient and in the edges were primarily associated with forest densification. However, the interaction between climate change and plant traits impacted tree species abundance changes at the climate edges. Tree species that invested more in their root systems and adopted more acquisitive leaf strategies showed greater increases in abundance in response to climate change. Synthesis : Our research emphasizes the crucial impact of historical forest management in shaping the current distribution of tree species. After controlling for the effect of forest management, climate change directly influenced tree demographics, favouring species based on their traits. As tree species regain their ecological niches after succession, climate change will determine their abundance by filtering for particular ecological strategies.