Julen Astigarraga

Following centuries of deforestation, many industrialized countries have experienced an increase in forest area and biomass due to changes in land- and forest-use since the mid-20th century. At the same time, the impacts of climate change on forests are aggravating, but the interplay between past land- and forest-use (i.e. land- and forest-use legacies) and climate change in tree growth remains elusive. Tree growth synchrony, defined as the coincident increase in annual tree growth between different tree individuals over time, represents a comprehensive ecological measure of the level of environmental stress faced by forests, and consequently, can assess forest vulnerability to global change. Here using network theory and generalized linear mixed models, we tested whether tree size heterogeneity, resulting from different land- and forest-use legacies (i.e. recently-established, long-established, recently-pruned pollards and old-pruned pollards), modulated tree growth synchrony in response to heatwaves frequency synchrony between 1970 and 2020. We analyzed tree growth data from European beech (Fagus sylvatica L.) stands with different histories of forest management at the species’ low-latitude margin. We found increased tree growth synchrony under more frequent heatwaves and late spring frosts, and reduced precipitation. Interestingly, tree growth synchrony in response to heatwave frequency was modulated by tree size heterogeneity, with the highest synchrony observed in stands with low tree size heterogeneity, mainly found in recently-established forests. Conversely, stands with high tree size heterogeneity did not show important changes in synchrony with increasing heatwaves frequency. Our results highlight the importance of maintaining structurally diverse forests to mitigate the negative effects of climate change on forest productivity, and thereby, increase forest resilience to future forest climate risks.