Stand age diversity weakens forests sensitivity to climate change

Abstract Forest age results from population dynamics, disturbance regimes, and silviculture playing an important role in the global Carbon (C) cycle. To examine the shaping effects of forest age on the forest productivity under current and future climate conditions, we used a biogeochemical-based model in three managed forest stands and then modeled their development as undisturbed systems. The model was forced with climate outputs of five Earth System Models under four representative climate scenarios plus one baseline scenario, over a matrix of 11 age classes for each stand. We find that the Net Primary Production (NPP) peak was reached in the young and middle-aged class (16- to-50-year-old) regardless of the climate scenario, while total C-woody stocks (tCWS) increased with age with different trajectories in the three sites, but not linearly as expected. Under climate change scenarios, the beech forest shows an expected increasing NPP with increasing atmospheric CO 2 and temperature as much for younger age classes as for older ones. Conversely, in the spruce and Scots pine-dominated sites NPP and tCWS decrease under climate change scenarios. A factorial ANOVA and Tukey’s Post hoc test (p = 0.05) were performed to verify if differences between age groups, climate scenarios, and their interactions during the simulation period were statistically significant and not just the result of different forcing in the model configuration. The ANOVA test showed that both age and, only for some sites, climate scenarios have effects with differences on the forest carbon balance only when taken individually, while when analyzed in combination, the analysis showed no effects due to the interaction of the two factors showing, then, a pervasive buffering effect of age on climate but also vice–versa. The consistent influence of age remains the primary determining factor across all sites for both NPP and tCWS. This calls for tailored management strategies to enhance the resilience and adaptability of forests in the face of changing climatic conditions, reflecting the different species and age-dependent response to climate.