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Recent changes commonly observed in forests growth could be the result of a combination of different climatic and non-climatic factors, such as rising atmospheric [CO2], temperature changes, atmospheric N deposition and drought stress. These effects are difficult to assess, however, due to the superimposition of age-related changes. After removing age effects through a novel approach, this study quantifies the effects on tree growth of global change, and assesses the relationship with individual environmental drivers and their relative importance. Generalized Additive Mixed Models (GAMMs) were applied to decouple the non-linear effects of age and co-occurring environmental changes on basal area increments (BAI) series, as derived from tree rings in a Pseudotsuga menziesii stand chronosequence of four different age classes (65-, 80-, 100- and 120-year-old). The model could explain about 57% of the overall variation in BAI as a function of age and a selected set of predictors, including water availability in the current summer and at the end of previous growing season; together with age, winter-spring mean temperature was found to be the most important predictor. After accounting for age-related effects, a significant decrease in BAI was observed in Douglas fir over the last decades. No significant impact of atmospheric [CO2] and atmospheric N deposition were detected.

Anthropogenic activities have resulted in a significant increase of reactive nitrogen (N) compounds in the atmosphere and a rise in N deposition on forest ecosystems. Increasing N loads impact forest productivity and health, altering tree physiological status and nutrient balance with possible beneficial and detrimental consequences. The impact of N deposition has received considerable attention by scientific research, covering medium and high latitudes, while experimental studies in the Mediterranean area are almost absent. The present study used a manipulative approach, through replicated N additions (background deposition, 30, 60 kg N ha -1 yr -1 ) to simulate the cumulative effects of N deposition in two beech (Fagus sylvaticaL.) forests located in contrasting climatic regions of Italy. Leaf nutrients and photosynthetic pigments were tested as monitoring indicators after four years of N fertilization. Foliar N and pigment concentrations indicated not limiting N conditions at both forest sites, although changes in chlorophylls and carotenoids showed an early response of the canopy to N additions. N-to-phosphorus (P) and sulfur (S) ratios increased under elevated N fertilization, which could be partly related to the relative enhancement of foliar N concentration, and partly associated with the reduction of foliar P and S. The two eutrophic beech forests monitored were not severely affected by chronic N addition, not showing critical nutritional and physiological impairments over the short to medium period. However, the modifications in leaf nutrient and pigment compositions showed an incipient stress response and accentuated the differences induced by climatic and soil characteristics at the two sites. The potential use of nutrients and photosynthetic pigments in monitoring forest N deposition under contrasting climatic conditions and the eventual limits of manipulative experiments are discussed., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have influenced the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)