Growth and Assemblage Dynamics of Temperate Forest Tree Species Match Physiological Resilience to Changes in...

Human-induced environmental changes are altering forest productivity and species composition, significantly impacting tree physiology, growth, water uptake, and nutrient acquisition. Investigating the intricate interplay between plant physiology and environmental shifts, we analyzed tree-ring isotopes (δ¹³C, δ¹⁸O, and δ¹⁵N) to track long-term trends in intrinsic water-use efficiency (iWUE) and nitrogen availability for European beech, Norway spruce, and silver fir in a unique old-growth temperate mountain forest since 1501 ce. Our findings reveal that Norway spruce, a dominant species, exhibited iWUE saturation, exacerbated by acidic precipitation, resulting in growth declines during periods of high acidic air pollution and increased drought frequency. In contrast, deep-rooted, deciduous European beech demonstrated physiological resilience to acid deposition, benefiting from lower dry deposition of precipitation acidity and thriving under conditions of increased nitrogen deposition and elevated air temperatures, thereby sustaining stem growth regardless of potential climatic limitations. Silver fir showed the most dynamic response to acidic air pollution, with contemporary adaptations in leaf gas exchange allowing accelerated stem growth under cleaner air conditions. These different species responses underscore shifts in species competition, with European beech gaining dominance as Norway spruce and silver fir decline. Furthermore, the influence of ontogeny is evident, as tree-rings exhibited lower initial iWUE values and higher δ¹⁵N, reflecting changes in nitrogen uptake dynamics and the ecological role of tree age. Our study integrates tree-growth dynamics with physiological and nutrient availability trends, revealing the pivotal role of atmospheric chemistry changes in shaping the competitive dynamics and long-term growth trajectories of dominant tree species in temperate forests.