Hardwoods on the rise: regeneration trends in Central European mountain forests over two decades

Mountain forests provide essential ecosystem services, including protection against natural hazards, carbon storage, and biodiversity habitat. Climate change threatens the continuous provision of these services and is driving anticipated shifts in tree species ranges across elevation gradients. Understanding current regeneration dynamics is critical for predicting future forest composition and guiding adaptive management.
We analyzed two decades (2004-2023) of tree regeneration data from 2,377 plots across seven forest types spanning 1.3 million ha and an elevation gradient from 200 to 2,300 m asl in the Swiss Jura mountains and Alps. Using data from the Swiss National Forest Inventory and statistical models accounting for repeated measurements (generalized estimating equations), we assessed trends in presence for 20 tree species in two height classes: small saplings (40-129 cm height) and tall saplings (≥130 cm, DBH <12 cm).
Broadleaf species showed widespread expansion (35% of models), particularly among small saplings in low- to mid-montane forests (Beech, Fir-Beech, and Fir-Spruce types). Sycamore maple (Acer pseudoplatanus) and goat willow (Salix caprea) expanded most consistently across elevation gradients, establishing extensively in historically conifer-dominated forests. European beech (Fagus sylvatica) increased its presence in mixed-montane forests. Silver fir (Abies alba) exhibited a notable pattern in Fir-Beech forests: an increase in small saplings and a decrease in tall saplings, suggesting potential recruitment bottlenecks. European ash (Fraxinus excelsior) declined significantly across its range. At low elevations, oaks (Quercus spp.) failed to expand beyond their current forest types, while at high elevations, European larch (Larix decidua) saplings decreased in Stone Pine and Larch forests.
To identify potential drivers of these trends, we tested multiple environmental and stand variables. Basal area, temperature, and species richness emerged as the most important explanatory factors associated with observed shifts in regeneration patterns.
These results reveal substantial compositional shifts toward more broadleaf species in Central European mountain forests, with important implications for future ecosystem service provision, forest management strategies, and climate adaptation planning.