Long-term forest development following afforestation of an alpine pasture and repeated thinning in the Central European Alps

The abandonment of alpine pastures in the European Alps has led to an expansion of forest cover in upper montane and subalpine zones, with afforestation playing an important role in steep alpine valleys where people and infrastructure require protection from natural hazards. Understanding growth trajectories in these high-elevation afforestations is therefore essential for guiding management decisions that support long-term forest stability. We present a 59-year case study of one of the longest continuously monitored high-elevation afforestation sites in the Austrian Alps (Tyrol), examining study plot-level variability in elevation, thinning history, and species composition, and their influence on stand structure (basal area, stem density) and tree growth traits. Height, diameter at breast height (DBH), and crown characteristics of Pinus cembra and Larix decidua were repeatedly assessed over multiple inventories. Across study plots, tree height, DBH, and crown dimensions generally declined with increasing elevation up to stand age 40. Thinned plots showed enhanced individual tree growth, lower stand basal area, and reduced stand density. Regular thinning was associated with higher structural stability and low tree mortality (< 1%), whereas the unthinned reference plot experienced 50% mortality by stand age 55, with remaining stems exhibiting high height-to-diameter ratios (75–82), indicating slender, mechanically less stable trees. Overall, our results indicate that afforestation on previously intensively used pastureland near the treeline can achieve sufficient growth to establish a protective forest, and that repeated thinning operations enhanced both single-tree stability and overall stand development.