Spatial and temporal variation of Fagus sylvatica growth in marginal areas under progressive climate change

The escalating decline in growth trends of European beech (Fagus sylvatica) observed across its distribution area pose a major ecological and economic challenge for countries with a high proportion of beech, such as Slovenia. In this study, the effects of climate change were examined at a high-resolution scale, encompassing the large climatic, orographic, and ecological variability of beech forests in Slovenia. Using basal area increment data (BAI) from a tree-ring network (48 sites in Slovenia), modelled climate data, and generalized linear mixed models (GLMM), we found an average growth decline of 11% between the 1953–1985 and 1986–2018 subperiods, affecting 90.5% of the forest stands. Based on climate data, we defined two contrasted marginal areas of beech presence (warm and cold) and analysed the growth changes over time. The warm marginal areas, which predominate near the geographical margin of beech distribution with a sub-Mediterranean climatic regime, were most affected by growth decline, threatening the survival of beech populations in the area. In contrast, cold marginal areas, mainly at high elevations in the Alps, where beech growth had previously been limited by low temperatures, turned out to be the only ones where growth of beech increased under prevailing warming conditions. Consequently, high elevation regions harbour climatic potential for increased beech growth performance, and may represent areas of future expansion of the species.