Abstract
Recent studies show that several tree species are spreading to higher latitudes and elevations due to climate change. European beech, presently dominating from the colline to the subalpine vegetation belt, is already present in upper montane subalpine forests and has a high potential to further advance to higher elevations in European mountain forests, where the temperature is predicted to further increase in the near future. Although essential for adaptive silviculture, it remains unknown whether the upward shift of beech could be assisted when it is mixed with Norway spruce or silver fir compared with mono-specific stands, as the species interactions under such conditions are hardly known. In this study, we posed the general hypotheses that the growth depending on age of European beech in mountain forests was similar in mono-specific and mixed-species stands and remained stable over time and space in the last two centuries. The scrutiny of these hypotheses was based on increment coring of 1240 dominant beech trees in 45 plots in mono-specific stands of beech and in 46 mixed mountain forests. We found that (i) on average, mean tree diameter increased linearly with age. The age trend was linear in both forest types, but the slope of the age–growth relationship was higher in mono-specific than in mixed mountain forests. (ii) Beech growth in mono-specific stands was stronger reduced with increasing elevation than that in mixed-species stands. (iii) Beech growth in mono-specific stands was on average higher than beech growth in mixed stands. However, at elevations > 1200 m, growth of beech in mixed stands was higher than that in mono-specific stands. Differences in the growth patterns among elevation zones are less pronounced now than in the past, in both mono-specific and mixed stands. As the higher and longer persisting growth rates extend the flexibility of suitable ages or size for tree harvest and removal, the longer-lasting growth may be of special relevance for multi-aged silviculture concepts. On top of their function for structure and habitat improvement, the remaining old trees may grow more in mass and value than assumed so far.
Highlights
For many tree species living on the edge of their distribution regarding longitude, latitude or elevation becomes increasingly difficult under climate change
1 was within the 95% confidence interval. This indicates no significant deviation from linear diameter growth over age for beech, as illustrated in Fig. 3, with a ranking regarding the steepness of the slope beech mono-specific > beech mixed mountain forests
Many of the sample trees are approaching this top age, their growth does not yet show a convergence to a maximum stem diameter (Fig. 3). This is in line with the results of Pretzsch et al (2020b), who found a linear diameter growth trend for Norway spruce, silver fir and beech trees in mixed mountain forests
Summary
For many tree species living on the edge of their distribution regarding longitude, latitude or elevation becomes increasingly difficult under climate change. Because of their multiple protection functions and ecosystem services, mountain forests demand high attention by global change research and require climate-smart forest management practices. European beech (Fagus sylvatica L.; hereafter referred to as beech) is one of the most important and successful tree species in Europe (Leuschner et al 2006) and contributes a high economic and ecological value across lowland, upland and mountain regions (Eurostat 2018). The current growth trends of beech and its response to stress events are important for a realistic assessment of its potential under future climate change
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