Zuwachsniveau, Zuwachstrend und episodische Zuwachseinbrüche. Ein zusammenfassendes Bild vom aktuellen Zuwachsgang in Rein- und Mischbeständen aus Fichte, Kiefer, Buche und Eiche

The presented here was based on seemingly contradictory findings of the growth behaviour of central European forest stands: On the one hand the course of the annual volume growth of forest stands is reported to proceed much higher than predicted by the yield tables. On the other hand, forest stands suffered considerable growth slumps below their long term trend. Here we show, how the growth rates of forest stands continuously exceed the level of the common yield tables since the middle of the last century, how they decrease in drought years such as 2003 and 2015, and report that even in years of severe drought growth losses are still continuously proceeding on a level above the yield tables of most forest types. The study is based on 57 middle aged, fully stocked, non-thinned or only slightly thinned mono-specific and mixed species stands of Norway spruce (Picea abies [L.] KARST.), Scots pine (Pinus sylvestris L.), European beech (Fagus sylvatica L.), and sessile oak (Quercus petraea (MATT.)) in Germany. We use the annual stand volume growth of the last 20 years. This period covers the extremely dry years 2003 and 2015. We found that the mean annual stand volume growth lay 5–77 % and on average 48 % above the level predicted by the respective yield tables (Figure 1–2 and Table 4). The deviations from the yield tables are most distinctive for sessile oak, Scots pine and European beech (+47–77%) and less for Norway spruce (+5%). Mono-specific stands (+51%) and mixed-species stands (47%) did not differ significantly regarding their deviation from the stand volume growth of the yield tables (Figure 2 and Table 4). The growth of most of the stands has increased in relation to the yield tables in the last 20 years (Figure 2 and Table 4). The deviation from the yield table increased on average by 2.60% per year. We found the steepest increase for Scots pine and the lowest for Norway spruce. There was a tendency that the increase was stronger in mixed than in mono-specific stands, however, the slopes did not differ significantly. In the years 2003, 2015 with the strongest growth reactions on drought stress the stand volume growth was reduced to 0.72 of the normal long-term level (1.0); this means a growth loss in relation to the long-term trend of 28% (Table 4). The growth reductions were most severe in mono- and mixed-species stands of Norway spruce (decrease to 0.63–0.68, i. e. losses of 32–37%). The slightest stress reactions were found in mixed stands of Scots pine and European beech and Scots pine and sessile oak (decrease to 0.78–0.81, i. e. losses of 19–22%) compared with the long-term trend. In dry years, growth decreased by 19–22% under the long-term trend. In the years with the strongest slumps below the long-term trend the growth still lied by 1.07 compared to the yield tables, i.e. by 7% above the growth level predicted by the yield tables (Table 4). This means that the growth falls significantly below the normal long-term trend in dry years, but growth was still significantly higher than suggested by the yield tables, which represent the historic growth level. Only in a few cases, namely in monospecific Norway spruce stands, growth decreased to 78% of the values reported in the yield table in dry years. In the most favourable cases, in monospecific sessile oak stands, the growth even in dry years still amounted to 133% of the values reported in the yield tables, indicating that the stands still exceed the yield tables by 33% in their stand volume growth despite the slumps caused by drought stress (Table 4). We discuss the relevance of both, the long-term deviation of the stand growth from the yield tables and the episodic deviation of the growth from the current upwards trend of stand growth. The long-term increase above the historic level of the yield tables indicates significant changes of the environmental conditions by rise of temperature, extension of the length of the growing season, eutrophic N-deposition and rise of the atmospheric CO₂-concentration. The episodic growth slumps indicate severe stress reactions to drought. The study underlines the relevance of long-term stand growth series for provision of forest management with productivity data and environmental monitoring with quantitative information of the current vitality and stress situation of forest stands. Against the background of the predicted increasing frequency of years with long drought periods during the vegetation period, such long-term data is urgently needed to adequately appraise possible future developments.