The imapct of drought and climate warming on Central European broad-leaved mixed forests

Climate warming is predicted to increase the frequency and severity of droughts in Central Europe through increasing temperatures and regional changes in the precipitation regime. Fagus sylvatica (European beech), the most abundant tree species of Central Europe’s natural forest vegetation and one of the key species in forestry, is thought to be particularly vulnerable to drought. As broad-leaved mixed forest stands in Central Europe are widely dominated by this species, the predicted climatic changes may have considerable impact on those forest ecosystems, what could lead to a shift in the competitive hierarchy and species composition of those forests. To evaluate how relevant changes may take shape, it is of great interest how co-occurring tree species react to drought compared to F. sylvatica and how mixed forest ecosystems are affected by drought in general. In the frame of the present study F. sylvatica as well as the four co-occuring species Acer pseuoplatanus (sycamore maple), Acer platanoides (Norway maple), Quercus petraea (sessile oak) and Fraxinus excelsior (common ash) were examined with regard to their drought sensitivity. For this purpose wood cores of the corresponding species were collected at three stands along a precipitation gradient (590-685 mm). The sampled cores were used for dendrochronological and xylem anatomical analysis to investigate the species specific response and adaption to climatic changes in the period from 1951-2010. Furthermore, the neighborhood of any target tree was recorded, to evaluate possible effects of neighborhood diversity and identity on the drought resistance of the target tree. This was done to (I) investigate the impact of drought on increment of the five species, (II) examine the adaption processes in the hydraulic architecture of the stem xylem to changes in climate and (III) identify how tree characteristics and neighborhood composition affect the drought resistance of the investigated trees. Increment of the investigated species was primarily determined by temperatures and precipitation amounts of the current and previous years spring and summer months. F. sylvatica was the only species showing growth decline since about 1980 at the driest stand. This growth decline was closely related to increasing temperatures and increasing drought intensity. Furthermore, F. sylvatica showed the strongest growth depression during extreme droughts and the frequency of corresponding growth depressions (negative pointer years) did increase during the end of the investigated period only in this species. Climate response analysis showed that hydraulically-weighted mean vessel diameter Dh was determined by the same climate conditions as tree-ring width, leading to smaller vessels under increased aridity. However, this trend was overlain by a marked age/size effect that led to a significant increase of Dh in all species except F. sylvatica over the investigated period and as a result in an increase of the sapwood area-specific hydraulic conductivity (Kp). Despite the missing increase in Dh, F. sylvatica was also able to increase its Kp through an increased vessel density (VD), reflecting a remarkably plasticity of the hydraulic architecture of this species. Yet, while Kann, the conductivity generated by annual ring growth, showed an even larger increase than Kp in the four other species, Kann of F. sylvatica decreased in conjunction with the observed growth decline. While the neighborhood diversity had no effect on the growth resilience to severe drought events, the proportion of F. sylvatica had a negative impact on resilience in some cases. Of greater importance for a high resilience was the size of the target tree. In all species growth resilience after two extreme drought events was negatively related to diameter at breast height (DBH). The higher sensitivity of large-diameter trees results in a shift in intra- as well as interspecific population structure with smaller, less competitive trees surpassing larger dominant trees in radial increment in the decades after a severe drought. This pattern was clearest in F. sylvatica and enabled small trees at the driest stand to even increase their BAI since 1980, contrary to the general trend. Those results indicate that extreme drought events represent an important structuring force in Central European mixed forests that trigger dynamic change in the population structure of the species and also in species composition. Despite its high plasticity, F. sylvatica was the species strongest affected by drought and it can be expected that this species will lose at least some of its competitive strength in the course of climate warming.