Response of European beech to decreasing summer precipitation under global climate change

Global warming-related summer precipitation reduction represents a major threat on tree vitality and persistence and hence on forest productivity over large parts of the temperate zone. European beech (Fagus sylvatica L.) is the most important broadleaf tree species of Central Europe’s natural forest vegetation and one of the economically most important trees in Germany. Although this forest tree shows a wide tolerance toward a wide range of climatic and edaphic growing conditions, beech is known to be drought sensitive. Accordingly, the future role of F. sylvatica in Europe is under controversial debate. In order to broaden the knowledge of the response and the adaption potential of European beech to a drier climate likely to occur in the future, a gradient study was conducted. Measurements were taken along a natural rainfall gradient (855-576 mm y-1) in 12 mature beech forest stands on comparable sandy geological substrate in the Central Northern German Lowlands was conducted. To consider the potential effect of soil water storage on the precipitation-response, this study was partly carried out in paired comparable beech stands growing on sites with contrasting soil texture (sandy vs. sandy-loamy). Influences on precipitation reduction during the growing season and the response of F. sylvatica trees were investigated by (i) a common garden experiment with provenances from moist to dry climates under defined moisture conditions to investigate the short-term adaptive drought responses of young beech trees. (ii) comparing tree ring chronologies along the gradient on two different soil types to define a critical precipitation limit under which growth decreases occur and to enlighten the role of the edaphic effect on growth dynamics (iii) characterizing the vulnerability but also the adaptation potential to drought of F. sylvatica of sun-exposed branches with cavitron spinning technique and anatomical investigations One option for forest managers to minimize the risk of failure is to use provenances of tree species which combine high productivity with drought tolerance. Beech sapling genotypes exposed to different precipitation regimes seem to have achieved only a limited number of specific drought adaptations. Surprisingly, provenance had only a minor influence on the morphology and growth. The main reason for this result may be the fact that genetic differences between populations were small, while genetic variation within populations was high in the study region. However, some physiological traits showed to be influenced by provenance: The modulus of leaf tissue elasticity ε was significantly higher in plants from moist origins. Hence, these plants may face a higher risk of hydraulic failure due to more rigid cell walls in leaf tissue. The high phenotypic plasticity observed in this study in terms of biomass partitioning, leaf and root morphology can be seen as the pivotal strategy of beech saplings to get along with the challenges of drought periods. The evolved adaptation of leaf water status adjustment has only minor effects on plant morphology and growth rate. Accordingly, selecting and translocating provenances across the investigated gradient is not recommended. Comparing tree-ring series across this gradient, revealed that global warming already acts as a major stressor in F. sylvatica stands in the center of the species’ distribution. Under a threshold of 350 mm precipitation during growing season or 600 mm during the year, basal area increment (BAI) has been constantly decreasing since the 1960s or 1970s. Even since the 1950s, sensitivity of mean ring width has increased, by decreasing autocorrelation of the forest stands. An increase in the occurrence of negative pointer years was observed since the 1960s. Surprisingly, the role of soil water storage for the growth dynamics of beech was not reflected in our results. These results call for careful tree species selection by foresters in forest regions where precipitation is already below the precipitation thresholds elaborated here. It is likely that F. sylvatica may lose part of its range in the managed and natural forest area of Central Europe upon climate warming. Mature beech trees adapt their hydraulic architecture to long-term decreases in water supply with diminution of their vessels. We observed an increased appearance of smaller vessels along the gradient, while the frequency of larger vessels declined or remained unaffected. Surprisingly, leaf-specific conductivity decreased with increasing drought, while pit membrane thickness increased. Branches showed a larger embolism resistance with increasing drought at tree origin. Contrary to expectations, leaf-specific conductivity declined with decreasing precipitation. Due to branch wood modification, trees at the dry end of the investigated gradient were capable of tolerating more negative leaf water potentials. It may therefore reasonably be concluded, that the beech’s branch hydraulic system has a distinct adaptive potential to respond to a precipitation reduction. The observed high variability of traits and the physiological adaption potential within mature and young F. sylvatica populations, could be seen as a valuable tool for withstanding a future drier climate. However, the observed radial growth declines at the drier stands in our study region in the last decades, casts doubt on the ability of F. sylvatica to withstand the predicted intensification of aridity during the growing season even in the centre of its distribution