Axel Göttlein

Tangel is an important humus form in mountain forests, particularly in areas of carbonate rock. To gain an initial understanding of its sensitivity to rising temperatures, a respiration experiment was conducted. The derived Q10 values (3.35–3.36) are comparatively high in relation to values reported in the literature, indicating that Tangel is highly sensitive to climate change and the opening of the forest canopy.

Information about soils and climate of the origin regions of the tree species is needed for the “assisted migration” of genotypes better adapted to the future climate as well as for planting of other native and non-native but better adapted tree species. Ecological classifications are seen as an important basis for “assisted migration”. However, up to now, all known approaches to classify regions of tree origin do not take into account soil properties, although soil is important for the selection of suitable origins of seed and planting material. The presented pedological-climatic classification of Central and Southern Europe can serve as a basis for a unified classification of forest growth regions that integrates soil properties for the first time. For the soil parameter of the pedoclimatic classification, the WRB soil map from the European Soil Data Centre 1:1.000.000 was used and classified for the base status of the soils. The climate was characterized by the water deficit in summer and a winter hardiness parameter. For this purpose, the CHELSA datasets for the periods 1981–2010, 2011–2040 and 2071–2100 were used, the latter with the SSP585 scenario. This gives us a 3 × 5 × 3 matrix with 45 units, with which we can fairly well map all seed and plant origins of forest tree species in Central and Southern Europe.

In Germany during several decades, emissions and thus the chemical climate affecting forests have changed significantly. The effects of these changes on the element balance of forests can be documented only by long‐term observations, as has been done at the Höglwald site (Southern Bavaria) since 1985. Since then, structural changes in agriculture have led to a reduction in emissions of reduced nitrogen (NH ₃ ). There was also a slight decrease in emissions of oxidized nitrogen (NO _x ). Air pollution control measures, especially in the 1980s, led to a particularly drastic reduction of sulfur emissions (SO ₂ ). Consequently, inputs to the ecosystem decreased by almost 95% between 1985 and 2020. Dry deposition nowadays plays practically no role for this element. High nitrogen inputs, dominated by reduced nitrogen, have led to a high proton production through N transformations. This has gradually reduced the buffering capacity of the topsoil. Comparing measured fluxes shows that with decreasing sulfur inputs, the sulfur stored in the topsoil from times of high deposition was remobilized. At the Höglwald, this process occurred rather clearly over a period of about 28 years and has resulted in only about 11% of the initial amount of sulfur being still present in the topsoil (humus layer + mineral soil down to 40 cm) in 2020. Forestry should take the changed chemical conditions into account in its nutrient management.

The climatic drought limit of common tree species depend on soil nutrient status In forest ecology, there is a huge experience in the ecological amplitude of tree species, which is commonly represented in so-called ecograms. However, the ecogram axes are purely qualitatively described and scaled. In the study presented here, we try to specify the drought limit in the ecogram for the ten most abundant and economically important tree species in Germany (Norway spruce, Scots pine, silver fir, European beech, pedunculate oak, sessile oak, sycamore maple, European elm, hornbeam and common ash) using generalized regression models. We use data on the occurrence of the tree species and on the soil of about 3300 plots of the second German forest soil condition survey (BZW II), combined with data from the ICP Forests Level I program (BioDiv and BioSoil) from 13 neighboring European countries as well as climate data from WorldClim. The focus is on the question to what extent the drought limit of the tree species depends on the soil nutrient status. As a predictor of nutrient status, we chose the base saturation type (BT) already introduced in practice. The warmth factor was included as Growing Degree Days, the water balance as climatic water balance in the forest vegetation period (KWB). The drought limit could be specified for all tree species except elm and pedunculate oak. It was found that not only in known nutrient-sensitive tree species (sycamore, ash, hornbeam), the drought limit depends on the BT, but also in beech and spruce. In the case of sycamore as an example of a nutrient-sensitive species, the dry climate limit on strongly acidic soils (BT 5) is already reached at a KWB of 20 l/m2, on base-rich (BT 1 and 0) only at a KWB of −150 l/m2. In beech, too, the drought limit on acidic soils has been reached more quickly than on base-rich sites (BT 5: −40 l/m2, BT 0: −120 l/m2). In contrast, the reaction of spruce is reversed (BT 5: −50 l/m2, BT 0: 10 l/m2). For pine, sessile oak and fir, the drought limit is independent of the base type and is −230 l/m2 (Scots pine), −150 l/m2 (sessile oak) and 0 l/m2 (silver fir). The drought limit specified by BT is a helpful quantity, especially in view of climate change, as it makes it possible to better estimate the potential of the different tree species.

The paradigm of sustainable forest management was initially aimed to ensure continuous wood supply but has gradually been extended for many functions and services of forest ecosystems, including the aspect of nutrient sustainability. The current tendency towards harvesting all year round and whole‐tree export for energy purposes raised the question of how the input : output balance of nutrients depends on site conditions, tree species, and harvest intensity. Possible differences in nutrient export between harvesting in autumn and summer have been more or less neglected. Based on compartment‐wise analyses of biomass and nutrient (Ca, K, Mg, and P) concentrations of adult European beeches (Fagus sylvatica[L.]), nine compartments were sampled in summer and nine in autumn. We found, for the majority of the compartments (especially in branch wood), increased levels of nutrient concentrations: 81% (Ca), 54% (K), 90% (Mg), and 96% (P) in autumn compared to summer. Upscaling our results to stand level revealed up to 10.0% and 1.6% less mineral nutrient exports of Ca and Mg, respectively, for whole‐tree harvest in forest stands when carried out in summer instead of autumn. Nutrient removal was increased by 0.8% and by 13.0% for K and P, respectively, in summer. Despite lower nutrient export, many ecological, logistic, and technical reasons discourage harvesting in summer. Our findings, however, deliver an additional yet so far missing mosaic piece for a better understanding and assessment of the frame conditions for sustainable nutrient management in beech forests.

Tree-ring stable isotopes, providing insight into drought-induced eco-physiological mechanisms, are frequently used to reconstruct past changes in growing season temperature and precipitation. Their climatic response is, however, still not fully understood, particularly for data originating from non-extreme, mid-latitude environments with differing ecological conditions. Here, we assess the response of δ¹³C, δ¹⁸O and tree-ring width (TRW) from a temperate mountain forest in the Austrian pre-Alps to climate and specific drought events. Variations in stem growth and isotopic composition of Norway spruce, common beech and European larch from dry, medium and moist sites are compared with records of sunshine, temperature, moisture, precipitation and cloud cover. Results indicate uniform year-to-year variations in δ¹³C and δ¹⁸O across sites and species, but distinct differences in TRW according to habitat and species. While the climate sensitivity of TRW is overall weak, the δ¹³C and δ¹⁸O chronologies contain significant signals with a maximum sensitivity to cloud cover changes (r = −0.72 for δ¹⁸O). The coherent inter-annual isotopic variations are accompanied by substantial differences in the isotopic signatures with offsets up to ∼3‰ for δ¹³C, indicating species-specific physiological strategies and varying water-use efficiencies. During severe summer drought, beech and larch benefit from access to deeper and moist soils, allowing them to keep their stomata open. This strategy is accompanied by an increased water loss through transpiration, but simultaneously enables enhanced photosynthesis. Our findings indicate the potential of tree-ring stable isotopes from temperate forests to reconstruct changes in cloud cover, and to improve knowledge on basic physiological mechanisms of tree species growing in different habitats to cope with soil moisture deficits.