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Central Europe has been experiencing unprecedented droughts during the last decades, stressing the decrease in tree water availability. However, the assessment of physiological drought stress is challenging, and feedback between soil and vegetation is often omitted because of scarce belowground data. Here we aimed to model Swiss forests' water availability during the 2015 and 2018 droughts by implementing the mechanistic soil-vegetation-atmosphere-transport (SVAT) model LWF-Brook90 taking advantage of regionalized depth-resolved soil information. We calibrated the model against soil matric potential data measured from 2014 to 2018 at 44 sites along a Swiss climatic and edaphic drought gradient. Swiss forest soils' storage capacity of plant-available water ranged from 53 mm to 341 mm, with a median of 137 ± 42 mm down to the mean potential rooting depth of 1.2 m. Topsoil was the primary water source. However, trees switched to deeper soil water sources during drought. This effect was less pronounced for coniferous trees with a shallower rooting system than for deciduous trees, which resulted in a higher reduction of actual transpiration (transpiration deficit) in coniferous trees. Across Switzerland, forest trees reduced the transpiration by 23% (compared to potential transpiration) in 2015 and 2018, maintaining annual actual transpiration comparable to other years. Together with lower evaporative fluxes, the Swiss forests did not amplify the blue water deficit. The 2018 drought, characterized by a higher and more persistent transpiration deficit than in 2015, triggered widespread early wilting across Swiss forests that was better predicted by the SVAT-derived mean soil matric potential in the rooting zone than by climatic predictors. Such feedback-driven quantification of ecosystem water fluxes in the soil–plant-atmosphere continuum will be crucial to predicting physiological drought stress under future climate extremes., Global Change Biology, 28 (20), ISSN:1354-1013, ISSN:1365-2486

Eutrophication due to increased nitrogen concentrations is known to alter species composition and threaten sensitive habitat types. The contribution of atmospheric nitrogen deposition to eutrophication is often difficult to determine. Various deposition models have been developed to estimate the amount of nitrogen deposited for both entire regions and different landscape surface types. The question arises whether the resulting deposition maps allow direct conclusions about the risk of eutrophication-related changes in the understory vegetation composition and diversity in nitrogen-sensitive forest ecosystems. We combined vegetation and soil data recorded across eutrophication gradients in ten oligo-mesotrophic forest types in southwest Germany with datasets from two different deposition models specifically fitted for forests in our study region. Altogether, 153 forest stands, with three sampling replicates each, were examined. Linear mixed-effect models and NMDS analyses revealed that other site factors, in particular the soil C/N ratio, soil pH and canopy cover, played a greater role in explaining vegetation gradients than nitrogen deposition. The latter only rarely had effects on species richness (positive), nitrophyte cover (positive or negative) and the cover of sensitive character species (negative). These effects varied depending on the deposition model used and the forest types examined. No effects of nitrogen deposition on average Ellenberg N values were found. The results reflect the complex situation in forests where nitrogen availability is not only influenced by deposition but also by nitrogen mineralization and retention which depend on soil type, pH and (micro)climate. This context dependency must be regarded when evaluating the effects of nitrogen deposition.

Severe drought events negatively affect tree growth and often cause legacy effects, expressed by smaller tree rings in the post-drought recovery years. While the pattern of reduced tree-ring widths is frequently described the processes underlying such legacy effects, i.e., whether it is due to shorter growth periods or lower growth rates, remains unclear and is investigated in this study. To elucidate these post-drought effects, we examined radial stem growth dynamics monitored with precision band-dendrometers on 144 Douglas fir, Norway spruce and silver fir sample trees distributed along four elevational gradients in the Black Forest (Southwest Germany) during the post-drought years 2019 and 2020. Growth onset of all investigated species occurred between 11 and 24 days significantly earlier in 2020 compared to 2019. Modelling growth onset based on chilling and forcing units and taking the study year into account explained 88-98 % of the variance in the growth onset data. The highly significant effect of the study year (p 0.001) led to the conclusion, that other factors than the prevailing site conditions (chilling and forcing units) must have triggered the earlier growth onset in 2020. On the other hand, for Douglas fir growth rates were significantly higher in 2020 compared to 2019 (2.9 μm d

Sustainable forest management requires understanding of ecosystem phosphorus (P) cycling. Lang et al. (2017) [Biogeochetnistry, https://doi.org/ 10.1007/s10533-017-0375-0] introduced the concept of P-acquiring vs. P-recycling nutrition strategies for European beech (Fagus sylvatica L.) forests on silicate parent material, and demonstrated a change from P-acquiring to P-recycling nutrition from P-rich to P-poor sites. The present study extends this silicate rock-based assessment to forest sites with soils formed from carbonate bedrock. For all sites, it presents a large set of general soil and bedrock chemistry data. It thoroughly describes the soil P status and generates a comprehensive concept on forest ecosystem P nutrition covering the majority of Central European forest soils. For this purpose, an Ecosystem P Nutrition Index (ENIp) was developed, which enabled the comparison of forest P nutrition strategies at the carbonate sites in our study among each other and also with those of the silicate sites investigated by Lang et al. (2017). The P status of forest soils on carbonate substrates was characterized by low soil P stocks and a large fraction of organic Ca-bound P (probably largely Ca phytate) during early stages of pedogenesis. Soil P stocks, particularly those in the mineral soil and of inorganic P forms, including Al- and Fe-bound P, became more abundant with progressing pedogenesis and accumulation of carbonate rock dissolution residue. Phosphorus-rich impure, silicate-enriched carbonate bedrock promoted the accumulation of dissolution residue and supported larger soil P stocks, mainly bound to Fe and Al minerals. In carbonate-derived soils, only low P amounts were bioavailable during early stages of pedogenesis, and, similar to P-poor silicate sites, P nutrition of beech forests depended on tight (re)cycling of P bound in forest floor soil organic matter (SOM). In contrast to P-poor silicate sites, where the ecosystem P nutrition strategy is direct biotic recycling of SOM-bound organic P, recycling during early stages of pedogenesis on carbonate substrates also involves the dissolution of stable Ca-P org precipitates formed from phosphate released during SOM decomposition. In contrast to silicate sites, progressing pedogenesis and accumulation of P-enriched carbonate bedrock dissolution residue at the carbonate sites promote again P-acquiring mechanisms for ecosystem P nutrition., Biogeochemistry, 2022 (158), ISSN:0168-2563, ISSN:1573-515X

Foliar phosphorus (P) concentrations in beech trees are decreasing in Europe, potentially leading to reductions in the trees’ growth and vitality. In the course of climate change, drying and rewetting (DRW) cycles in forest soils are expected to intensify. As a consequence, P leakage from the root zone may increase due to temporarily enhanced organic matter mineralization. We addressed the questions whether sites with different soil properties, including P pools, differ in their susceptibility to DRW-induced P leaching, and whether this is affected by the DRW intensity. A greenhouse experiment was conducted on naturally structured soil columns with beech saplings from three sites representing a gradient of soil P availability. Four DRW cycles were conducted by air-drying and irrigating the soils over 4 hours (fast rewetting) or 48 hours (slow rewetting). Leachates below the soil columns were analyzed for total P, and molybdate reactive P (considered as inorganic P). The difference was considered to represent organically bound P. Boosted regression trees were used to examine the effects of DRW and soil characteristics on P leaching. Contrary to a first hypothesis, that P leaching increases upon rewetting with the intensity of the preceding desiccation phase, intense soil drying (to pF 3.5 to 4.5) did not generally increase P leakage compared to moderate drying (to pF 2 to 3). However, we observed increased inorganic P concentrations and decreased organic P concentrations in leachates after drying to matric potentials above pF 4. Also against our expectations, fast rewetting did not lead to higher leakage of P than slow rewetting. However, the results confirmed our third hypothesis that the site poorest in P, where P recycling is mainly limited to the humus layer and the uppermost mineral soil, lost considerably more P during DRW than the other two sites. The results of our experiment with naturally structured soils imply that intensified drying and rewetting cycles, as predicted by climate-change scenarios, may not per se lead to increased P leaching from forest soils. Soil properties such as soil organic carbon content and texture appear to be more important predictors of P losses.

Heavy harvesting leads to harmful soil compaction and negative effects of soil physical properties and rootability. We monitored the recovery of soil structure in a compacted forest soil, which had been treated with a combination of regeneration techniques (mulching, liming, planting of alder trees or a combination of those). Soil physical properties (relative apparent gas diffusion coefficient (Ds/D0), bulk density and the macropore volume) were measured in three successive campaigns, with two years between each, with fine root densities taken as a biological target variable. Distinct changes of soil physical properties were detected during the study period, mainly caused by the effects of the applied regeneration techniques. After four years, higher values of Ds/D0 and macropores indicated significant improvements of soil aeration in the topsoil. The deeper soil showed no distinct evidence of regeneration, regardless of the regeneration technique applied. However, rather a negative impact of mulching on soil physical properties was detected. The measured physical parameters, Ds/D0, bulk density and macropore volume correlate highly with observed fine root densities. In the topsoil, root density increases with increasing soil gas permeability, while in the deeper horizons only few macropores are occupied by fine roots for gas exchange.

Phosphorus (P) is a limiting factor of primary productivity in most forest ecosystems but little is known about retention within and losses of P from forests. Subsurface flow (SSF) is one of the important pathways of P export but few attempts exist to quantify it. We present results of sprinkling experiments with ca. 150 mm, 2H labelled, total rainfall conducted at 200 m2 plots on hillslopes with slopes between 14° and 28° at three beech forests in Germany in summer and spring. We aimed at quantifying vertical and lateral SSF and associated P transport in the forest floor, the mineral soil and the saprolite. The study sites differed regarding soil depth, skeleton content and soil P stocks (between 678 g/m2 and 209 g/m2, in the first 1 m soil depth). Vertical SSF in the mineral soil and in the saprolite was at least two orders of magnitude larger than lateral SSF in the same depth. Vertical and lateral SSF consisted mainly of pre-event water that was replaced by sprinkling water (piston flow mechanism). Short spikes of event water at the beginning of the experiment at two of the sites with high skeleton content indicate that preferential flow occurred in parallel to matrix flow. We observed a significant decrease in P concentrations in SSF with increasing soil depth suggesting effective retention of P by adsorption to soil particles in all three forest ecosystems. Higher P concentrations in SSF at the beginning of the experiments indicate nutrient flushing but P concentrations were nearly constant thereafter despite strong increase in SSF. P concentrations did also not change significantly with increasing share of event water in SSF. These chemostatic transport conditions suggests that P mobilization rates were similar to transport rates in both, P-rich and P-poor sites. The observed first flush effect implies that P export by SSF will increase as rainfall events with high transport capacity are predicted to occur more frequent under future climatic conditions.

The warm and dry years of the recent past have once again impressively shown that bark beetle outbreaks are among the most dangerous natural hazards that occur in forests of Central Europe and North America.The European spruce bark beetle (Ips typographus L.) in particular is one of the most important pests in Central European forests. Induced by the ongoing climate change, it seems to be quite likely that the growing conditions of Norway spruce (Picea abies L.) will deteriorate considerably due to predicted rising temperatures and increasing frequency and intensity of droughts and further extreme weather events. In contrast, the spruce bark beetle is favored by the same trends. As a result, it tends to mass outbreaks and can thereby also infest healthy spruces, causing forests to die off over large areas. Since management resources and warning tools needed for a just-in-time detection of infested trees will remain limited, efficient operational systems are highly desired to enhance and to facilitate bark beetle risk management.For this reason, we developed the prototype of an operational early warning system to assess the current risk of potentially endangered spruce stands to bark beetle infestations at a high temporal (daily) and spatial (≤ 250-m-grid) resolution.The system considers the following input layers:(a) a quasi-static base-risk layer that is calculated from stand and site characteristics;(b) an annually updated layer determining the bark beetle population density; anddaily-updated layers for increased host tree susceptibility by (c.1) drought stress or (c.2) storm damage and (c.3) the swarming activity of the bark beetle.From these inputs a daily overall infestation risk plus a 7-day-forcast is calculated and made available online to forest owners and managers in the form of a risk map providing different risk levels (e.g., low – medium – high).As one of the main driving factors, the (c.1) drought stress induced disposition of spruce forests to bark beetle infestation is assessed by applying a grid-based soil water balance model at daily resolution. The plausibility of the model is checked via representative soil hydrological measuring areas in the three German project areas Black Forest National Park, Saxon Switzerland National Park, and Hunsrück-Hochwald National Park. At the same time, suitable water scarcity indicators are identified and defined for these threshold values, below or above which an increased susceptibility of spruce trees to bark beetle attack is to be expected. Hence, in connection with daily updated weather forecasts, the water-related disposition of spruce stands to bark beetle infestation can be predicted with reasonable accuracy.The developed early warning system or implemented sub-systems have the flexibility to be adapted to other bark beetles or further forest pests and can be applied at local, regional and national scales. Furthermore, its functionality can be extended by integrating novel modern approaches, e.g. machine-learning methods or remote sensing technologies.

Excessive atmospheric nitrogen deposition is known to alter nutrient cycles and species composition in temperate forest ecosystems. Foliar N concentrations reflect the nutrient status of plants and can be used to assess the impact of atmospheric N deposition on ecosystems. Our objective was to test whether the foliar N content reflects increasing N deposition in the understory vegetation of temperate forests on acidic soil. Therefore, we analyzed the foliar N content of seven plant taxa representing different functional groups of the understory vegetation and correlated them to modeled deposition datasets. Linear mixed models revealed no relationship of N deposition with foliar N concentrations across all species examined but decreasing foliar concentrations with increasing soil C/N ratio. Nevertheless, we found positive correlations of foliar N concentrations with increasing deposition in conifer saplings (Picea abies and Abies alba), the dwarf shrub Vaccinium myrtillus and the graminoid Luzula luzuloides. Such an increase was, however, not found in saplings of broad-leaved trees (Fagus sylvatica and Quercus petraea/robur) and the moss Polytrichum formosum. This suggests marked species-specific differences in nitrogen uptake and allocation. Foliar N concentrations were more strongly correlated to NOy than NHx and to wet than dry deposition. Since foliar N concentrations were correlated to deposition while the species’ cover was not, they may provide a valuable indicator of N deposition.

Spatial patterns in biotic reactions of forest trees (secondary growth, defoliation, nutrition) in Germany and their associations with conditions and changes in forest soils and with other environmental variables were investigated. The study aimed to identify areas at risk and risk factors. Growth ring widths and defoliation indicated drought stress as main risk factor. High annual mean temperature deviations of approximately 1.5 °C from the long-term mean (1961–1990), particularly in combination with high negative precipitation deviations, were associated with increased defoliation in all four main tree species. Defoliation development types that showed clear large-scale spatial distribution patterns were identified for all species. The defoliation development types were in good accordance with the landscape regions of Germany. Weather conditions, in particular deviations of temperature and precipitation from the long-term mean, explained a large proportion of differences among defoliation development types. South-western Germany was the region at highest risk for defoliation in recent years. This finding was mainly attributed to severe drought stress in this region in the previous years. Nutrition types did not display a strong spatial pattern. In regions affected by chronic high N deposition or where intensive liming has been conducted, antagonistic effects of K with other nutrients (\( {\mathrm{NH}}_4^{+} \), Ca) probably played a role in frequently observed deficient nutrition of forest trees with K and also in defoliation. Mitigation of climate change should have top priority. Environmental policy and forest management should further aim to reduce stress factors originating from air pollution and nutrient deficiencies in order to facilitate the regenerative capability of forest trees and adaption to climate change.

The availability of water to roots is a central property for the growth and productivity of forests. Because nearly all plant nutrients are taken up from the soil solution, there is a close connection between forest nutrition and the water budget. Furthermore, seepage water transports substances between the atmosphere, the soil, and the hydrosphere and determines the soils’ filter function. Information on the water budget is therefore an important basis for the assessment of forest sites. Through processing and combination of data on soil, root and forest stand parameters in the NFSI data set, it was possible to reliably estimate the factors that control the soil water budget. Soil hydraulic properties such as water retention and water conductivity were derived from measured and estimated soil properties using pedotransfer functions. Additionally, the depth profiles of fine roots for each of the NFSI profiles were estimated in a correlation analysis between fine root density and soil properties (Hartmann and Wilpert 2014). Based on the processed NFSI data, the soil water budget model LWF-Brook90 was parameterized, and the dynamic water budget for all NFSI profiles was modelled. The aim was to identify in what way water flow and availability are affected by the variability of soil properties. All simulations were carried out in daily resolution, and model output, such as seepage water rates, soil water storage and evapotranspiration, was derived in daily resolution. This chapter gives an overview on (1) the soil water budget model LWF-Brook90, (2) the methodologies used to derive the input parameters for the soil water budget modelling (specifically the soil hydraulic and rooting properties) and (3) the results of the soil water modelling. The discussion of results focusses on the assessment of drought indices. It is shown that the plant-available soil water storage and other drought indices display similar trends. According to the model results, the intensity of water shortage has increased since 1990 and years with above-average soil water supply.

Phosphorus (P) is essential for sustainable forest growth, yet the impact of anthropogenic impacts on P leaching losses from forest soils is hardly known. We conducted an irrigation experiment with 128 mesocosms from three forest sites representing a gradient of resin extractable P of the A-horizon. On each site we selected a Fagus sylvatica and a Picea abies managed subsite. We simulated ambient rain (AR), anthropogenic nitrogen input (NI) of 100 kg (ha · a)−1 and forest liming (FL) with a dolomite input of 0.3 Mg (ha · a)−1. Soil solution was extracted from the organic layer, 10 cm depth and 20 cm depth of the mesocosms, and analyzed for molybdate reactive phosphorus (MRP) and molybdate unreactive phosphorus (MUP). Additionally, we separated colloids from the soil solution using Asymmetric Field Flow Fractionation for assessing the colloidal fraction of total element concentrations. NI increased MRP and MUP concentrations for all plots with one exception, while FL decreased MRP and MUP with the exception of another plot. While the irrigation treatments had little impact on the P-richest site, MRP and MUP concentrations changed strongly at the poorer sites. The colloidal fraction of P in the soil solution equaled 38–47% of the total P load. Nitrogen input and liming also affected the Fe, Al, Ca, and Corg contents of the colloidal fraction.

Understanding and quantification of phosphorus (P) fluxes are key requirements for predictions of future forest ecosystems changes as well as for transferring lessons learned from natural ecosystems to croplands and plantations. This review summarizes and evaluates the recent knowledge on mechanisms, magnitude, and relevance by which dissolved and colloidal inorganic and organic P forms can be translocated within or exported from forest ecosystems. Attention is paid to hydrological pathways of P losses at the soil profile and landscape scales, and the subsequent influence of P on aquatic ecosystems. New (unpublished) data from the German Priority Program 1685 ‘‘Ecosystem Nutrition: Forest Strategies for limited Phosphorus Resources’’ were added to provide up-to-date flux-based information. Nitrogen (N) additions increase the release of water-transportable P forms. Most P found in percolates and pore waters belongs to the so-called dissolved organic P (DOP) fractions, rich in orthophosphate- monoesters and also containing some orthophosphate-diesters. Total solution P concentrations range from ca. 1 to 400 mg P L–1, with large variations among forest stands. Recent sophisticated analyses revealed that large portions of the DOP in forest stream water can comprise natural nanoparticles and fine colloids which under extreme conditions may account for 40–100% of the P losses. Their translocation within preferential flow passes may be rapid, mediated by storm events. The potential total P loss through leaching into subsoils and with streamswas found to be less than 50mgPm–2 a–1, suggesting effects on ecosystems at centennial to millennium scale. All current data are based on selected snapshots only. Quantitative measurements of P fluxes in temperate forest systems are nearly absent in the literature, probably due to main research focus on the C and N cycles. Therefore, we lack complete ecosystem-based assessments of dissolved and colloidalP fluxeswithin and fromtemperate forest systems. peerReviewed

Climate change is expected to impact the water cycle and severely affect precipitation patterns across central Europe and in other parts of the world, leading to more frequent and severe droughts. Usually when projecting drought impacts on hydrological systems, it is assumed that system properties, like soil properties, remain stable and will not be affected by drought events. To study if this assumption is appropriate, we address the effects of drought on the infiltration behavior of forest soils using dye tracer experiments on six sites in three regions across Germany, which were forced into drought conditions. The sites cover clayey-, loamy- and sandy-textured soils. In each region, we compared a deciduous and a coniferous forest stand to address differences between the main tree species. The results of the dye tracer experiments show clear evidence for changes in infiltration behavior at the sites. The infiltration changed at the clayey plots from regular and homogeneous flow to fast preferential flow. Similar behavior was observed at the loamy plots, where large areas in the upper layers remained dry, displaying signs of strong water repellency. This was confirmed by water drop penetration time (WDPT) tests, which revealed, in all except one plot, moderate to severe water repellency. Water repellency was also accountable for the change of regular infiltration to fingered flow in the sandy soils. The results of this study suggest that the drought history or, more generally, the climatic conditions of a soil in the past are more important than the actual antecedent soil moisture status regarding hydrophobicity and infiltration behavior; furthermore, drought effects on infiltration need to be considered in hydrological models to obtain realistic predictions concerning water quality and quantity in runoff and groundwater recharge.

This study is concerned with the transport of Phosphorus (P) with lateral subsurface flow in the organic layer and topsoil of three forested headwater sites in Germany. Sampling frequency was set proportional to the incident flow rate in high temporal resolution. With this approach we want to investigate intra-event dynamics of P transport in lateral subsurface flow to establish initial process understanding about this potentially relevant pathway of P loss in forested hillslopes. With the organic layer being an important transfer site in the P cycle of temperate forests, availability and transportability of P in short timescales may reveal details about the overall balance of P in theses ecosystems. Our results demonstrate that P concentrations in lateral flow are highly variable within and in between distinct flow events as well as among our study sites. To determine possible controls of the P transport we constructed multiple linear models of the P concentration in lateral flow as a function of site specific environmental datasets. Site affiliation was responsible for more than half of total explained variability regarding P concentration in lateral flow, followed by flow rate, electric conductivity of subsurface lateral flow

In response to a wide-spread decline in forest vitality associated with acid rain in the 1980s, liming of soils has been implemented in many federal states in Germany to buffer further acid deposition and improve availability of nutrients such as calcium and magnesium. As a consequence, it may also increase vitality and depth of fine-root systems and hence improve the drought tolerance of species such as Norway spruce [Picea abies (L.) Karst.], which occurs mostly on acidic forest soils. However, the influence of repeated liming on drought tolerance of trees has never been studied. Here we compared the resistance, recovery and resilience of radial growth in P. abies in relation to drought in limed and control stands and assessed how the dosage and interval between lime application and drought year influences the radial growth response of P. abies. We analyzed radial growth in 198 P. abies trees of six experimental sites in south–west Germany. An analysis of the radial increment over the last 30 years allowed the analysis of drought events shortly after the first liming (short-term effect) as well as posterior drought events (mid- to long-term effects). Generalized linear models were developed to assess the influence of drought intensity, site and period since first liming on the drought tolerance of Norway spruce. Regardless of drought intensity, there was no general increase in drought resistance of Norway spruce in response to liming. However, drought resistance of radial growth improved on a loamy site that was additionally treated with wood ash 30 years after the first lime application. Furthermore, recovery and resilience of radial growth after severe drought events were generally better in spruce trees of limed treatments. This indicates a shorter stress period in spruce trees growing on limed soil, which may reduce their susceptibility to secondary, drought-related pests and pathogens.

Key message Understory plant communities are essential for the recruitment of trees making up future forests. Independent of plant diversity, the understory across different forest ecosystems shows considerable physiological acclimation and structural stability towards drought events, which are expected to occur more frequently in future. Context Understory plant communities are essential for the recruitment of trees making up the future forest. It is so far poorly understood how climate change will affect understory in beech and conifer forests managed at different intensity levels. Aims We hypothesized that drought would affect transpiration and carbon isotope discrimination but not species richness and diversity. Moreover, we assumed that forest management intensity will modify the responses to drought of the understory community. Methods We set up roofs in forests with a gradient of management intensities (unmanaged beech—managed beech—intensively managed conifer forests) in three regions across Germany. A drought event close to the 2003 drought was imposed in two consecutive years. Results After 2 years, the realized precipitation reduction was between 27% and 34%. The averaged water content in the top 20 cm of the soil under the roof was reduced by 2% to 8% compared with the control. In the 1st year, leaf level transpiration was reduced for different functional groups, which scaled to community transpiration modified by additional effects of drought on functional group leaf area. Acclimation effects in most functional groups were observed in the 2nd year. Conclusion Forest understory shows high plasticity at the leaf and community level, and high structural stability to changing climate conditions with drought events.

The leaching of P from the upper 20 cm of forest topsoils influences nutrient (re-)cycling and the redistribution of available phosphate and organic P forms. However, the effective leaching of colloids and associated P forms from forest topsoils was so far sparsely investigated. We demonstrated through irrigation experiments with undisturbed mesocosm soil columns, that significant proportions of P leached from acidic forest topsoils were associated with natural colloids. These colloids had a maximum size of 400 nm. By means of Field-flow fractionation the leached soil colloids could be separated into three size fractions. The size and composition was comparable to colloids present in acidic forest streams known from literature. The composition of leached colloids of the three size classes was dominated by organic carbon. Furthermore, these colloids contained large concentrations of P which amounted between 12 and 91% of the totally leached P depending on the type of the forest soil. The fraction of other elements leached with colloids ranged between 1% and 25% (Fe: 1–25%; Corg: 3–17%; Al

The combination of technical treatments and planting of alder trees in a compacted forest soil improves the circulation of air and water through the pore system. This leads to decreases in CO 2 concentrations and increases in root growth in the soil. Both are indicative of an initial recovery of soil structure. The compaction of forest soils, caused by forest machinery, has as a principal consequence: the destruction of soil structure and thus the reduction of the soil aeration status. Thus, the gas exchange between soil and atmosphere is reduced and the depth propagation of roots is limited, resulting in the shortage of water and nutrient supplies for trees. This research aimed at detecting the first stages of recovery of soil structure in a compacted forest soil, which was treated with a combination of techniques (i.e., planting tree species, mulching, addition of lime), which could presumably accelerate the regeneration of soil structure. The variation of CO2 concentrations and the dynamics of root growth were repeatedly measured. Linear mixed models were developed in order to test the effects of the treatments and the planting of trees on soil aeration, as well as to identify the influence of the different environmental effects on CO2 concentration in soil. The planting of root-active trees showed significant effects on decreases in CO2 concentrations. However, during the short-term observation, some negative effects occurred especially for the mulched sites. Nevertheless, all applied technical treatments promoted an improved soil aeration and a higher root growth compared to untreated sites which points to an initial enhanced recovery of soil structure. Pronounced seasonal and interannual variations of soil respiration were highly influenced by soil temperature and soil water content variations. An initial regeneration of soil structure is indicated by distinct changes of the soil aeration status. This regeneration is partially enhanced by the applied treatments. The quantitative potential of the regeneration techniques needs a longer observation period for mid- and long-term soil recoveries.

Knowledge of soil moisture dynamics in the unsaturated soil zone provides valuable information on the temporal and spatial variability of groundwater recharge. This is especially true for the Mediterranean region, where a substantial fraction of long-term groundwater recharge is expected to occur during high magnitude precipitation events of above-average wet winters. To elucidate process understanding of infiltration processes during these extreme events, a monitoring network of precipitation gauges, meteorological stations, and soil moisture plots was installed in an area with a steep climatic gradient in the Jordan Valley region. In three soil moisture plots, Hydrus-1D was used to simulate water movement in the unsaturated soil zone with soil hydraulic parameters estimated by the Shuffled Complex Evolution Metropolis algorithm. To generalize our results, we modified soil depth and rainfall input to simulate the effect of the pronounced climatic gradient and soil depth variability on percolation fluxes and applied the calibrated model to a time series with 62 years of meteorological data. Soil moisture measurements showed a pronounced seasonality and suggested rapid infiltration during heavy rainstorms. Hydrus-1D successfully simulated short and long-term soil moisture patterns, with the majority of simulated deep percolation occurring during a few intensive rainfall events. Temperature drops in a nearby groundwater well were observed synchronously with simulated percolation pulses, indicating rapid groundwater recharge mechanisms. The 62-year model run yielded annual percolation fluxes of up to 66% of precipitation depths during wet years and of 0% during dry years. Furthermore, a dependence of recharge on the temporal rainfall distribution could be shown. Strong correlations between depth of recharge and soil depth were also observed.

Precipitation patterns across Central Europe are expected to change over the 21st century due to climate change. This may reduce water availability during the plant-growing season and hence affect the performance and vitality of forest ecosystems. We established a novel rainfall reduction experiment on nine sites in Germany to investigate drought effects on soil–forest–understory ecosystems. A realistic, but extreme annual drought with a return period of 40 years, which corresponds to the 2.5% percentile of the annual precipitation, was imposed. At all sites, we were able to reach the target values of rainfall reduction, while other important ecosystem variables like air temperature, humidity, and soil temperature remained unaffected due to the novel design of a flexible roof. The first year of drought showed considerable changes in the soil moisture dynamics relative to the control sites, which affected leaf stomatal conductance of understory species as well as evapotranspiration rates of the forest understory., Biogeosciences, 12 (4), ISSN:1726-4170

Future climate projections for Central Europe indicate a decrease in summer precipitation which might range between 15% and 50%, and equally important, changes in the climate variability, resulting in consecutive years with drought periods. With respect to Central European forests, we asked to which degree realistic drought conditions are tolerated by the recruits of the dominant tree species Fagus sylvatica L., and how the effects depend on biotic interactions. To test the combined effects of drought, competition and provenance of F. sylvatica recruits we set up a rain shelter experiment at three sites in different regions of Germany. Transposable roof panels allowed a flexible precipitation reduction between 10% and 70% corresponding with a return period of 40 years. We planted F. sylvatica saplings of three provenances, exposed them to drought and competition. We tested if understorey herbaceous competitors have a negative impact on F. sylvatica saplings, and thus, exacerbate drought effects and that F. sylvatica provenances from drier regions are adapted to drought conditions and cope better with drought conditions. Six months after the drought treatment started, we encountered significant drought effects, seen in a reduced leaf stomatal conductance, although there was not yet a response in growth rates. Overall, the site had the greatest impact on phytometer performance, while we found no indication of adaptation to drought of the different provenances. Furthermore, drought effects increased in interaction with site effects, being highest at the driest site. At the driest site, leaf stomatal conductance decreased in the presence of competition but increased in the control subplots, while the site of intermediate moisture conditions showed the opposite pattern and the wettest site displayed no differences. Our results highlight the fact that biotic interactions can mitigate or exacerbate drought effects, depending on regional site conditions.

The hydraulic properties of soils, i.e., their ability to store and conduct water, mainly govern the availability of soil water for plants. Information on the hydraulic properties is needed, e.g., for the quantification of drought risk at a given site. Furthermore, knowledge of the water transport is the precondition for the estimation of element fluxes in the soil, e.g., when predicting element leaching from the root zone to the groundwater. For forest soils, only few systematic investigations of their hydraulic properties exist. Within the 2nd forest-soil survey of Germany, soil samples were taken along a regular 8 km × 8 km grid in the forests of the State of Baden-Wurttemberg and the hydraulic properties were estimated in the laboratory by multistep outflow experiments. Besides the soil-hydraulic measurements, numerous additional soil chemical and physical analyses were carried out and comprehensive profile descriptions were compiled and integrated in a hydraulic database. Based on this database, multiple-linear-regression techniques were used to develop pedotransfer functions for the water-retention curve and the unsaturated-hydraulic-conductivity curve using the parametric models of Mualem/van-Genuchten. Our work fills a gap since to our knowledge, no pedotransfer functions for the unsaturated hydraulic conductivity for forest soils exist so far. The predictive accuracy of the established pedotransfer functions, both for the water-retention curve and the hydraulic-conductivity curve, is in the range of (and in some cases better than) other published pedotransfer functions that were mostly derived for agricultural soils.

Forest soils are under-represented throughout the existing soil hydraulic databases. Classical measurement methods for the hydraulic soil characteristics are very time-consuming and are not suited for routine application in the course of soil survey. We are presently engaged in establishing a database of the hydraulic properties of forest soils using multistep outflow experiments together with an inverse optimization of the Mualem/van Genuchten parameters from the tension and outflow measurements. The lower boundary in the multistep outflow experiments was modified such that, firstly, the experiment can be conducted as fast as possible and hence can be incorporated in the framework of routine soil monitoring. Secondly, the experimental design should be identical for a large texture range and therefore allow for a statistical comparison of the hydraulic properties of a broad variety of soils. Thirdly, the experiments must be designed such that they yield sufficient measurements for a unique and stable inverse parameter estimation. The inverse parameter estimates of the presently available 141 multistep outflow experiments were compared with predictions of two pedotransfer functions. For most soil samples, the PTF predictions differed highly from the MSO estimates. Reasons for this are, among others, insufficient hydraulic measurements included in the datasets used for establishing the PTFs and, specifically, insufficient information on water retention/conductivity in the range of the air entry value. Here, the MSO experiments can provide much more detailed information on the water retention and conductivity curves because they yield measurements over a continuous pressure range and are conducted such that the pressure range of the air entry value is covered by sufficient tension and outflow measurements. The presented research aims to: (i) introduce the measurement of soil hydraulic properties in an ongoing soil monitoring campaign, (ii) complement existing soil hydraulic databases with data from forest soils, and (iii) enhance the reliability of soil hydraulic databases through a standardization of the measuring approaches.

This study evaluates the acidification status and trends in streams of forested mountain ranges in Germany in consequence of reduced anthropogenic deposition since the mid 1980s. The analysis is based on water quality data for 86 long-term monitored streams in the Ore Mountains, the Bavarian Forest, the Fichtelgebirge, the Harz Mountains, the Spessart, the Black Forest, the Thuringian Forest, and the Rheinisches Schiefergebirge of Germany and the Vosges of France. Within the observation period, which starts for the individual streams between 1980 and 2001 and ends between 1990 and 2009, trends in chemical water quality were calculated with the Seasonal Mann Kendall Test. About 87% of the streams show significant (p < 0.05) negative trends in sulfate. The general reduction in acid deposition resulted in increased pH values (significant for 66% of the streams) and subsequently decreased base cation concentrations in the stream water (for calcium significant in 58% and magnesium 49% of the streams). Reaction products of acidification such as aluminum (significant for 50%) or manganese (significant for 69%) also decreased. Nitrate (52% with significant decrease) and chloride (38% with significant increase) have less pronounced trends and more variable spatial patterns. For the quotient of acidification, which is the ratio of the sum of base cations and the sum of acid anions, no clear trend is observed: in 44% of the monitored streams values significantly decreased and in 23% values significantly increased. A notable observation is the increasing DOC concentration, which is significant for 55% of the observed streams.

Soil microbial communities play an important role in forest ecosystem functioning, but how climate change will affect the community composition and consequently bacterial functions is poorly understood. We assessed the effects of reduced precipitation with the aim of simulating realistic future drought conditions for one growing season on the bacterial community and its relation to soil properties and forest management. We manipulated precipitation in beech and conifer forest plots managed at different levels of intensity in three different regions across Germany. The precipitation reduction decreased soil water content across the growing season by between 2 to 8% depending on plot and region. T-RFLP analysis and pyrosequencing of the 16S rRNA gene were used to study the total soil bacterial community and its active members after six months of precipitation reduction. The effect of reduced precipitation on the total bacterial community structure was negligible while significant effects could be observed for the active bacteria. However, the effect was secondary to the stronger influence of specific soil characteristics across the three regions and management selection of overstorey tree species and their respective understorey vegetation. The impact of reduced precipitation differed between the studied plots; however, we could not determine the particular parameters being able to modify the response of the active bacterial community among plots. We conclude that the moderate drought induced by the precipitation manipulation treatment started to affect the active but not the total bacterial community, which points to an adequate resistance of the soil microbial system over one growing season.

Introduction Conceptual hydrological models are useful tools to support catchment water management. However, the identifiability of parameters and structural uncertainties in conceptual rainfall-runoff modeling prove to be a difficult task. Here, we aim to evaluate the performance of a conceptual semi-distributed rainfall-runoff model, HBV-light, with emphasis on parameter identifiability, uncertainty, and model structural validity. Results The results of a regional sensitivity analysis (RSA) show that most of the model parameters are highly sensitive when runoff signatures or combinations of different objective functions are used. Results based on the generalized likelihood uncertainty estimation (GLUE) method further show that most of the model parameters are well constrained, showing higher parameter identifiability and lower model uncertainty when runoff signatures or combined objective functions are used. Finally, the dynamic identifiability analysis (DYNIA) shows different types of parameter behavior and reveals that model parameters have a higher identifiability in periods where they play a crucial role in representing the predicted runoff. Conclusions The HBV-light model is generally able to simulate the runoff in the Pailugou catchment with an acceptable accuracy. Model parameter sensitivity is largely dependent upon the objective function used for the model evaluation in the sensitivity analysis. More frequent runoff observations would substantially increase the knowledge on the rainfall-runoff transformation in the catchment and, specifically, improve the distinction of fast surface-near runoff and interflow components in their contribution to the total catchment runoff. Our results highlight the importance of identifying the periods when intensive monitoring is critical for deriving parameter values of reduced uncertainty.