Droughts affect the vitality of trees and their central role in regulating ecosystems and the climate. Moreover, climate change is expected to increase the frequency and intensity of dry periods endangering the stability and functionality of forests. It is therefore essential to adequately represent drought impacts on forests in catchment scale eco-hydrologic models.

In this study, we aim at assessing the effects of water stress on tree vitality and evapotranspiration with the eco-hydrologic model SWAT+. To this end, the catchment of the Ellerbach (182 km² area) at gauge Schleifmühle and its tributary the Gräfenbach at gauge Argenschwang (32 km²) in the low mountain ranges of the Soonwald in southwest Germany were modelled. A spatially distributed parameterisation was applied to represent the spatial heterogeneity of the catchment. Model calibration was based on Latin Hypercube Sampling to derive 1000 parameter sets for eleven model parameters. From these model runs the best model run in terms of the smallest absolute percentage bias at both gauges was chosen. The model showed a good performance at the daily time scale at the catchment outlet and a lower but still acceptable performance in the upstream indicated by Kling-Gupta efficiencies of 0.81 (Ellerbach) and 0.66 (Gräfenbach) in the calibration period (2011 to 2016) and 0.83 (Ellerbach) and 0.64 (Gräfenbach) in the validation period (2017 to 2021).

Plant water stress was identified on a daily basis when the actual plant transpiration deviated from the potential plant transpiration. The highest water stress was found for forests in the low mountain range areas in the years 2011 and 2022, with durations ranging from 62 to 119 days in 2011 and from 43 to 130 days in 2022 for different tree species. These values are up to 2.1 times (2011) and 1.5 times (2022) higher than the long-term average. Generally, coniferous trees are more affected by water stress (long-term average: 91 days) than deciduous trees (long-term average: 31 days). However, in the two analyzed years deciduous trees experienced 2.1 (+34 days, 2011) and 1.5 (+15 days, 2022) times more water stress as compared to their long-term average, whereas coniferous trees experienced 1.3 (+25 days, 2011) and 1.4 (+34 days, 2022) times more water stress. 

Water stress affected tree vitality in the respective years indicated by the development of the leaf area index. Moreover, drought conditions led to a reduction in evapotranspiration by 14% (2011) and 12% (2022) when compared to mean annual evapotranspiration. Spatio-temporal differences in evapotranspiration patterns can be explained by an interplay between tree species, soil properties and precipitation patterns. As tree species perform different strategies for coping with water stress, future research shall focus on evaluating drought-induced transpiration decreases for the main tree species based on independent species-specific transpiration data.

How to cite: Wagner, P., Wendell, A.-K., Müller, E. V., and Fohrer, N.: Modelling the impact of drought-induced water stress on tree vitality and evapotranspiration , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7057, https://doi.org/10.5194/egusphere-egu25-7057, 2025.

Northern forests exhibit strong sensitivity to recent climate change, which brings risks of carbon reemission from mature forests and threatening biodiversity. Understanding the role of these forests in regional carbon and hydrological cycle is fundamental for implementing effective forest management strategies and projecting future terrestrial dynamics. Currently, most studies investigating vegetation growth or responses to climate change rely on the fraction of photosynthetically active radiation (fPAR) as an indicator of plant productivity. However, while fPAR primarily reflects carbon assimilation within the current growing season, it provides limited insights into long-term carbon storage, such as above-ground biomass (AGB). This discrepancy arises from the complex vertical structure of forests, leading to an incomplete understanding of how forest AGB responds to local climate conditions. In this study we utilize a wall-to-wall AGB dataset derived from microwave remote sensing observations to investigate the asymmetric responses of AGB and fPAR to climatic factors and explore the underlying mechanisms driving these differences. The key contributions of this study are twofold: (1) demonstrating that AGB and fPAR exhibit distinct and asymmetric responses to local climate conditions, and (2) elucidating the factors contributing to the mismatch between AGB and fPAR. Using annual mean precipitation, temperature, and radiation data from 2015 to 2020, we analyzed the climatic responses of AGB and fPAR. Results reveal an asymmetric relationship with precipitation: AGB is negatively correlated with local precipitation, while fPAR exhibits a positive correlation. Temperature and radiation, however, show no significant constraints on either AGB or fPAR. To further investigate this asymmetry, we introduced the difference between normalized AGB and fPAR (AGB-fPAR) as an indicator and divided the study area into 12 sub-regions of 1° × 1° where precipitation and downwelling radiation energy were treated as invariant. Our findings demonstrate that topographic factors account for approximately 60% of the variation in AGB-fPAR, driven by the redistribution of energy caused by local terrain. Additionally, surface runoff reallocates water availability beyond local precipitation, with proximity to open surface water showing a significant positive relationship with higher AGB-fPAR. Forest structural complexity, such as mixed species composition and older tree age, further amplifies the AGB-fPAR gap due to their influence on vertical structure. This study highlights the importance of considering the divergence between fPAR and AGB in assessing forest responses to climate change. Local topographic effects, hydrological dynamics, and forest structural traits jointly drive the disparity between these indicators, which evidences the need for integrated approaches to understand and predict forest-climate interactions.

How to cite: Tan, S., Liu, Q., Huang, H., and Gao, G.: Asymmetric Responses of AGB and fPAR in Northern Forest to Climate Condition, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9598, https://doi.org/10.5194/egusphere-egu25-9598, 2025.

Precipitation is partitioned and redistributed by vegetation when it passes through a forest canopy resulting in interception, stemflow and throughfall. Throughfall is known to be spatially highly heterogeneous beeing lower than precipitation in certain areas and higher in others. As these emerging spatial throughfall patterns infiltrate, they may propagate into soil moisture patterns and influence root water uptake, percolation, runoff generation and ultimately the entire forest water balance. Despite the relevancy of throughfall in water resources research, there is a scarcity on experimentally-derived high-quality datasets on its spatio-temporal dynamics. Sampling procedures changed little over the past decades and are often not optimal for systems under study. Large sampling efforts especially for complex vegetation structures limit most studies to investigate on either high temporal or spatial resolution of throughfall.

We present an innovative throughfall sampling approach for continuous measurement of throughfall at high spatial resolution. The sampling scheme allows to quantify the spatio-temporal throughfall variability at both intra-event and intra-stand levels and assess spatial throughfall patterns and their temporal persistence across precipitation events of varying size during leafed and non-leafed periods. 60 self-built throughfall sampler boxes featuring a cost-efficient design with four troughfall collectors and tipping bucket units each, were distributed in a stratified sampling design in forest plots of pure and mixed Beech, Douglas and Silver fir (total area 0,4 ha). The tipping buckets are controlled with newly developed micro boards connected to data loggers so that the network measures continuously, automatically and requires minimal maintenance during precipitation events. The sampler boxes operate with an inlay of litter material on top of a grid and mesh, which allows to include forest floor interception as part of the overall throughfall process and reduces throughfall splash. The placement of the boxes at the forest floor boxes is minimally invasive as quantified throughfall can percolate into the soil below allowing to monitor soil moisture patterns in the same plot. The network of sampler boxes is supplemented with classical throughfall samplers, stemflow- and lysimeter measurements at every plot. Throughfall data were analysed from the network at a Beech, Douglas fir, Silver fir and mixed plot for the first three months of measurement. Using spatial correlation and temporal stability analysis of the observed data, we show spatio-temporal throughfall dynamics of different tree species, during and among precipitation events of varying size and for a stand that develops from dormancy to vegetation period.

How to cite: Dedden, L. and Weiler, M.: An innovative monitoring approach to measure spatio-temporal throughfall patterns in forests, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10636, https://doi.org/10.5194/egusphere-egu25-10636, 2025.

Water fluxes in forests inherit complex dynamics remains poorly understood. Precipitation in forests is intercepted by the canopy and spatially redistributed, resulting in distinct patterns of throughfall and stemflow. Canopy throughfall creates spatially heterogeneous water flux patterns beneath the forest canopy, which leads to 'hot spots' of water and nutrient input to the ground and affects soil infiltration, groundwater recharge and runoff. Despite its importance, the influence of forest structure on vegetation driven water partitioning and on the emerging spatio-temporal patterns remains poorly understood. The quantification of forest morphology in high spatial and temporal resolution is a challenge. as direct approaches are labour-intensive and often require destructive sampling (e.g. count total leaf number). Light Detection and Ranging (LiDAR) sensing from Uncrewed Aerial Vehicles (UAVs) has emerged as an effective technique to measure the three-dimensional forest structures.

Previous studies considering LiDAR derived structural metrics investigated throughfall on forest stand level with airborne laser scanning (ALS)(Schumacher & Christiansen (2015)), developed models on throughfall kinetic energy (Senn et al. (2020)) and identified  water drip points in branch architecture with TLS (Wischmeyer et al. (2024)). However, these studies are limited in their spatio-temporal resolution. Recent advances of UAV based LiDAR sensor technologies (ULS) enabled the representation of forest structures both with adequate temporal and spatial detail. Such data may be the key to track and understand precipitation dynamics in forests.

Here, we present an innovative approach that combines ULS-derived forest structure metrics and in-situ-derived throughfall measurements to explore the relationship between changes in forest structure and spatio-temporal throughfall dynamics. The ULS datasets was collected starting from April 2024 with 1-2 flights per month over a forest plot in the black forest, Germany. The dataset captures forest morphology variation within the year including tree growth and changes in structure and foliage density. Continuous throughfall measurements were collected in the center (0,4 ha area) of the forest plot. 100 tipping bucket units of an automated throughfall sampling network were mounted along transects on the ground of a mixed and pure Beech and Douglas fir stand monitoring throughfall from precipitation events of different sizes, starting November 2024. Classic trough throughfall measurements starting summer 2024 complement the dataset, which covers throughfall during dormant- and vegetation period. From the LiDAR data, we derive different metrics describing forest morphology, from voxel based point densities to experimental occlusion-related permeability metrics.

In a combined correlation analysis of density- and permeability metrics with corresponding daily spatial throughfall, the influence of phenological changes on throughfall patterns at a high spatial and temporal resolution is investigated. With this study, we aim to identify the potential of LiDAR-derived metrics of forest structure from multi-temporal datasets for forest hydrology research and to develop approaches on how to integrate metrics that are suitable descriptors for complex forest canopy structures into investigation of water fluxes in forest.

How to cite: Gassilloud, M., Dedden, L., Koch, B., Kattenborn, T., Weiler, M., and Göritz, A.: Elucidating spatio-temporal throughfall dynamics with ULS derived forest structure density metrics, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19485, https://doi.org/10.5194/egusphere-egu25-19485, 2025.

Climatic change, including an increasing frequency of large-scale disturbances, is progressively threatening the resilience and productivity of forests. Against this backdrop, the LabForest project, funded by the German Federal Ministry of Education and Research (BMBF), investigates the effectiveness and efficiency of silvicultural measures following such calamities. LabForest includes a living lab in a forest of the Ludwig-Maximilians-University (450 ha), facilitating the comparison of forestry and timber management effects (e.g., advance regeneration, clearance, and planting versus natural regeneration) on disturbed areas. The silvicultural measures are examined and evaluated for their impacts on important ecosystem services (e.g., hydrology, carbon sequestration, and timber production) and biodiversity. Ultimately, the project aims to establish a multidimensional assessment matrix to support decision-making in forestry, balancing climate change mitigation, economic interests, biodiversity, and hydrology.

The Hydrometeorology work package develops high-resolution hydrometeorological models of varying complexity to accurately represent and compare water and matter fluxes relative to the differently managed forest plots. For this purpose, a newly setup measurement network captures components of the water balance (evaporation, soil moisture, soil temperature, fraction of photosynthetically active radiation), water quality, and micrometeorological variables. The measured quantities serve as the basis for the parameterization and validation of the models.

The modeling results and analysis of the hydrological conditions of the different forest management strategies will be incorporated into the assessment matrix for silvicultural practices. This will enable regional recommendations and an improved understanding for establishing economically and ecologically resilient forests to cope with anticipated climatic changes.

How to cite: Svanidze, D., Ludwig, R., Obermeier, W., and Lehnert, L.: Hydrometeorological Aspects of Sustainable Forest Regeneration Management in Climate Change, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12627, https://doi.org/10.5194/egusphere-egu25-12627, 2025.

This study summarized the results of long-term monitoring of various functions obtained from forest restoration, including increased plant diversity and water resource recovery. The study area is a long-term monitoring area for degraded forests, and the characteristics of valley runoff and vascular flora have been continuously observed since 1970, and the annual changes in valley runoff and plant species diversity were compared. As a result of the study, the average runoff was confirmed to be approximately 90 days in the 1980s, approximately 310 days in the 1990s, and 365 days in the 2000s. Compared to the number of days of runoff in the initial degraded area, it increased 3.7 times in 1990 and 4.4 times after 2000. In particular, it was confirmed that the valley flow was maintained throughout the year regardless of rainfall characteristics after 2000. In the case of vascular plants, there were 30 species including Miscanthus sinensis, Lespedeza bicolor, and Arundinella hirta in the 1980s, 62 species including Quercus serrata, Callicarpa japonica, and Rhus chinensis in the 1990s, and 80 species including Quercus mongolica, Viburnum dilatatum, and Fraxinus sieboldiana in the 2000s. In a recent survey, there were 116 taxa including Maackia amurensis, Alnus japonica, and Sorbus alnifolia, and the number has continuously increased. In terms of vegetation structure, in the 1980s, simple communities of shrubs and herbs with a height of about 5 m were the main focus, but in the 1990s, they developed into 10 m tall sub-tree vegetation, and after the 2000s, broad-leaved forests and coniferous-broadleaf mixed forests with a height of about 15 m were formed. It is expected that this study can be used as basic data for estimating ecosystem services and evaluating public interest functions according to the restoration of desolate and damaged forests.

How to cite: Choi, B., Li, Q., Lim, H., and Nam, S.: Monitoring study on plant diversity and water resource recovery characteristics due to restoration of desolate mountain areas, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13549, https://doi.org/10.5194/egusphere-egu25-13549, 2025.

Studies on climate change effects reveal that the natural water cycle will continue to shift, often intensifying pressure on water resources. Understanding the interaction between forest ecosystems, soil water dynamics, and groundwater recharge under these changes is essential. This study investigates the forested areas of the Hessian Ried in Hesse, Germany—a low-lying, fertile region crucial for agriculture, forestry, nature conservation, and groundwater supply to the Frankfurt/Rhine-Main metropolitan region. Ensuring sustainable water management for this vital resource zone is necessary to meet future demands.

The LWFBrook90 model, a 1D Soil-Vegetation-Atmosphere-Transfer (SVAT) tool, was utilized to examine future soil water variability and its impact on groundwater resources in the study area. The model simulates the soil water balance using Richards’ equations and, with its latest version (LWFBrook90R 0.6.0), incorporates capillary rise. Historical daily climate data (1960 to present) and climate projections based on the RCP8.5 scenario (extending to 2100) were used. The forested area was discretized into 500 m x 500 m cells, each with representative vegetation, soil, and climate data. Vegetation data was obtained from forest inventories and used in combination with forest yield tables to derive leaf and stem area index using allometric functions. Calibrated model parameters for the main tree species—oak, beech, and pine—were obtained from Weis et al. (2013). A detailed discretization of soil profiles was performed and the corresponding soil physical properties were assigned layer-wise using pedotransfer functions (Wessolek et al., 2009).

The cell-based framework effectively captured spatial variability in tree species and soil properties, ensuring computational efficiency despite excluding lateral flow. Future projections (especially in 2081–2100) indicate a significant decline in soil water availability during summer months (July–September), increasing water stress and potentially impairing plant growth. In winter, soil moisture recovery may still occur but is less pronounced. Monthly transpiration ratios, averaged across periods 2021–2050, 2051–2080, and 2081–2100, revealed severe stress across all projections. Even scenarios with wetter conditions suggest that increased rainfall and infiltration may not sufficiently mitigate tree stress in vulnerable areas.

The model successfully simulated vertical water movement in the profile and groundwater recharge dynamics. Groundwater recharge during winter is projected to maintain current rates under moderate scenarios or slightly increase under wetter ones. However, under the driest scenario, the average rate was less than 50 mm per year for the period 2081–2100. These findings highlight significant stress on forest ecosystems under changing climate conditions, emphasizing the need for adaptive water management strategies. Ongoing research seeks to refine this prototype by examining vertical fluxes at sites with shallow groundwater depths and further investigate recharge rates under various forest conversion scenarios. These advancements will contribute to a more comprehensive understanding of forest ecosystem responses and guide water management efforts in the Hessian Ried and similar regions.

How to cite: Morales, A. G., Köhler, M., Sutmöller, J., Ahrends, B., and Meesenburg, H.: Modelling Climate Change Effects on Soil Water Dynamics and Groundwater Recharge in Temperate Forests in Germany, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14995, https://doi.org/10.5194/egusphere-egu25-14995, 2025.

Vegetation interacts with both soil moisture and atmospheric conditions, contributing to water flow partitioning at the land surface. Therefore, both climate and land cover changes impact water resource availability. This study aimed to determine the differential effects of climate change on the soil water regime of two common Central European forest types: Norway spruce (Picea abies L.) and European beech (Fagus sylvatica L.) stands.

A unique dataset, including 22 years (2000–2021) of measured soil water potentials, was used with a bucket-type soil water balance model to investigate differences in evapotranspiration and groundwater recharge both between the forest types and across years. While long-term column-averaged pressure head indicated drier soil at the spruce site overall, this was driven by the wettest years in the dataset. Seasonal and interannual variability of meteorological conditions drove complex but robust differences in flow partitioning between the forest types. Higher snow interception by spruce (27 mm season^-1) resulted in drier soil below the spruce canopy in the cold season. Higher transpiration by beech (70 mm season^-1) led to increasingly drier soils over the warm seasons. Low summer precipitation inputs exacerbated soil drying under beech as compared to spruce. Estimated summer recharge was lower under beech (25 mm season^-1) due to its lower transpiration. The difference was more pronounced (over 40 mm season^-1) during wetter summers.

These suggest that expected trends in regional climate and forest species composition may interact to produce a disproportionate shift of recharge from the summer to the winter season.

How to cite: Sipek, V., Zelikova, N., Touskova, J., Kocum, J., Vlcek, L., Tesar, M., Pátek, K., and Bouda, M.: Future changes in water availability: Insights from a long-term monitoring of soil moisture under two tree species, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18908, https://doi.org/10.5194/egusphere-egu25-18908, 2025.