As a consequence of global change, forests worldwide are undergoing a restructuring process. The expectations for forests of the future are ambitious: Providing resilient forest ecosystems that capture large amounts of carbon but also provide stable groundwater recharge rates. To balance the interests of both forestry and water management authorities, scientists and practitioners need to be able to investigate and predict which forest types and combinations of tree species are most likely to fulfill these needs.

Methods based on the analysis water stable isotopes have been used extensively for studying water uptake depths of vegetation, groundwater recharge, transit times, and water sources in general. Despite being arguably the superior tool when not only amounts but also knowledge of the sources of an (eco-)hydrological flux are needed, the highly dynamic nature of water transport processes within the soil-plant-atmosphere continuum (SPAC) could hardly be captured in the past. With the advent of  laser spectroscopy in the last decade, we are now able to measure water stable isotopes continuously and in all compartments of the SPAC.

In this keynote, we present and review the most recent advances (2016-now) of combined soil and plant in-situ water isotope measurements carried out in different ecosystems worldwide. We then critically discuss the gain in process-understanding of in-situ monitoring approaches and demonstrate how in-situ methods could be integrated with traditional and novel methods to advance forest hydrology.

In-situ and semi-in-situ (i.e., sampling of water vapor) water stable isotope methods have been greatly improved within the last five years. Initial disadvantages (e.g., comparability to traditional methods, complicated & laborious setup & maintenance, expensive) have been carefully addressed, and improvements have been implemented. Recent research has proven that i.) highly dynamic and heterogenous processes (e.g., stem flow, groundwater recharge through preferential pathways, change of uptake depths in response to rainfall/drought, disentangling water use of different tree species in mixed forests) can be captured exceptionally well using in-situ isotope methods; ii.) water-vapor equilibration methods represent the isotope composition of mobile water better compared to destructive methods, and iii.) using continuous water isotope data reduces parameter uncertainties in SPAC modeling.

In summary, we state that the benefits of using in-situ or semi in-situ techniques outweigh the disadvantages by far and strongly encourage the water stable isotope community to integrate them regularly into studies of dynamic soil-plant-atmosphere feedbacks.

How to cite: Beyer, M., Kuehnhammer, K., van Haren, J., Kuebert, A., Birkel, C., Sanchez-Murillo, R., and Marshall, J.: Dynamic processes require appropriate methods to capture them: Why in-situ water stable isotope monitoring needs to become a standard method in forest hydrological research, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10827, https://doi.org/10.5194/egusphere-egu23-10827, 2023.

Throughout the Caribbean, hillside quarrying has become a common practice. While these activities remove large sections of the critical soil zone, very little work has been done on how hillside quarrying impacts rainfall runoff response and catchment water storage. We hypothesised that the removal of the critical soil zone during hillside quarrying will increase the timing and magnitude of streamflow response to storm events due to its close proximity to the river, while also reducing the overall storage of the watershed. The aim of this study is to understand the landuse impacts on rainfall runoff response and catchment storage. A paired catchment study between the 3.6 km² Acono (forested) and the adjacent 3.6 km² Don Juan (quarried) watersheds in Trinidad and Tobago was conducted using a hydrometric and geochemical approach. Direct measurements of rainfall and streamflow and bi-weekly water sample collections for geochemistry and stable isotopes of ¹⁸O and ²H from rainfall, baseflow, soils, springs and groundwater were done. Fraction of young water (Fyw) an inverse transit time proxy was computed along with the mean transit time distributions (MTTDs) by sine wave fitting were used as important descriptors of runoff generation and the catchment storage. The quarried watershed had higher streamflow levels during the wet season than the forested watershed. However, during the dry season there is a reversal.. The quarried watershed responded faster to rainfall events with a lag time between 1–3 hours with a higher peak rate of streamflow versus a lag time of 2-4 hours in the forested watershed with a lower peak rate of flow. In the upper quarried watershed 18.4 % of the stream water were younger than 0.46 years and 20.3% were younger than 0.55 years in the lower portion of the catchment. In the upper forested catchment 5.2 % of the stream water was younger than 2.71 years whereas 4.7% of the stream water was younger than 3.04 years in the lower catchment. The data suggest that the quarry leads to the faster delivery of water during storm events while also reducing the overall storage in the catchment. With an anticipated increase in hillside quarrying, this study provides important information for land use and water resource managers.

How to cite: Farrick, K. and Mathura, N.: Hillside Quarry Impacts on Streamflow and Stormflow response in a Tropical Watershed:  A Geochemical and Hydrometric Investigation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9930, https://doi.org/10.5194/egusphere-egu23-9930, 2023.

Rainfall is a key determinant of tropical rainforest resilience in South America and Africa, of which a substantial amount originates from terrestrial and forest evaporation through moisture recycling. Both continents face deforestation that reduces evaporation and thus dampens the water cycle, and climate change that increases the risk of water-stress induced forest loss. Hence, it is important to understand the influence of forest moisture supply for forest rainfall during dry periods. Here, we analyze mean-years and dry-years dry-season anomalies of moisture recycling in the South American (Amazon) and African rainforests (Congo) over the years 1980-2013. Annual average reliance of forest rainfall on their own moisture supply (ρ_for) is 26 % in the Amazon and 28% in the Congo forest.  In dry seasons, this ratio increases by 7% (or ~2 percentage points) in the Amazon and up to 30 % (or ~8 percentage points) in Congo. Dry years further amplify dry season ρ_for in both regions by 4-5 %. In both Amazon and Congo, dry season amplification of ρ_for are strongest in regions with a high mean annual ρ_for. In the Amazon, forest rainfall self-reliance has declined over time, and in both Amazon and Congo, the fraction of forest evaporation that recycles as forest rainfall has declined over time. At country scale, dry season ρ_for can differ drastically from mean annual ρ_for (e.g., in Bolivia and Gabon, mean annual ρ_for is ~30% while dry season ρ_for is ~50 %). Dry period amplification of ρ_for illuminates additional risks of deforestation as well as opportunities from forest conservation and restoration, and is essential to consider for understanding upwind forest change impacts on downwind rainfall at both regional and national scales.

How to cite: Wang-Erlandsson, L., van der Ent, R., Staal, A., Keys, P., Zemp, D. C., Fetzer, I., Taniguchi, M., and Gordon, L.: Dry seasons and dry years amplify the Amazon and Congo forests’ rainfall self-reliance, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14167, https://doi.org/10.5194/egusphere-egu23-14167, 2023.

Forest cover influence the isotopic composition of precipitation before it eventually reaches the ground, especially through rainfall interception processes. Many plot scale recent studies focusing on throughfall and stemflow fluxes have demonstrated the role of forest canopy cover in (mostly) enriching their isotopic signature. However, the common approach in small catchments (even forested ones) remains to sample rainfall only at one single location (generally in an open area), assuming that the spatial variability of the isotopic composition of precipitation is small. Only a handful of studies have focused on the spatial variability of the isotopic composition of precipitation, and very few have included the role of forest cover. Nonetheless, a correct characterization of the isotopic composition of the incoming precipitation is essential in isotope-based catchment hydrology, for example to proceed hydrograph separation, as well as for process understanding or models development.

The aim of this study is to investigate the spatio-temporal variability of the isotopic composition of precipitation in a small Mediterranean catchment (0.6 km²) where forest cover roughly 2/3 of the catchment. Precipitation was sampled at the event scale in 31 locations across the catchment with bulk collectors consisting of plastic funnels (130mm diameter) connected to a 0.5-L plastic bin positioned 100cm above ground before each rain event and collected the day after. The sampling locations were distributed ±80m along 5 elevation lines every 50m (from 1150 to 1350m), 15 in open areas and 16 under forest (i.e., collecting throughfall). The percentage of canopy cover above each sampling location was determined using hemispherical photographs. For all events, rainfall was also measured every 5min at 3 locations with tipping bucket rain gauges and meteorological variables at 2 locations (at the ground level and above the forest canopy). Sampled events were analysed both altogether and separating open areas and under forest locations to determine the factors affecting the spatio-temporal variability of the isotopic composition of precipitation at the catchment scale and their relative influence.

Results show that mean δ¹⁸O of the events for the whole catchment varied from -11.96 to -3.6‰ along the year, with a mean coefficient of variation of 39%. Locations under forest were always more enriched than in open areas at the same elevation (+0.67‰ on average). Data analysis using the time stability approach (Vachaud et al., 1985) showed that forest locations had lower persistence of δ¹⁸O spatial patterns than open areas, indicating that spatial variability of isotopic composition was less predictable in forest locations compared to open areas. The elevation effect on δ¹⁸O, often observed in open area locations, was much less apparent in forest locations, confirming that forest introduced additional complexity on the spatial variability of the isotopic signal. Our findings highlight the actual need of taking into account the effect of both elevation and forest cover to assess a catchment scale representative isotopic composition of precipitation.

How to cite: Saurat, P., Llorens, P., Alharfouch, L., and Latron, J.: Influence of forest cover on the spatio-temporal heterogeneity of the isotopic composition of precipitation at the small catchment scale, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-496, https://doi.org/10.5194/egusphere-egu23-496, 2023.

Ecosystem functions of temperate forests are expected to be severely impacted by future climate change - particularly hydro-meteorological extremes (heavy precipitation events, droughts, and heat waves) that will increase in frequency, duration, magnitude, and extent. Previous studies have shown that both structural and tree species diversity may act as buffers against the impacts of climate extremes.

To better understand the influence of structural and tree species diversity, we use two models to analyze the influence of species and structural diversity on hydrologic dynamics during recent drought events. The well-equipped test sites are located in central Germany and represent typical forests in this area. One model is the individual forest gap model FORMIND. Using a newly developed technique, it allows us to analyze local heterogeneous patterns on a 2 meter scale of carbon and water cycling, flow, and water stress, and their relationship to structural and species diversity. The second model is mHM, a mesoscale hydrological model. We parameterize mHM for two catchments in central Germany (Nägelstedt and Upper Saale). We further analyze the relevance of local heterogeneity in structural and species diversity and the resulting local heterogeneity in water fluxes on the fluxes at the mesoscale.

This two-model and interdisciplinary workflow allows us to consider various soil-plant-atmosphere interactions in drought disturbed ecohydrological systems

How to cite: Bohn, F. J., Attinger, S., Kihm, C., and Hildebradt, A.: Modeling interactions of water fluxes with species and structural diversity in temperate forests, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6746, https://doi.org/10.5194/egusphere-egu23-6746, 2023.

Ongoing warming due to climate change has generally led to lengthened growing seasons and subsequent changes in evapotranspiration (ET) and streamflow seasonality. This has been well studied in seasonally dry, snowmelt dominated watersheds, but not in humid, temperate forested watersheds without significant seasonal snowmelt. In this study, we investigate how seasonal streamflow patterns have responded to variability in vegetation phenology in the southern Appalachians over the last four decades. We characterize seasonal shifts in low-frequency streamflow peaks using 50^th percentiles of cumulative daily precipitation, streamflow, and soil moisture measurements, and investigate interactions with remotely sensed, long-term greenup anomalies in the deciduous-dominated forested watersheds. After removing a dominant precipitation control, one-day earlier greenup is associated with about one-day early spring flow peak at the low-elevation deciduous catchment. This indicates that the strong dependency of seasonal flow regimes on precipitation is mediated by warming-induced extended growing season, especially by early greenup. In contrast, we find less significant correlations of the greenup anomalies on flow percentiles of an adjacent evergreen and a high-elevation deciduous catchment. At a plot scale, similar correlations of cumulative soil moisture days were found only at an upslope topographic position, where precipitation also showed tighter coupling with soil moisture patterns than downslope. This indicates that early greenup in deciduous forests leading to early ET increase, in turn results in early soil moisture dry-down patterned by hillslope positions, and earlier seasonal streamflow peaks and subsequent declines. Our study suggests that spring flow peaks are likely to shift earlier by warming-induced early greenup even in snow-free regions, which has great implications for future freshwater availability in the southeastern US.

How to cite: Hwang, T., Band, L., Oishi, C., and Kang, H.: Early greenup impact on seasonal streamflow and soil moisture dynamics in humid, temperate forests, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2852, https://doi.org/10.5194/egusphere-egu23-2852, 2023.

Temperate forests provide ecosystem services such as protecting water environment, timber and fuel production, carbon sequestration, and reduction of nutrient loss. During the last decades, large forest areas were decimated world-wide by bark beetle attacks. Under climate change, drought and higher temperatures increase the risk of infestation. Future forest ecosystem services are therefore at risk of deterioration and it is essential to understand how bark beetle attacks and following management practices influence nutrient cycling, nitrate (NO₃) and dissolved organic carbon (DOC) fluxes in seepage water. After dieback, accelerated mobilization of nutrients can be expected due to an increase in mineralization rates and the lack of plant nutrient uptake, whereas the lack of litter input may reduce nutrient leaching. The significantly reduced interception and evapotranspiration might furthermore increase soil water contents and seepage fluxes. Regeneration strategies (e.g. site clearance vs. keeping the dead trees, natural vs. artificial regeneration, regeneration with nurse crops) are decisive for the extent and persistence of the impact of calamities on water quality and quantity. We use a meta-data-analysis to gather knowledge out of approx. 60 studies around the world, to assess the expected behaviours of DOC and NO₃ concentrations in seepage water and streams after bark beetle outbreaks in temperate forests and to identify gap of knowledge. Most studies focussed on nitrate leaching and only few on DOC. Overall, DOC concentrations increase in seepage water and streams directly after dieback, reaching a peak 2 to 3 years after disturbance. In the opposite, the first evidences of increased NO₃ concentrations are visible approximately one year after disturbance and peak is reached within 3 to 10 years (on average after 5 years), when DOC decreases. NO₃ maxima never exceeded drinking water limit. In all studies, DOC and NO₃ concentrations recovered to pre-event or, in some cases, were even below the pre-dieback conditions only few years after the peak. Forest ecosystems seem therefore to be resilient to disturbances showing overall rapid recovery of ecosystem functions. However, the timing and duration of the concentration peaks largely differed among the studies, which might be explained by the extent and velocity of tree dieback in the studied areas, the harvest management practices and the type of vegetation re-growth after disturbance, but also by the local climatic conditions and the catchment size. Only few studies specifically analysed these effects on nutrient fluxes and their results differ considerably. More research is needed for assessing the influence of different regeneration strategies after calamities on water quality risks in forested catchments. A bark beetle attack currently decimating the Norway Spruce forest in the well-monitored Lange Bramke catchment (Harz, Germany), offers a unique opportunity to answer this question. With long-term datasets of NO₃ and DOC concentrations in stream and the recent installation of a network of lysimeters at three soil depths in a) a healthy forest area, and infested areas b) with dead trees standing, and c) with site clearance, we will be able to better understand the effect of regeneration strategies on nutrient fluxes.    

How to cite: Freudiger, D., Ahrends, B., Meedenburg, H., Scheler, B., and Talkner, U.: How do bark beetle outbreaks impact water quality in temperate forested catchments?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17269, https://doi.org/10.5194/egusphere-egu23-17269, 2023.

Tree mortality triggered by bark beetle infestation can significantly affect terrestrial carbon and water balances. However, how to improve the parameterization of the stomatal conductance to express the dynamics of ecosystem disturbance remains unclear. A subalpine forest located in the Rocky Mountains experienced a severe bark beetle outbreak in 2008, which provided a unique opportunity to investigate carbon and water flux changes covering the periods of pre-infestation (2005-2007), infestation (2008-2009), and post-infestation (2010-2014). Affected by bark beetle infestation, the stomatal conductance during the summer season (July and August) significantly reduced from 0.0018 m/s in the pre-infestation period to 0.0011 m/s in the infestation period. The decrease in stomatal conductance was not solely caused by the decrease of LAI, but also related to variation in parameter g1 in three commonly-used models of Ball-Barry, Leuning, and Medlyn. The parameter g1 was related to water use efficiency (WUE), and WUE values increased in the infestation period and decreased in the post-infestation period providing evidence that the physiological behavior was significantly changed due to bark beetle infestation. As for the simulation of transpiration, the fitted parameter significantly improved the accuracy in comparison with recommended parameterization. With the inclusion of temporally varied stomatal conductance, estimated transpiration during the infestation period and post-infestation period was improved by 4.3%~13.6% in comparison with the unvaried parameterization fitted in the pre-infestation period. Accounting for the temporally varied stomatal conductance parameters in response to disturbed environments may improve the description of stomatal conductance leading to better model performance in estimated water and carbon balances.

How to cite: Su, Y., Li, M., Shao, W., and Jarsjö, J.: Large-scale shifts in transpiration dynamics following bark beetle infestation: Stomatal conductance responses, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13089, https://doi.org/10.5194/egusphere-egu23-13089, 2023.