Carbon and water cycling are tightly linked in forest ecosystems but severe droughts bare the potential to disrupt the sensitive balance and ecohydrological interactions between different species in forest ecosystems. To unravel complex ecosystem carbon-water dynamics we imposed a 9.5-week drought on the Biosphere 2 tropical rainforest, a thirty-year old enclosed forest. We traced ecosystem scale interactions through a whole-ecosystem 13C and 2H-labelling approach in the Biosphere 2 Tropical Rainforest, the B2 Water, Atmosphere, and Life Dynamics (B2WALD) experiment. We analysed total ecosystem exchange, soil and leaf fluxes of H₂O, CO₂ and BVOCs, and their stable isotopes over five months. To trace changes in soil-plant-atmosphere interactions we labelled the ecosystem with a ¹³CO₂-isotope and investigated the importance of deep water sources under drought by 2H-labelling at the end of the drought. The tropical rainforest exhibited highly dynamic, non-linear responses during both dry-down and rewetting phases.

Drought sequentially propagated through the vertical forest strata, with a rapid increase in vapor pressure deficit, the driving force of tree water loss, in the top canopy layer and early dry-down of the upper soil layer but delayed depletion of deep soil moisture. This induced a two-phase response of ecosystem fluxes: gross primary production (GPP), ecosystem respiration (Reco), and evapotranspiration (ET) declined rapidly during early drought and moderately under severe drought.

Ecosystem ¹³CO₂-pulse-labeling showed that drought enhanced the mean residence times of freshly assimilated carbon- indicating down-regulation of carbon cycling velocity and delayed transport form leaves to trunk and roots. Ecosystem carbon and water fluxes were determined by different ecohydrological responses of the dominant plant functional groups: while drought sensitive canopy trees dominated total ecosystem water fluxes under well-watered conditions, they showed the largest decline in response to top-soil moisture decline. Drought tolerant canopy trees exhibited lower fluxes but also higher resistance to soil water decline. Interestingly, all dominant canopy trees had access to deep water reserves, serving as a crucial water source during drought but not sustaining high transpiration rates.

Recovery of ecosystem carbon and water fluxes was slow after drought release, which reflected the by long water transit times within the soil-plant-atmosphere system. Thus, we found highly diverse responses of carbon and water fluxes, driven by the interplay of hydraulic regulation of different vegetation compounds and ecohydrological feedbacks in the forest. This study highlights the importance of ecohydrological responses for overall ecosystem resilience and carbon sequestration potential, providing valuable insights into the complex interplay between climate change, water availability, and carbon cycling in terrestrial ecosystems. We need to develop a comprehensive understanding of the multifaceted ecohydrological factors shaping ecosystem responses to climate change, with implications for sustainable ecosystem management and carbon mitigation strategies.

Werner et al. 2021, Science 374, 1514 (2021), DOI: 10.1126/science.abj6789

How to cite: Werner, C., Meredith, L., and Ladd, N. and the B2WALD: Ecohydrological Responses of Different Functional Groups Driving Ecosystem Carbon Cycling under Experimental Climate Change Drought, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12775, https://doi.org/10.5194/egusphere-egu24-12775, 2024.

With climate change, accelerated increases in potential evapotranspiration (PET) and decreases in relative extractable water (REW) are increasingly affecting stand transpiration (T). However, the responses of T to PET under limited REW are still unclear, especially in forests of dryland region. In the present study, we partitioned the effects of REW and PET on T in oak (Quercus wutaishansea) forest stands in the Liupan Mountains, northwest China. The results showed that the reduction in REW due to drought resulted in a significant decrease in T. When REW was higher, i.e., above 0.5, there was a linear relationship of T with PET but an exponential relationship when REW was lower than 0.5. Moreover, REW in the soil layer of 20-60 cm rather than that in the soil layer of 0-20 cm plays a decisive role in T during drought. More REW, such that at the mid- and downslope sites, would be helpful to mitigate the decline in T under drought to some extent compared with less REW that at the upslope sites. These remind us that the soil moisture in dryland regions should be paid more attention in forest management and vegetation restoration in future.

How to cite: Yu, P., Liu, B., Wan, Y., Wang, Y., and Wu, Y.: Soil moisture shapes the responses of Quercus wutaishansea forest stand transpiration to potential evapotranspiration in the Liupan Mountains, Northwest China, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21796, https://doi.org/10.5194/egusphere-egu24-21796, 2024.

Terrestrial vegetation plays a crucial role for water-energy-carbon interactions between the land and the atmosphere but is experiencing above-average temperature increases due to climate change. This contributes to widespread increases in vapor pressure deficit (VPD). High VPD and low soil moisture are considered the two main drivers of plant water stress, triggering the downregulation of vegetation-atmosphere fluxes, such as transpiration and photosynthesis. While ecosystems are initially driven by energy availability, soil drying below a critical soil moisture threshold (θ_crit) shifts them to a water-limited regime. However, the relative importance of VPD versus soil moisture limitation and the relative role of soil versus plant hydraulic conductance are highly debated. Understanding the key mechanisms controlling these relative roles is therefore crucial to predict vegetation-atmosphere exchanges under changing environmental conditions. Here, by analysing global observations of θ_crit, we demonstrate the central role of soil texture in shaping the importance of VPD versus soil moisture limitation by mediating the magnitude of soil hydraulic conductance relative to that of the plant. On average, we find that loss in soil rather than plant hydraulic conductance determines the onset of water limitation across climates and biomes globally. This implies that ecosystems in fine textured soils are more sensitive to VPD than ecosystems in coarse textured soils, while ecosystems in coarse soils are more sensitive to soil drying than in fine soils. This is a consequence of the steeper decline in soil hydraulic conductivity in coarse soils, resulting in the dominant control of soil hydraulics in these soils. Our analysis explains the emergent control of soil texture on ecosystem water limitation and unifies long-standing controversies about the relative importance of VPD versus soil moisture and soil versus plant hydraulic limitation. We demonstrate the global relevance of soil texture for land-atmosphere exchanges and open new paths to understanding the impacts of climate change on terrestrial ecosystems.

How to cite: Wankmüller, F. J. P., Delval, L., Lehmann, P., Baur, M. J., Wolf, S., Or, D., Javaux, M., and Carminati, A.: Soil Texture Controls the Relative Importance of Vapor Pressure Deficit and Soil Moisture for Ecosystem Water Limitation Globally, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17348, https://doi.org/10.5194/egusphere-egu24-17348, 2024.

The semi-arid and semi-humid region of China occupies an area of more than 200 million km2. Since years of ecological restoration implementation, land use cover changes (LUCC) significantly with the vegetation continually recovered in the recent decades. During this period, climate change is significant in the region with air warming and precipitation extremes intensification. How climate change and LUCC play different roles in the land surface – atmosphere exchanges and catchment hydrological processes is still not clear.  Based on VIP (Vegetation Interface Processes) distributed eco-hydrological model, the eco-hydrological changes are predicted integrated with remotely sensed vegetation information. Fourteen catchments in the study region are selected to explore the diversified responses and feedbacks of the vegetation recovery to climate change. It is found that evapotranspiration (ET) and vegetation gross primary productivities (GPP) are increasing steadily in the study period from 2000 to 2020, in which ET and GPP from the forest and cropland are more distinguished with each other. However, terrestrial water storage is decreasing in the southern catchment, especially those over the Loess Plateau. Although the water consumption from vegetation is increased, water availability  is still increasing in most of the study area due to enhanced precipitation, which implicated the intensification of the hydrological cycle with climate change and global greening. The complex interactions and feedback between re-vegetation and climate change in the water limited region posed challenges to the water resources management and ecosystem stability, in need of paying much more special attention.

How to cite: Mo, X., Hu, S., Sang, J., Huang, L., and Liu, S.: Impacts of climate change and LUCC on eco-hydrological processes in semi-arid and semi-humid regions of China  , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17690, https://doi.org/10.5194/egusphere-egu24-17690, 2024.

Sun-induced chlorophyll fluorescence (SIF) measurements have shown unique potential for quantifying both plant physiological and structural stress under compound drought and heatwave events. However, there is a lack of understanding and well explore of the differences in the sensitivity of vegetation physiological and structural information based on solar-induced chlorophyll fluorescence (SIF) to compound drought and heatwave stresses and the driving mechanisms behind it. The aim of this study was to assess whether SIF-derived physiological information (eΦF) and structural information (NIRvP) could improves the quantification of physiological and structural aspects of vegetation sensitivity to compound drought and heatwave stress at the mid-high latitudes of northern hemisphere by using contiguous sun-induced fluorescence (CSIF) data, respectively. We found that, compared to the vegetation sturctural information (NIRvP), the relative importance of vegetation physiological information (eΦF) to eCSIF variability increases 6.5% to14.8% under compound drought a heatwave stresses in all regions, which confirms the contribution of physiological variation to eSIF. We further demonstrated that vegetation physiological information (eΦF) can better detect compound drought and heatwave stress in humid regions and forest ecosystems, which is mainly driven by physiological (LCC and VCmax) and environmental (VPD and SR) factors; whereas vegetation physiological information (eΦF) and structural information (NIRvP) have similar ability to capture compound drought and heatwave stresses. The lines of evidence suggested that utilizing eΦF for physiological investigations and NIRvP for structural information will contribute to improve our comprehensive understanding of vegetation physiological and structural responses to simultaneous high-temperature and high-drought stresses.

How to cite: Diao, C., Wu, X., Li, Y., and Zhao, L.: Sensitivity of vegetation structural and physiological information to compound drought and heatwave stress based on Solar-induced chlorophyll fluorescence (SIF), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3248, https://doi.org/10.5194/egusphere-egu24-3248, 2024.

Global change drivers, such as climate change and land-use intensification, pose imminent threats to the functioning of agricultural ecosystems, by disrupting vital ecosystem processes. In this context, understanding the interplay between carbon cycling and ecohydrological processes becomes important for the development of adaptive strategies that enhance the resilience of agricultural ecosystems in response to the dynamic challenges imposed by global change drivers. Process-based simulation models are a powerful tool to disentangle the complex interactions of microbiota-plant-soil interactions and provide a basis for long-term predictions and scenario simulations.

Our study focuses on the "Global Change Experimental Facility (GCEF)" (https://www.ufz.de/index.php?en=42385), where comprehensive data on plant physiology, soil nutrients, soil microbes, fauna, soil structure and moisture were collected across various agricultural land-use types. These include conventional and organic cropping systems, intensively and extensively farmed meadows, and extensively grazed sheep pastures, all under ambient and simulated future-climate conditions.

Here we present an extended version of the process-based soil model BODIUM, which captures the dynamics of soil functions, responding to soil management or changes in climatic conditions. The model was parametrized for different land-use types on the GCEF. First simulation results, compared with measured data for validation, reveal promising agreement in carbon data for cropland systems. However, an overestimation of water content within the soil profile after the vegetation phase needs further investigation. Additional simulations, alongside experimental findings are employed to discuss the impact of climate-change and land-use types on carbon and water dynamics and their potential interactions. The successful validation of the model across varied treatments provides the foundation for a potential application of the model to other boundary conditions that are not covered by the GCEF experiment.

How to cite: Kanagarajah, L., Reitz, T., Schädler, M., Taubert, F., Vogel, H.-J., Weller, U., and König, S.: Bridging Carbon and Water Dynamics: Insights from Process-Based Modeling in Agricultural Ecosystems Under Global Change Drivers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10378, https://doi.org/10.5194/egusphere-egu24-10378, 2024.

The accelerating pace of climate change has led to unprecedented shifts in surface temperature and precipitation patterns worldwide. Regions particularly susceptible to these changes include African savannas. With increased climate anomalies and human impacts, comes increased disturbance events in the form of fires, overgrazing from elephants, invasive species etc. Consequently, there is a continual, large-scale transformation in vegetation patterns. Long-term ecosystem characteristics, such as soil traits, topography, and species composition, also contribute to the susceptibility of ecosystems to these climate and disturbance pressures. Understanding the complex interaction between climatic pressures, local disturbance drivers and underlying ecosystem traits, and their impact on ecosystem functioning and stability, is thus crucial.

This research pursued a dual objective: firstly, detecting shifts in ecosystem functioning within the African savanna biome and analysing their spatial and temporal patterns; secondly, exploring the interplay between climate legacy, disturbance legacy, and underlying ecosystem characteristics, and how these factors influence savanna ecosystem resilience at regional and local scales in the context of global change. Remote sensing time series analysis and breakpoint detection algorithms were employed to identify change hotspots in the South African savanna biome. Combining this ethodwith survival analysis, a novel approach, also helped uncover large-scale climatic drivers and underlying ecosystem traits facilitating these changes. Bayesian hierarchical modeling, coupled with field data collected in the Kruger National Park of South Africa, was then utilised to delve into the complex interactions between disturbance legacy (e.g., drought legacy, history of extreme rainfall events, increased fire frequency, and elephant activity) and the resistance and resilience of the savanna landscape at a local scale. The outcomes of this research contribute to prioritising conservation efforts and enhancing our understanding of the future of savannas in the face of global change.

How to cite: Vermeulen, L. M., Verbist, B., Van Meerbeek, K., Slingsby, J., Negri Bernardino, P., and Somers, B.: Climate change, disturbance legacy and abrupt shifts in ecosystem functioning: what makes arid African savannas more resilient?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17222, https://doi.org/10.5194/egusphere-egu24-17222, 2024.

Geoengineering strategies can be classified into two primary categories: i) Solar Radiation Management (SRM), which aims to mitigate the absorption of sunlight by the Earth, and ii) Carbon Dioxide Removal (CDR), involving the active extraction of carbon from the atmosphere for storage in terrestrial or marine environments. The ongoing discourse on geoengineering, particularly SRM on a global scale, is marked by polarization, primarily due to the challenging nature of predicting remote consequences.

This presentation endeavors to demonstrate two key points. Firstly, it will present a range of evidence indicating that local mitigation and adaptation, employing ecohydrological processes in regional models, yield more pronounced effects on regional temperatures and moisture compared to studies that use global climate models. Secondly, it will highlight that various bottom-up interventions in the energy-carbon-water nexus significantly impact maximum temperatures and moisture availability. For instance, a recent review (van Woesik et al., 2024) identifies over 50 of such interventions for East Africa.

While advocating for the efficacy of local solutions, this presentation acknowledges that such interventions, including reforestation and afforestation (e.g. Staal et al. 2024), can lead to remote consequences due to the interconnected energy-carbon-water dynamics, affecting for instance shifts in local precipitation patterns (e.g. van Theeuwen et al. 2024). Consequently, local-scale CDR solutions influence both local and remote energy balances, blurring the distinction from SRM. This challenges the applicability of conventional IPCC terminologies for climate mitigation and adaptation at the local scale. The prevalent global focus of IPCC research, derived from global models, has impeded the analysis of local ecohydrological interventions.

The central proposition of our research is that Targeted Climate Modification should be approached and analyzed from a bottom-up perspective rather than a top-down one. Therefore, we propose terminology shifts from mitigation and adaptation to Targeted Climate Modification. We hypothesize that such locally targeted interventions can benefit humanity and biodiversity by inducing cooling, enhancing agricultural productivity, and mitigating extremes in droughts and floods.

However, our research also calls for ethical and governance discussions. Acknowledging that local Targeted Climate Modification may yield negative remote consequences and substantial impacts on biodiversity loss, it advocates for the development of a new framework to analyze ethical, social, and environmental issues associated with Targeted Climate Modification.

References:

Staal A, Theeuwen JJE, Wang-Erlandsson L, Wunderling N, Dekker  SC (in press) Targeted rainfall enhancement as an objective of forestation. 2024. Global Change Biology.

Theeuwen JJE, Dekker SC, Hamelers BVM, Staal A,  Ecohydrological variables dominate local moisture recycling in Mediterranean-type climates, 2024, submitted to JGR-Biogeosciences

van Woesik FM, Dekker SC, van Steenbergen F, de Boer HJ. A review of Local Climate measures to increase Resilience of East African Agroecological Systems. To be submitted to Journal of Environmental Management

How to cite: Dekker, S. C., de Boer, H. J., Koren, G. B., Staal, A., Theeuwen, J. J. E., and van Woesik, F. M.: Targeted Climate Modification on land – A matter of scale, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4740, https://doi.org/10.5194/egusphere-egu24-4740, 2024.

Recent decades have been characterized by increasing temperatures worldwide, resulting in an exponential climb in vapor pressure deficit (VPD) and soil droughts. Heat and VPD have been identified as increasingly important drivers of plant functioning in terrestrial biomes and are significant contributors to recent drought-induced tree mortality. Despite this, few studies have isolated the physiological response of plants to high VPD, heat, and soil drought, thus limiting our understanding and ability to predict future impacts on terrestrial ecosystems. I will present diverse experimental approaches to disentangle atmospheric and soil drivers of plan functions across scales in this presentation. I will further discuss recent findings suggesting that high temperature and VPD can lead to a cascade of impacts, including reduced photosynthesis, foliar overheating, and higher risks of hydraulic failure, independently of soil moisture changes. Moreover, I will highlight how species interactions can modulate the adverse impacts of soil and atmospheric droughts.

How to cite: Grossiord, C.: Impact of rising temperature and drought on forest ecosystems across scales, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3085, https://doi.org/10.5194/egusphere-egu24-3085, 2024.

Heatwaves are becoming more frequent, with higher temperatures, drier air and reduced soil water availability. However, as increasing temperature, increasing VPD and soil drought are often coupled in nature, especially during heat waves, their isolated effects on tree physiology and, ultimately, mortality are not fully understood.

To disentangle the effects of these factors, we conducted a climate chamber experiment on nine tree species from different biogeographical backgrounds (three conifer species: Pinus sylvestris L., Pinus halepensis Mill. and Cupressus sempervirens L.; three temperate broadleaved species: Alnus cordata (Loisel.) Duby, Acer platanoides L. and Phillyrea angustifolia L.; three tropical broadleaved species: Terminalia microcarpa Decne., Syzygium jambos L. (Alston) and Trema orientale (L.) Blume). We exposed the trees to three different treatments which were imposed for two-day periods for a total of twelve days; (1) increasing temperature (20 to 40°C) with constant VPD (1.2 kPa), (2) constant temperature (35°C) with increasing VPD (1.2 to 4.7 kPa), and (3) increasing temperature (20 to 40°C) and VPD (1.2 to 6 kPa) but with constant vapor pressure (1.2 kPa). Each treatment was also divided into two groups: well-watered to field capacity (~35% soil moisture) and soil drought (~10% soil moisture). On the second day of each step, total water consumption, gas exchange (A_max, g_s, E), leaf temperature, and the maximum photochemical efficiency (Fv/Fm) were measured, and leaves were sampled for abscisic acid (ABA) analysis. The occurrence of stem and leaf embolism for conifer and broadleaf trees, respectively, was continuously monitored with the optical vulnerability method. G_min and P₅₀ were measured for all the tree species.

With this data set, we can study the isolated and combined effects of high temperature and VPD on plant gas exchange and xylem embolism, and how this response varies in plants with different biogeographic backgrounds and environmental adaptions. These findings help us better understand the underlying physiological drivers of globally rising tree mortality and improve models to better predict tree responses in a hotter and drier world.

How to cite: Schuler, P., Didion-Gency, M., Johnson, K., Hoch, G., Kahmen, A., and Grossiord, C.: Disentangling the impact of air temperature, vapor pressure deficit, and soil drought on photosynthesis, transpiration, and embolism, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7766, https://doi.org/10.5194/egusphere-egu24-7766, 2024.

In non-stressed conditions, majority of water loss from plants occurs through stomata in leaves. During drought, plants close their stomata or even drop leaves to prevent massive embolism formation that disconnects leaves and above-ground parts hydraulically from roots and can ultimately lead to hydraulic failure. However, plants loose water also through leaf cuticle and bark. While water loss from leaves after stomatal closure has received increasing attention in recent years, water loss through bark has been largely ignored although bark covers 30 to 50% of whole tree surface area. The outer bark layers are practically impermeable to gases and water, but they are pierced by lenticels that provide channel for exchange of water and gas with ambient air to allow oxygen intake for the metabolic processes of the stem. In contrast to active stomatal control in leaves, gas exchange through bark cannot be actively regulated by plants and therefore water loss through bark continues after stomatal closure (together with water loss through leaf cuticle).

Stomatal closure in leaves also reduces photosynthesis; thus, drought can cause both hydraulic failure and carbon starvation and these processes are strongly linked. When leaf photosynthesis is minimized, bark photosynthesis can locally compensate the decreasing leaf photosynthesis. This helps to avoid carbon starvation, because bark photosynthesis utilizes recycled CO₂ released from internal respiration resulting in more efficient carbon fixation in terms of water use. Bark thus plays a role in tree water and carbon balance, and it is crucial to understand the bark water and carbon dynamics in trees under changing climate.

We will show results and discuss three different aspects of bark gas exchange: 1) Drivers and seasonality of water loss and CO₂ exchange through bark. These results are based on continuous stem chamber measurements of Pinus sylvestris in boreal environment. We successfully partitioned stem CO₂ exchange into bark photosynthesis driven by light, respiration driven by temperature, and transport of CO₂ dissolved in xylem sap; 2) Species-specific differences in water loss rate through bark and bark photosynthesis. These results are based on sampling branches from 11 coniferous and 4 broadleaved species grown in a boreal arboretum and comparing their bark characteristics regarding water loss and photosynthesis; 3) The role of water loss through bark in the whole tree water loss in dry conditions. These published results show that water loss rate per bark area was typically ~76% of the shoot transpiration rate (on projected needle area basis) in Pinus halepensis growing in semi-arid conditions but could even surpass the shoot transpiration rate during the highest evaporative demand. Irrigation of trees did not affect bark water loss rate, whereas shoot transpiration was greatly increased due to stomatal control.

The role of bark in tree water and carbon balance is often neglected in research, because its share of the whole tree water and carbon balance is negligible in good growing conditions, but when trees get stressed and stomata in leaves are closed, the role of bark may become dominant.

How to cite: Lintunen, A., Koivu, K., Dukat, P., and Hölttä, T.: Do we need to care about water loss through bark and bark photosynthesis in trees under changing climate?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10395, https://doi.org/10.5194/egusphere-egu24-10395, 2024.

Knowledge of hydraulic strategies is essential to understand the response of forest ecosystems to changing environmental conditions. Species-specific regulation mechanisms of water fluxes and water storage in the xylem are important drivers of drought tolerance in trees. Differences in the regulation may also occur along the radial profile of the xylem leading to a differential drought response of water fluxes within the tree xylem. To this end, we investigated sap flow and stem water content in two xylem depths, variations in stem radius and water potentials of Abies alba, Fagus sylvatica, Picea abies and Quercus petraea in a mature forest stand in SW-Germany during three years with varying environmental conditions.

Generally, hydraulic strategies varied between the four investigated species: A. alba was generally the most water-saving species, while drought tolerance was highest in Q. petraea and lowest in P. abies. Under moist conditions F. sylvatica was the most water-spending species, whereas sap flow was strongly reduced under drought. Overall, sap flow of all four species responded more pronounced to high vapor pressure deficits (VPD) than to decreasing soil moisture. We found a varying contribution of stem water storage to daily sap flow between the four species, which might be a crucial trait explaining drought tolerance.

A dynamic radial shift of sap flow in ring-porous Q. petraea was observed, which was tightly linked to corresponding stem water content: under high VPD sap flow in the inner xylem (10 mm beneath the cambium) exceeded sap flow in the outer xylem (20 mm beneath the cambium), while the inverse was observed under low VPD. Such a differential response within different xylem depths might enhance drought tolerance of ring-porous Q. petraea.

Moreover, the relationship between declining stem water reserves (expressed as tree water deficit) and water potentials was analyzed by a generalized additive model incorporating VPD and soil moisture. The model was trained with measured water potentials and subsequently used to predict water potentials from tree water deficits. Model predictions represented well measured values, if environmental variables were considered, and thus our modelling approach might be a useful tool in the future to predict water potentials on a high temporal scale without excessive measurement intensities.

In summary, our study demonstrated that availability of stored stem water influences species-specific response of sap flow to drought conditions and that regulation mechanisms may vary along the radial profile. Further investigations are needed to determine if differential responses in different xylem depths are species-specific or part of a general protection mechanism, e.g. to preserve hydraulically more important xylem vessels from cavitation. Furthermore, predicting water potentials from tree water deficits might be a useful and cost-efficient approach to gain insights into stomatal regulation processes without the need to reach the tree canopy.

How to cite: Dumberger, S., Kinzinger, L., Haberstroh, S., and Werner, C.: Dynamics in radial sap flow and stem water reserves under varying environmental conditions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16544, https://doi.org/10.5194/egusphere-egu24-16544, 2024.

Ecological studies place great importance on understanding the profound significance of plant diversity in maintaining the functioning of grassland ecosystems. However, despite decades of research, ecologists have faced persistent challenges in fully comprehending the intricate relationships between biodiversity and ecosystem functioning, as well as the influence of dominant plant functional groups on overall ecosystem function. To investigate two key aspects, namely the association between species richness and above- and below-ground biomass (AGB and BGB), as well as the relative contributions of functional groups in maintaining ecosystem function, we investigated the grassland biomass productivity in meadow steppe and alpine meadow ecosystems in the Qinghai-Tibet Plateau (QTP) for the period 2015-2019. 

The pristine grassland sites belonging to the alpine meadow and meadow steppe were maintained for data collection over 5 years. Shoot biomass was harvested during the peak growing season (mid-late September) by clipping vegetation samples from 3 typical 0.25 × 0.25 m² quadrats within the central 4 m² (2 × 2 m²)of the plot (5 × 5 m²) at the respective site of the total 36 sites. Root biomass was sampled using soil cores at depths of 0-40 cm (0-10, 10-20, 20-30, and 30-40 cm) in 3 typical 0.25 × 0.25 m² quadrats over the 5 years. Before harvesting the biomass, the number of species and functional groups in the selected quadrat was counted. We used multiple tests including Mann-Kendall, Generalized Linear Model, Kruskal-Wallis, and Wilcoxon texts for analyzing the data.

The AGB of both grasslands exhibited an increasing trend over 5 years, while the BGB remained stable. This rise in AGB was attributed to the upward trajectory observed in AGB for forbs and grasses, which are the dominant functional groups in the QTP. These results underscore the crucial role of dominant species and functional groups in maintaining ecosystem functioning. Higher species richness plays a crucial role in ecosystem stability, as evidenced by the significantly positive relationships between biodiversity and AGB, and the stable relationships between biodiversity and BGB. In both grasslands, the top soil layer (0-10 cm) exhibited a dominant contribution to the observed BGB. This can be attributed to the abundance of nutrients present in the topsoil layer, which creates favorable conditions for root proliferation. Within the meadow steppe, there was an isometric allocation pattern observed in biomass, indicating that the BGB increased proportionally with the AGB.

The findings of this study demonstrate the influence of species richness on ecosystem functioning, with forbs and grasses playing a dominant role in biomass productivity. Notably, the top soil layer was responsible for three-quarters of the BGB. These empirical results provide valuable evidence that higher species richness enhances ecosystem functioning, serving as a scientific basis for informing policymaking regarding ecosystem stability.

How to cite: Hossain, M. L. and Li, J.: Increasing aboveground biomass and stable belowground biomass are controlled by greater species richness and dominant functional groups in Qinghai-Tibet Plateau, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5532, https://doi.org/10.5194/egusphere-egu24-5532, 2024.

Tree processes belowground are highly complex and strongly affect the soil carbon. Trees assimilate carbon and allocate up to 20% of the carbon to the rhizosphere as root exudates. Rhizosphere processes in the forest soil might have a significant global effect. Abiotic factors such as intensified drought and elevated atmospheric CO2 (eCO2) will influence tree carbon fluxes as predicted under climate change. As trees play a vital role in maintaining Earth’s carbon balance, ecological studies on trees are crucial, especially in light of climate change.

The effect of the predicted elevation of atmospheric CO₂ and drought on the rhizosphere was studied on 2-years-old pine saplings in climate-controlled growth rooms. The results showed up to a twofold increase in assimilation, increasing shoot biomass, while root exudation rate remained unchanged in eCO₂ compared to ambient CO₂. Root exudation increased under drought, despite reduced assimilation. Under combined drought and eCO₂ treatment, exudation rate increased even more by 56%, suggesting assimilated surplus carbon might have been stored in the roots under eCO₂. In addition, we found an increase of soluble sugars in the stem under combined drought and eCO₂ treatment, specifically glucose and fructose, indicating that indeed surplus carbon is stored during well-watered times under eCO₂.

Our results are unique because they show for the first time that pines were able to increase their root exudation under drought due to eCO₂, potentially enhancing their resilience during drought recovery.

How to cite: Obersteiner, S., Oppenheimer - Shaanan, Y., and Klein, T.: Pine root exudation increased under drought and even more under eCO2 and drought, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16967, https://doi.org/10.5194/egusphere-egu24-16967, 2024.

Biogenic volatile organic compounds (BVOCs), primarily emitted into the atmosphere by terrestrial vegetation through biochemical processes, have key ecological functions in protecting vegetation from biotic or abiotic stresses. However, accurately quantifying and predicting changes in BVOC emissions in response to long-term environmental changes large spatial scales remain challenging. The appropriate tools for observing the BVOC emissions at large scales are still missing. Remote sensing of optical signals is a promising solution to fill spatial knowledge gap. We hypothesize that the carotenoid-related vegetation index, such photochemical reflectance index (PRI), is a promising method to investigate BVOCs emitted by plants based on their functional links with carotenoids and photosynthetic activity.

We conducted a leaf-level experiment in greenhouse during the summer of 2022 to investigate how the relationships between PRI and BVOC emissions change in response to drought or heat stresses in Scots pine and English oak saplings during the peak of growing season. We aim to address the following questions: (1) What factors control the relationships between PRI and BVOC emissions in response to mild/extreme drought or heat; (2) Will these controlling factors differ between vegetation species or BVOC emission types (e.g., isoprene and monoterpenes)? (3) Can PRI or other carotenoid-related vegetation indices capture the changes of BVOC emissions in response to drought or heat stresses?

We will present our preliminary results. The expected outcomes will give new insight into leaf-level mechanistic links between PRI and BVOC emissions for plants in response to climate drought or warming.

How to cite: Zhang, C., Miettinen, I., Porcar-Castel, A., Atherton, J., Rissanen, K., Aalto, J., Tykkä, T., Hellén, H., López-Pozo, M., Fernández-Marín, B., García-Plazaola, J. I., Kohl, L., and Bäck, J.: Photosynthetic optical signals studying biogenic volatile organic compound emissions in Scots pine and English oak saplings under drought or warming, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16543, https://doi.org/10.5194/egusphere-egu24-16543, 2024.

Increasing drought stress has triggered various negative impacts on forests worldwide, including growth reduction, defoliation, crown dieback, and even tree mortality, with unavoidable consequences for forest ecosystems. However, how reductions in both precipitation and soil moisture progressively lead to tree mortality remains largely unknown. Here, we define relative soil water (RSW) as the ratio of the actual soil moisture to the field capacity, which can reflect the fraction of water in the root zone, to reveal how soil moisture reduction leads to progressive tree mortality. Based on field measurements of tree behaviors, including transpiration, tree growth, defoliation, crown dieback and other behaviors, before, during and after an extreme drought in the Larix principis-rupprechtti plantations in 2021, we found that the variability in precipitation and soil moisture affect tree behaviors, but soil moisture is the dominant driver of drought stress on progressive tree mortality, with prolonged and severe soil moisture reduction leading to widespread tree mortality. RSW thresholds for different stages of progressive tree mortality and drought stress levels are identified as follows: Level I (RSW > 0.7), no detectable hydraulic limitations; level II (0.7 to 0.45), persistent stem shrinkage and onset of transpiration reduction; level III (0.45 to 0.35), onset of slight discoloration and defoliation; level IV (0.35 to 0.25), onset of crown dieback and tree mortality; and level V (< 0.25), severe defoliation, 20% crown dieback and tree mortality. Our results shed light on predicting tree mortality and distribution in forests under increasing climate warming, particularly in semiarid areas with warming-induced tree mortality.

How to cite: Wan, Y., Yu, P., Wang, Y., Bai, Y., Yu, Y., and Li, J.: Reduction in soil moisture dominates progressive tree mortality in semiarid areas, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21790, https://doi.org/10.5194/egusphere-egu24-21790, 2024.