The intensifying global warming scenario leads to a notable increase in atmospheric evaporative demand, presenting novel threats to plant and ecosystems. Thus, it is crucial to comprehend the intricate relationship among temperature, vapor pressure deficit (VPD), and ecosystem processes to effectively address the challenges presented by climate change. Plants have evolved a myriad of strategies to cope with heat, drought, and high VPD. Thus, the impact of these environmental stressors will strongly depend on species-specific adaptations and prevailing environmental conditions.

This study delves into the intricate dynamics of ecological responses to vapor pressure deficit (VPD), heat, and soil moisture, with a focus on species-specific adaptations, and their consequential effects on carbon and water fluxes in diverse climatic regions. By examining dominant plant responses from field studies in semi-arid, temperate, and tropical forests, examples of species-specific adaptations to VPD, heat and drought and the ecological responses across various spatial and temporal scales will be presented. Utilizing field data, the role of soil moisture in modulating the impacts of VPD and heat on carbon and water fluxes will be explored, considering the seasonal dynamics that influence these interactions.

The findings highlight the importance of species-specific adaptations in influencing ecosystem responses to environmental stressors, emphasizing the need for a nuanced understanding of these adaptations across different climatic zones. A comprehensive understanding of the interplay between VPD, heat, soil moisture, and species-specific adaptations will be important for improved ecosystem management and climate change mitigation strategies tailored to specific regions and vegetation types.

How to cite: Werner, C.: Ecological Responses to Vapor Pressure Deficit, Heat, and Soil Moisture: A Species-Specific Perspective across Climatic Regions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13085, https://doi.org/10.5194/egusphere-egu24-13085, 2024.

Strong covariation between temperature and vapour pressure deficit (VPD) in nature creates challenges for understanding the direct effects of the two on leaf gas exchange. Measurements of stable isotope discrimination in CO₂ and H₂O provide additional insights into physiological and biochemical processes during leaf gas exchange. We investigated mechanistic causes of variations in net photosynthesis rate (A_n) and stomatal conductance (g_s) with increasing temperature at constant VPD and increasing VPD at constant temperatures.

We conducted combined leaf gas exchange and online isotope discrimination measurements on four common European tree species (Fagus sylvatica, Picea abies, Quercus petraea, and Tilia cordata) (1) across a temperature range of 5–40°C, while maintaining a constant VPD (~0.8 kPa) and (2) across a VPD range of 1–4 kPa, while maintaining a constant temperature (~30°C). The experiments were conducted without soil water limitation. The whole plant along with the whole instrumental setup were heated to prevent condensation when the dew point temperature within the leaf cuvette was higher than the room temperature.

Above the optimum temperature for photosynthesis (~30°C), we observed a decoupling of g_s and A_n across all tested species, with g_s increasing but A_n decreasing. Measurements of carbon and oxygen isotope discrimination indicated that during this decoupling, mesophyll conductance to the chloroplast decreased consistently and significantly among species; however, this reduction did not lead to reductions in CO₂ concentration at the chloroplast surface or the chloroplast stroma. Both g_s and A_n decreased, while the transpiration rate increased with increasing VPD. The relative humidity inside the leaf, derived from the oxygen isotope discrimination measurements, decreased from 100% to around 70% with increasing VPD, suggesting a progressive unsaturation of vapour pressure in the substomatal cavity. Accounting for the unsaturation, we found decreased CO₂ concentration in the intercellular air spaces and at the chloroplast stroma with increasing VPD; however, mesophyll conductance and CO₂ concentration at the chloroplast surface remained relatively stable.

We conclude that the effects of temperature and VPD on leaf gas exchange are distinctly different. The reduction in A_n at higher temperatures, unlike that at higher VPD, was not associated with stomatal closure and thus a restricted supply of CO₂ to the chloroplasts. Instead, it was more likely caused by Rubisco deactivation and/or a reduction of the electron transport rate. The unsaturation of vapour pressure inside leaves must not be ignored at VPD higher than 1 kPa, as it is vital for accurate estimations of g_s and the CO₂ concentration in the internal air spaces of leaves. Under non limiting soil water supply, the increases in leaf water loss due to increased leaf transpiration at higher temperature and VPD are important for plants to strategically cope with severe heat and dry conditions.

How to cite: Diao, H., Cernusak, L. A., Saurer, M., Gessler, A., Siegwolf, R. T. W., and Lehmann, M. M.: Disentangling direct effects of temperature and vapour pressure deficit on leaf gas exchange: mechanistic insights from online stable isotope techniques, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7683, https://doi.org/10.5194/egusphere-egu24-7683, 2024.

Tropical forests host Earth’s highest biodiversity and act as global climate regulators more than any other biome. In a world where heat waves become increasingly severe and recurrent, especially in the Amazon, it becomes crucial to know how close tropical species are getting to their critical temperatures. Indeed, while plant leaves operate within a broad range of air temperatures, they must stay under a species-specific critical temperature (T_crit) to sustain their function. In this context, we study how quickly the canopy temperature (T_c) approaches the critical temperature of tree species in the tropics.

We specifically focus on the evolution of the thermal safety margin (Δ = T_c − T_crit) during the period 2001- 2020 across tropical forest biomes in South America, Southeast Asia, and Central Africa. Data analysis (~ 1km resolution) was conducted by combining (1) tree species distribution maps, (2) T_crit databases, (3) MODIS-derived maximal Land Surface Temperature per month, and (4) a dense vegetation map to obtain the T_c maps. Given that the exposure time to T_crit required to cause the leaves to die is short, we focus on the heatwaves and select the warmest month per year to study the Δ evolution. We fit a linear regression at each geographic coordinate to obtain trend maps based on their coefficients.

Our results indicate a consistent trend wherein T_c progressively approaches critical thresholds. On average over the three studied regions, the median increase in T_c is around 0.11 °C per year, with a median Δ of 9.5 °C in 2020.

More sensitive locations exhibiting initial proximity to T_crit show an accelerated rate of gap closure. For instance, in South America, while the median increase per year in T_c is about 0.1 °C, the 90^th percentile is about 0.19 °C: in 2001, 10% of the locations were 4.5 °C away from the critical temperature of their most sensitive species, dropping to 0.3 °C in 2020. Central Africa has a less severe but still concerning trend, with the hottest 10% experiencing a 0.16 °C increase per year and a thermal safety margin of 2.5 °C in 2020.

Our findings suggest that, although most areas in the tropical biomes still have a rather large safety margin before reaching the T_crit associated with their species, sensitive areas are getting dangerously close to these critical thresholds, suggesting enhanced vulnerability to global warming.

How to cite: Lenczner, G., van Tiel, N., Tuia, D., and Grossiord, C.: How quickly are canopy temperatures approaching their critical limit in the tropics?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1763, https://doi.org/10.5194/egusphere-egu24-1763, 2024.

Heat waves and co-occurring droughts are increasing in frequency and magnitude in Central Europe with major impacts on tree water and carbon dynamics. So far, knowledge on how common temperate broadleaved tree species respond to combined heat and drought remains limited. To understand if tree species might tolerate future climate conditions, knowledge on their ability to regulate canopy temperature, sustain gas exchange and prevent leaf damage is essential. In our study we explored the impacts of increasing temperature and vapour pressure deficit (VPD) combined with a mild soil drought on three broadleaved tree species Fagus sylvatica, Acer platanoides and Quercus robur. Seedlings were subjected to a gradual temperature increase from 25°C to 45°C in a greenhouse, experiencing either well-watered control or mild drought conditions. Using single tree chambers, we assessed changes in leaf temperature, gas exchange and leaf senescence.

We found that with higher temperatures and VPD transpirational cooling of the leaves increased in all species, even though it was lowest for A. platanoides. Under mild drought conditions leaf cooling was strongly limited and leaf temperatures mostly exceeded air temperatures. Drought limitations were also reflected in the gas exchange with overall lower net assimilation, stomatal conductance and transpiration rates.  Still, drought and control trees both exhibited increased transpiration rates with higher temperature and VPD. At the same time stomatal conductance and net assimilation decreased with increased heat. Consequently, water-use efficiency strongly declined under well-watered and drought conditions for all species, emphasizing the crucial role of water during heat stress. For Q. robur these declines in gas exchange only happened at temperatures beyond 38°C under well-watered conditions, indicating higher thermal thresholds. This was also reflected in the leaf senescence as Q. robur avoided visible leaf damage entirely. In contrast, F. sylvatica and A. platanoides demonstrated high leaf senescence particularly in combination with drought.

In summary, our study highlights the importance of water availability for thermal regulation and sustaining positive net carbon uptake. Even though all species showed similar trends in their heat response, sustained net assimilation and avoidance of leaf damage point towards a potentially better heat resistance of Q. robur compared to the other two species.

How to cite: Zeppan, J., Huelsmann, L., and Ruehr, N.: Water availability determines heat stress response in temperate broadleaved tree species, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15883, https://doi.org/10.5194/egusphere-egu24-15883, 2024.

Extreme heatwaves, that are increasing in intensity and duration around the globe, are causing many locally adapted plant populations to rapidly become maladapted to climate conditions in ways that are likely to impact forest carbon storage, biogeochemical cycling, and biodiversity. One species that may be of particular risk from excess heat exposure is Populus fremontii (Wats.): a dominant riparian tree species that occupies extremely arid riparian ecosystems in western North America. We used an experimental common garden of two-year old P. fremontii genotypes sourced across a broad climate gradient to evaluate leaf thermal regulation and thermal tolerance of trees exposed to daytime summer temperatures that regularly exceeded 45 °C. Traits were measured to evaluate patterns of hydraulic and thermal safety, including leaf temperature (T_leaf), stomatal conductance, leaf water potentials, leaf turgor loss point, stem xylem cavitation vulnerability and leaf thermal tolerance - defined as the critical temperature that triggers rapid reductions in electron transport capacity of Photosystem II (T_crit; °C). Three major results emerged. First, T_leaf of genotypes from the warmest locations were 4 to 5 °C cooler than air temperatures, even on days where air temperatures exceeded 48 °C. Second, short-term reductions in soil water availability - even reductions that were largely undetectable from predawn leaf water potentials - disrupted leaf cooling patterns in all genotypes, resulting in periods in which T_leaf exceeded T_crit. And third, during the warmest period of the summer, a clear tradeoff was detected between leaf thermal safety and hydraulic safety, with warm-adapted genotypes risking hydraulic safety to maximize leaf thermal safety. Results not only improves our understanding of tree thermal limits in the face of episodic heat exposure, but also advances our understanding of how short-term changes in soil moisture availability can alter plant thermal regulation and subsequent exposure to long-term heat stress.

How to cite: Hultine, K., Posch, B., Bush, S., Koepke, D., Anderegg, L., Aparecido, L., Blonder, B., Guo, J., Kerr, K., Moran, M., and Schuessler, A.: Hydrological limits to leaf cooling during a record summer heat wave, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13339, https://doi.org/10.5194/egusphere-egu24-13339, 2024.

Spring wheat (Triticum aestivum L.) yields in continental cropping systems of the U.S. Midwest have shown a consistent upward trajectory over the past century due to successful breeding efforts and improvements in crop management. However, the looming threat of increasingly extreme temperature trends and the rising evaporative demand (vapor pressure deficit, VPD) during the cropping season in that region may require adaptive water use strategies to maintain or further increase wheat yield potential. Therefore, our objective was to investigate whether the observed increase in yield over the last 122 years coincides with a shift in plant water use strategies, i.e., the transpiration rate (TR) sensitivity to rising VPD.

In this study, we selected 15 spring wheat genotypes from Minnesota with a year of release (YOR) spanning from 1898 to 2020 to capture the genetic yield gains achieved during that period by the local breeding program. We tested plant transpiration rate in response to rising VPD ranging from 0.5 to 2.8 kPa in a climate chamber in wet soil and potted conditions. Additionally, we measured several traits that capture plant hydraulic properties, including stomatal conductance, plant hydraulic conductance, leaf area, above- and belowground biomass, and stomatal morphological properties.

Our investigation revealed that at a critical VPD beyond 1.83 ± 0.17 kPa, a significant portion of the tested genotypes expressed a limited increase in TR with increasing VPD, indicating a decline in stomatal conductance. No discernible correlation was observed between parameters characterizing plant water use strategies or the plant hydraulic system and YOR over the whole 122-year window. However, a moving window analysis unveiled that post the green revolution (around 1960 ± 15 years), breeding for yield indirectly favored less hydraulically conductive plants with a reduced leaf area and a linearization of the transpiration rate response to increasing VPD, as evidenced by a decreasing difference in slopes beyond a critical VPD. This resulted in a less pronounced reduction in water use due to a restricted TR response to increasing VPD and, thus, a lower sensitivity to rising VPD. 

Our study indicates that hydraulic traits such as the TR sensitivity to VPD might have been under the control of a cryptic selection mechanism by breeders as they increased wheat yield potential in the region, at least from the 1960s onwards. This points to the promising possibility of using such traits to improve yields under drier climates.

How to cite: Köhler, T., Ossola, E., Anderson, J., Carminati, A., and Sadok, W.: Shifts in plant water use patterns during increasing VPD across 122 years of breeding in U.S. Midwest spring wheat (Triticum aestivum L.) genotypes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4771, https://doi.org/10.5194/egusphere-egu24-4771, 2024.

The balance between terrestrial carbon assimilation and water loss is optimized by stomatal control in leaves. The response of stomatal conductance to increasing vapor pressure deficit (VPD) is critical in the context of climatic change and highly variable between tree species and environments. Why this variability in VPD sensitivity exists is still largely unknown. Yet, as the regulatory cues initiating closure remain unclear, current simulations of forest water use face significant uncertainty.

To address this knowledge gap for most of the common European tree species, we present data measured regularly in the crowns of mature trees growing in a natural forest at the Swiss Canopy Crane II (SCCII) site (Basel, Switzerland). We used three years of repeated stomatal conductance measurements across the growing season performed at the leaf level (over 1000 measurements), in combination with concurrent diel leaf water potential measurements and VPD monitoring of over 80 individuals.

We show that pre-dawn, rather than the expected midday, tree water status is more critical in adjusting the stomatal closure responses to increasing VPD. A striking reduction can be found in this VPD sensitivity when pre-dawn leaf water potential approaches -1.2 MPa, independent of the species. Only above this threshold, i.e., when trees were well-hydrated, did the species show variance in midday stomatal sensitivity to VPD. This aligns with the commonly adopted hydraulic safety-efficiency theorem for explaining species-specific variance.

We argue that daytime canopy conductance does not solely optimize assimilation against the risk of cavitation, which commonly happens during high midday VPD. Rather, our novel finding suggests that mature trees might adjust their water-use strategy to sustain high nighttime turgor pressure (as required for sugar transport and growth), although the regulating mechanisms are yet unknown. The discovery of this uniform pre-dawn threshold across species is particularly critical for reducing uncertainty when modeling forest water use responses to VPD.

How to cite: Peters, R. L., Arend, M., Zahnd, C., Hoch, G., and Kahmen, A.: Pre-dawn water potential determines stomatal sensitivity to vapor pressure deficit in trees, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10789, https://doi.org/10.5194/egusphere-egu24-10789, 2024.

High atmospheric vapor pressure deficits lead to high transpiration demands which can induce plant water stress because of large dissipation of water potential within plant tissues, but also because the transpiration demand exceeds the possible root water uptake rate from the soil. The latter point might be particularly critical in coarse textured soils which display poor soil-root contact and an unsaturated hydraulic conductivity curve steeply decreasing with matric potential. Root hairs can increase the possible root water uptake rate by increasing the root-soil contact and the effective root radius. Thus, we hypothesize that plants with functional root hairs display (1) less negative leaf water potentials at midday at high transpiration rates in comparison to plants without root hairs; and (2) that this effect is more pronounced in coarse textured soils at low soil matric potentials.

To test these hypotheses, we grew two maize (Zea mays) genotypes (wildtype and its root hairless mutant) in two contrasting soil textures (Sand vs Loam). We measured leaf water potential (leaf Psychrometer), transpiration rate (sap flow), atmospheric vapor pressure deficit, soil water potential and soil water content every ten minutes for 30 consecutive days in summer of 2023.

The root hair bearing wildtype consistently maintained a higher transpiration rate at relatively less negative leaf water potentials when the atmospheric vapor pressure deficit was high. This effect was more pronounced in coarse textured soil (sand) even in relatively wet soils (soil matric potentials > -100 kPa).

We concluded that root hairs enabled plants, in relatively wet soils to maintain high transpiration rates without excessive leaf dehydration. This suggests that at high atmospheric vapor pressure deficit, losses in the hydraulic conductivity of the rhizosphere can already limit transpiration even when the soil would be typically considered wet. Our findings highlight the importance of rhizosphere processes and their relevance for plant water use at the field scale.

How to cite: Stoll, F., Mustafa, O., Akale, A., Carminati, A., Vanderborght, J., Javaux, M., and Ahmed, M.: Root hairs prevent excessive losses in leaf water potential of field grown maize at high atmospheric vapor pressure deficits even in wet soils, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18268, https://doi.org/10.5194/egusphere-egu24-18268, 2024.

Photosynthetic responses of Pinus densiflora seedlings to warming in different seasons were investigated. In March 2023, four temperature treatments with five replicates were conducted in an open-field nursery site located in Seoul, South Korea: constant warming (April 15th-October 15th), spring and fall warming (April 15th-May 31st and September 1st-October 15th), summer warming (June 1st-August 31st), and control. In each plot, 108 1-year-old P. densiflora seedlings were planted. The temperature of the warming plots was set to increase by 4°C compared to control plots using infrared heaters. Photosynthetic responses of seedlings were measured on the 10th of each month from May to October and linear mixed-effect models were used to analyze the effect of treatments on photosynthetic responses. Net photosynthetic rate was not affected by any of the treatments until June but decreased by 12.7% under summer warming compared to spring and fall warming in July. In August, both constant warming and summer warming treatments decreased the net photosynthetic rate by 18.0% and 12.3%, respectively, compared to the control. However, in September, following the cessation of the summer warming and the initiation of the fall warming treatment, seedlings only subjected to constant warming exhibited a significant reduction in net photosynthetic rate, with a 33.0% decrease compared to the control. This reduction increased to 41.2% in October, whereas summer warming and spring and fall warming treatments did not affect photosynthesis in the same month. This study indicates that warming might result in losses in plant photosynthesis, and those losses could be higher during summer. In addition, although the spring and fall treatment did not independently affect net photosynthetic rate of seedlings, the accumulated heat during spring and fall appears to have attributed to the photosynthetic reduction under the constant warming treatment.

* This research was carried out with the support of the Korea Forest Service Government (KFSG) as [Graduate School specialized in Carbon Sink].

How to cite: Jo, H., Kim, G.-J., Kim, J., Kim, G., Kwon, M., Lee, J.-M., and Son, Y.: Photosynthetic responses of Pinus densiflora seedlings are affected by accumulated heat under artificial warming, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14328, https://doi.org/10.5194/egusphere-egu24-14328, 2024.

‘Water potential’ is the biophysically relevant measure of water status in vegetation relating to stomatal conductance, hydraulic conductance, and mortality thresholds; yet this cannot be directly related to fluxes of water at plot- to landscape-scale without understanding its relationship with ‘water content’.  The relationship between water content and water potential has long been of interest to plant and soil scientists and is typically determined at small scale on excised plant parts or soil samples.  But how can water potential be scaled from leaf to canopy, branch to stem, tree to forest?  How does water content vary throughout a plant, and across individuals and species?

In this talk I will outline some practical considerations for deriving representative values for ecosystem water potential and content: the ecosystem pressure-volume curve.  I will discuss how ecosystem water status is influenced by the boundaries we apply to the system, which differ depending on whether we are interested in interpreting remote sensing data, models, or field measurements.  And I will describe the concept of the ‘state-based’ model which relates to steady-state vegetation types that emerge predictably in response to a given climate or hydraulic environment.

To support this discussion, I will present ecosystem pressure-volume curves from nine sites including tropical rainforest, savanna, temperate forest, and a long-term Amazonian rainforest drought experiment.  The results from these preliminary analyses show that the relationship between the water stored in biomass consistently scales with biomass across systems, as does the vegetation-level hydraulic capacitance; while the relative measures of water storage and hydraulic capacitance show no trend across ecosystems.  Such cross-biome relationships in water relations hold promise for improving our understanding of vegetation-climate feedbacks over large spatial and temporal scales, and enhancing our capacity to interpret remote sensing data.

How to cite: Binks, O.: Ecosystem water relations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12052, https://doi.org/10.5194/egusphere-egu24-12052, 2024.

Climate extremes like drought are threatening forests worldwide. Record breaking forest mortality has been observed in central Europe in the past five years. Meanwhile, more and more experiments are being set up that enable measurements of  the hydraulic states of dying trees under extreme drought stress. These experimental data can be exploited by mechanistic vegetation models, offering  the possibility to disentangle environmental drought stressors, e.g. atmospheric and soil moisture dryness, and their effects on a plant’s hydraulic system, such as stomatal closure and loss of hydraulic conductivity. 

Here, we show how a next generation plant hydraulic modelling is able to accurately reproduce the water potential dynamics of dying trees. We apply this plant hydraulic model to European drought experimental sites, including the canopy crane experiment II in Basel, Switzerland, and the KROOF experiment in Freising, Germany. We find that soil heterogeneity, rooting depth and stem hydraulic capacity are critical in determining whether a tree survives or succumbs to drought. Furthermore, good knowledge of four parameters is crucial to accurately capture the magnitude and temporal development of observed leaf and stem water potential: (1) stem hydraulic capacitance, (2) P50 (the water potential at which 50% of a plant’s hydraulic conductivity is lost), (3) saturated xylem hydraulic conductivity, and (4) the reference leaf water potential associated with full stomatal closure. Finally, when implemented into the terrestrial biosphere model QUINCY, our hydraulic scheme produces a clear mortality signal associated with recent drought events, giving confidence in our capacity to project the impact of future droughts on European forests

How to cite: Papastefanou, P., Arend, M., De Kauwe, M., Grams, T., Kahmen, A., Rammig, A., Sabot, M., and Zaehle, S.: Vital role of hydraulic capacity uncovered by mechanistic modelling of extreme drought impacts on European forests, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18114, https://doi.org/10.5194/egusphere-egu24-18114, 2024.

The frequency and severity of droughts is expected to increase as climate change develops, which negatively affects forests and their numerous benefits. However, some species may be adapted better to the new conditions than others. Hence, evaluating the stress level of trees under ranges of water-limited conditions is crucial to judge and predict forest health. While water potential measurements are valuable for indicating stress, they are hardly applicable to monitoring continuous developments or investigating large numbers of individuals simultaneously. An alternative attempt is to utilize high-resolution dendrometer measurements of stem shrinkage, which is caused by the reduction of the tree's internal water storage (tree water deficit; TWD) and is thus an indirect indicator of stress. There is first proof of correlations between TWD and water potential under moderate drought stress. However, we still lack observations that relate stem diameter variations to severe drought stress when embolism formation (air bubbles in the xylem preventing water flow) occurs. Also, recovery responses after re-wetting, and the respective linkages to physiological states and processes are fairly unknown.

Here, we present results from a greenhouse experiment on potted tree saplings. Two widespread temperate conifers (Pinus sylvestris, Larix decidua) were exposed either to drought-recovery cycles or to lethal drought until complete dehydration under controlled experimental conditions. We found strong relations between TWD and both midday water potential and gas fluxes across the full range of dehydration, with only minor differences between the two species. Re-wetting after a short drought period had no effect. Conversely, re-wetting after a drought severe enough to cause hydraulic damage significantly affected the correlations due to different recovery times.

Our results indicate the great potential of dendrometers to provide continuous and cost-efficient time series that allow valuable insights into the water status and thus drought stress of trees. While all stages of dehydration can be covered, the dependencies of re-wetting responses after severe droughts are still unclear and more species-specific investigations are required. Nevertheless, applying TWD seems to be a promising way forward to improve our understanding of drought-stress-induced forest decline and drought-recovery dynamics. 

How to cite: Ziegler, Y., Alongi, F., Knüver, T., Grote, R., and Ruehr, N.: Tree water deficit as a drought stress indicator, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16104, https://doi.org/10.5194/egusphere-egu24-16104, 2024.

Plant water stress has major implications for vegetation productivity, mortality, and global carbon cycle variations. Yet, its representation in models constitutes a major source of uncertainty. This is related to uncertainties in the representation of water stress exposure due to unknown belowground rooting structure and water storage, and uncertainties in the representation of interactive responses to atmospheric dryness (vapour pressure deficit, VPD) and belowground moisture.

Recent theoretical developments have given rise to several hydraulically explicit models for predicting plant physiological responses to VPD and belowground moisture at the leaf level. The P-hydro model that we have recently developed, accounts for the simultaneous acclimation of stomatal conductance and photosynthetic capacity using an eco-evolutionary optimality approach. Based on three “first principles” (balance of water demanded by transpiration and that supplied from the soil, coordination of the carboxylation-limited and light-limited photosynthesis rates, and maximization of net profit after accounting for the costs of maintaining photosynthetic and hydraulic capacities), it correctly predicts the responses of stomatal conductance, assimilation rates, leaf water potentials, and photosynthetic capacities to changing hydroclimatic environments. However, the hydraulic strategies of plants depend critically on their belowground rooting environments, such as soil properties and rooting depth.

Here, we implement the P-hydro model for modelling ecosystem-level fluxes and couple it with a simple model of soil water balance within the rsofun modelling framework. The water-balance model accounts for variations in root-zone water storage capacity of plants, thus allowing us to characterize both above-ground and belowground hydraulic strategies. We investigate (1) the power of P-hydro in simulating gross primary production and evapotranspiration at globally distributed FLUXNET sites under conditions of simultaneous atmospheric and belowground dryness, benchmarked against a non-hydraulically explicit version of the model (P-model), (2) apply a Bayesian data assimilation approach to infer plant and soil hydraulic traits (plant conductivity and vulnerability, root zone water storage capacity, and cost parameters of the optimality model), (3) assess the environmental dependencies of the inferred traits and the generalisability of the model for global simulations.

The P-hydro model, together with the inferred trait relationships, promises a simple yet robust approach to predicting the global environmental dependencies of ecosystem productivity.

How to cite: Joshi, J., Arán Paredes, J., and Stocker, B.: Inferring geographic and climatic variation in plant hydraulic traits from flux data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17872, https://doi.org/10.5194/egusphere-egu24-17872, 2024.

From heightened canopy dieback to tree die-off, many forest ecosystems are showing signs of poorly coping with more severe, more frequent, or hotter droughts. Understanding forest resilience to drought has become paramount, and eco‐physiological optimisation approaches that test behavioural hypotheses have been proposed as a means to build this understanding in global terrestrial models. Here, we used a land-surface model that considers competing optimality principles to simulate canopy gas exchange and leaf nitrogen investments into the photosynthetic apparatus, whilst also accounting for sustained hydraulic impairment (Sabot et al., 2022). We applied this model to a pristine observational site of the Caatinga, Brazil’s drought-hardy, seasonally deciduous, and exceptionally diverse dry tropical forest. Six woody species dominate 80% of the study area whilst displaying contrasting functional strategies – for example, their respective P50s (the water potential at which 50% of a plant’s hydraulic conductivity is lost) range between -1 MPa and -5 MPa. Model predictions were assessed against species-specific leaf-level observations of stomatal conductance and photosynthetic uptake, as well as eddy covariance measurements of ecosystem carbon and water fluxes spanning a period with high interannual rainfall variability (and including a severe multi-year regional drought). We found that none of the six species could, in isolation, explain the magnitude and dynamics of the observed surface fluxes. However, taken together and accounting for their relative contribution to total ecosystem fluxes, they did. Further, our analysis emphasises the vital role of phenology in mitigating seasonal and inter-annual hydraulic risks, with foliage reductions triggered by a 10 to 20% loss of hydraulic conductivity in the canopy. On the whole, accounting for diverging species-level responses and their relative influence at the ecosystem-scale appears key to improving model predictions in functional diverse forests.

Reference: Sabot, M.E.B., De Kauwe, M.G., Pitman, A.J., Ellsworth, D.S., Medlyn, B.E., Caldararu, S. et al. (2022) Predicting resilience through the lens of competing adjustments to vegetation function. Plant, Cell & Environment, 45, 2744–2761.

How to cite: Sabot, M., Nobrega, R., Moura, M., De Kauwe, M., Majcher, B., Cosme, L., Miatto, R., Ferreira Domingues, T., Pitman, A., Prentice, I. C., and Verhoef, A.: Modelling the functionally diverse Caatinga: insights into a unique tropical forest, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13238, https://doi.org/10.5194/egusphere-egu24-13238, 2024.

Vegetation water content (VWC) plays a key role in transpiration, plant mortality, and wildfire risk. Although land surface models now often contain plant hydraulics schemes, there are few direct VWC measurements to constrain these models at global scale. A potential solution to this data gap is passive microwave remote sensing, which is sensitive to temporal changes in VWC. Here, we test that approach by using synthetic microwave observations to constrain VWC and surface soil moisture within the Climate Modeling Alliance Land model. We further investigate the possible utility of sub-daily observations of VWC, which could be obtained through a satellite in geostationary orbit or combinations of multiple satellites. These high-temporal-resolution observations could allow for improved determination of ecosystem parameters, carbon and water fluxes, and subsurface hydraulics, relative to the currently available twice-daily sun-synchronous observational patterns that cannot single-handedly capture the two most informative times of the diurnal cycle (i.e. pre-dawn and mid-day). We find that incorporating observations at four different times in the diurnal cycle (such as could be available from two sun-synchronous satellites) provides a significantly better constraint on water and carbon fluxes than twice-daily observations do. For example, the root mean square error of projected evapotranspiration and gross primary productivity during drought periods was reduced by approximately 40%, when using four-times-daily relative to twice-daily observations. Adding hourly observations of the entire diurnal cycle did not further improve the inferred parameters and fluxes.

How to cite: Konings, A., Holtzman, N., Wang, Y., Wood, J., and Frankenberg, C.: Constraining a plant hydraulics-enabled land surface model with microwave radiometry: impact of temporal resolution, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7158, https://doi.org/10.5194/egusphere-egu24-7158, 2024.

Optimization models of stomatal conductance (g_s) provide a conceptual framework to understand stomatal responses to environmental changes in terms of tradeoffs between the benefits and costs of stomatal opening. Theoretically, stomata maximize the benefits (carbon gain, A) and minimize the costs (water loss through transpiration, E). However, during heat waves, there can be conflicts between the need to maintain a high E to control leaf temperature via evaporative cooling and avoid heat damage, and the decrease in g_s in response to increased VPD and reduced soil water availability. Under these conditions, the balance between the gains and the associated costs remains unclear. We measured leaf gas exchange (A, E and g_s) in potted grapevines, cv Sauvignon Blanc, before, during and after a simulated six-day heat wave (T_max = 40 °C), in the morning (10 to 12) and the afternoon (15 to 17), using heated well-watered (HW), heated drought-stressed (HD), non-heated well-watered (CW) and non-heated dry (CD) vines. We also measured plant transpiration (E_lys) and leaf temperature (T_leaf) continuously during the heat wave using lysimeters and infrared cameras. We test the hypothesis that under combined stress, in addition to the stomatal limitation to A, there is an additional non-stomatal cost for A caused by the heat damage in the photosystem. We explore whether this potential additional cost is captured by two g_s optimization models that include soil-to-leaf hydraulic conductance (model 2) or not (model 1). We also hypothesize that g_s in HW vines may not follow the optimal predictions because these models do not include stress-related risks such as heat damage due to higher-than-optimal T_leaf resulting from stomal closure and reduced E. There were no significant differences in g_s in HD and CD vines, during the morning or the afternoon measurements. Consequently, there were no differences in measured E and A between HD and CD vines during the peak of the heat wave. This was probably due to the stronger effect of water stress (soil water potential from -400 to -800 kPa) than high VDP (5 kPa) on g_s during the peak of the heat wave. Under well-watered conditions, measured g_s, E and A in the morning were much higher in HW than in CW vines and the values decreased from the morning to the afternoon. Preliminary results suggest that optimal models including dynamic responses to soil water potential can correctly integrate plant responses to heat and drought stress. The incorporation of stress-related risks (such as heat damage to the photosystem) into these models will be discussed.

How to cite: Asensio, D., Hammerle, A., Niedrist, G., Shtai, W., Kadison, A., Schwarz, M., Raifer, B., Andreotti, C., Zanotelli, D., Haas, F., Tagliavini, M., and Wohlfahrt, G.: Applicability of optimal stomatal conductance models during a simulated heat wave in grapevine, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8365, https://doi.org/10.5194/egusphere-egu24-8365, 2024.

Stress caused by high temperatures is a critical limiting factor of crop growth and development. Accurately identifying heat stress is crucial to assess and mitigate the negative impact of high temperatures on crop growth. However, isolating the independent effects of heat stress from other factors, such as moisture stress, poses a challenge in field conditions. This study developed an innovative approach to distinguish crop heat stress periods from normal growth conditions, disentangling them from moisture stress and light limitation in croplands. Utilizing FLUXNET data, including air temperature, gross primary productivity, soil water content, and shortwave radiation observations, we identified 78 heat periods and corresponding normal growth conditions. The identified heat and normal periods were further related with remote sensing to extend the identification process to a large scale. Single bands and spectral vegetation indices (VIs) derived from MODIS were employed to evaluate the capability of multispectral data in detecting heat stressed crops from healthy crops. The analysis revealed a significant increase in the reflectance of red band during heat stress. VIs, in general, enhanced the visibility of heat-induced spectral variations and exhibited sufficient capability in distinguishing crops at heat and normal conditions. Visible bands-based indices (EVRI, GLI, and NGRDI) exhibited the highest distinguishability (p-value < 0.01 in the Mann–Whitney U test). These findings underscore the significance of visible bands, especially the red band, in advancing large-scale crop heat stress detection, agricultural monitoring, and crop modeling considering heat stress.

How to cite: Lai, P., Marshall, M., Darvishzadeh, R., Tu, K., and Nelson, A.: Identifying crop heat stress with MODIS and FLUXNET data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2626, https://doi.org/10.5194/egusphere-egu24-2626, 2024.

In recent decades, forests have faced increasingly severe droughts due to rising temperatures and longer dry spells. These conditions intensify atmospheric and soil drought, putting forest ecosystems under considerable stress. The extreme drought years in Germany, particularly in 2018 and 2022, have had a profound impact on forests and thus offer the potential to gain important insights into the responses of different tree species to hotter droughts. Given the pivotal role of forests in mitigating climate change and the long rotation periods of managed forests, understanding species-specific drought responses is crucial for developing effective strategies to adapt to evolving climate scenarios.

Currently, our understanding of species-specific drought responses relies heavily on dendroecological analysis and plot-based ecophysiological monitoring networks. While these methods provide insights into tree growth and physiology, their spatial constraints limit widespread replication. To address these limitations and quantify drought responses of specific tree species at a larger scale, our study integrated tree-species maps from the Thünen Institute with remotely sensed canopy greenness and environmental variables, including soil moisture (PAWC), atmospheric vapor pressure deficit (VPD), and climatic water balance (SPEI). Specifically, we focused on four dominant species: two with more anisohydric characteristics (beech and oak, which keep their stomata largely open under drought) and two with more isohydric strategies (pine and spruce, which close their stomata already under less extreme drought). Using statistical methods such as linear regression and machine learning within a gradient-boosting framework, we aimed to explore the factors influencing changes in canopy greenness for different species from 2018 to 2022.

We found that nearly all trees of these species had lower PAWC in 2022 than in 2018, while only one-third of beech, oak, and pine trees and more than 70% of spruce trees had higher VPD in 2022. More isohydric species showed a greater decline in canopy greenness over this period compared to more anisohydric species, despite similar soil moisture conditions. Our models suggest that more isohydric species were primarily affected by extremely low soil moisture, whereas more anisohydric species were primarily affected by atmospheric moisture deficit. Our statistical analysis showed that oak is the only species with significantly higher canopy greenness in 2022 compared to 2018. Linear regression models showed very low importance of PAWC for oak canopy greenness but much higher importance of VPD. However, we hypothesize that all species are still susceptible to carry-over effects from previous drought years or secondary factors related to biotic pathogens.

Our study provides critical insights into the diverse responses of different tree species to changing environmental conditions over large spatial scales. It elucidates the complex interactions between soil moisture, climate variables, and canopy greenness. These findings contribute significantly to our understanding of the resilience of forest ecosystems to climate variability and provide invaluable guidance for informed forest management and conservation strategies.

How to cite: Wang, Y., Rammig, A., Blickensdörfer, L., Wang, Y., Zhu, X., and Buras, A.: Tree-species-specific response of canopy greenness to the extreme droughts of 2018 and 2022, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8545, https://doi.org/10.5194/egusphere-egu24-8545, 2024.