BG3.23

Acclimation processes in a changing climate require the ability to tolerate and survive abiotic stress events (e.g. drought), challenging especially immobile and long-living species and ecosystems, such as forests. The drought years 2018/2019 in Central Europe have laid bare the vulnerability of temperate forest ecosystems to drought and heat. The recovery phase after the stress event represents a crucial phase, especially after intense and repeated drought periods, and might be different for anisohydric and isohydric species.

The second phase of the Kranzberg Forest Roof (KROOF) experiment in southeast Germany focused on the watering of mature more anisohydric European beech (Fagus sylvatica L.) and more isohydric Norway spruce (Picea abies (L.) H. Karst.) after five years of repeated experimental summer drought. Both treatments, the former throughfall-exclusion (TE, recovering trees) and control (CO) plots were labeled with ²H enriched water by controlled watering, to end the experimental drought. Pre-dawn leaf water potential, stomatal conductance, xylem sap flow density (at breast height and crown base) and leaf osmoregulation were recorded for two growing seasons after drought release and the resilience and recovery times were calculated.  

All measured parameters were strongly reduced by on average 30% in both species due to the drought treatment. While the distribution of the labeled water upon irrigation across the soil profile occurred within a few days in both treatments, the water uptake and distribution within the trees was delayed by several days in recovering trees compared to control trees and in recovering spruce compared to recovering beech. Additionally, upon drought release recovering beech reached full resilience (i.e. same level as control trees) earlier than recovering spruce in water potential, stomatal conductance and xylem sap flow density and even showed signs of overcompensation by surpassing the control trees. No differences were found between the two species in the recovery of leaf osmoregulation.

The “opposing” drought mitigation strategies seem to be responsible for the differences detected between more anisohydric beech and spruce during the recovery period. For example, the lack of recovery of xylem sap flow density at crown base in TE spruce indicates a re-filling of the stem water reservoirs upon watering. Additionally, we found fast responding parameters as water potential (hours to days) and stomatal conductance (days to weeks) compared to slow responding parameters such as osmoregulation (weeks to months) and full hydraulic recovery, i.e. xylem sap flow density, may even take years. Rapid physiological recovery after drought events, which are expected to increase in frequency and intensity with ongoing climate change, will be beneficial for overall recovery and might put faster-reacting trees in favor over slower responding species.

How to cite: Hesse, B. D., Gebhardt, T., Hafner, B. D., Hikino, K., Häberle, K.-H., and Grams, T. E. E.: High resilience of water related physiology after five years of repeated summer drought of mature beech and spruce., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3835, https://doi.org/10.5194/egusphere-egu22-3835, 2022.

Climate change is expected to increase the frequency and intensity of extreme weather events like heat waves (HW), with potential negative impacts on grapevine productivity and wine quality in several regions. High temperatures during heat waves are usually accompanied by decreases in soil moisture, but few studies have explored the single and combined effects of temperature and drought on grapevine physiology. Using fully controlled environmental chambers we simulated a 6-day heat wave. Each chamber was assigned to one temperature treatment (control or heat). Inside each chamber, four grapevine plants (cv. Sauvignon b. on SO4 rootstock) were placed on weighing lysimeters. Half of the plants in each chamber were well-watered (watered plants) and the other half were subjected to drought (dry plants). Radiation intensity and air temperature mimicked average summer conditions near Bolzano (Italy). In the control chambers, the maximum daily temperature (Tmax) was 30°C for the entire period. In the heat chambers, Tmax reached 40° C on day 6, then it decreased on day 12 to 30° C.  Instantaneous leaf photosynthesis (Pn) as well as chlorophyll fluorescence parameters (Fv/Fm, Y(NPQ), qN, qP and qL) were measured manually in the late morning (10:00-12:00) and in the afternoon (15:00-17:00), together with stem water potential (SWP; 15:00-17:00). Plant transpiration (T), soil water potential and leaf temperature were continuously monitored throughout the experiment. During the HW, well-watered plants showed a marked reduction of Pn from the morning to the afternoon, which was not visible in the vines under control temperature. Drought significantly reduced Pn and, when combined with the heat stress, further decreased Pn with respect to the control temperature, both in the morning and in the afternoon. Daily T of watered plants during the HW was about 50% higher than T under the control temperature, while the HW had no effect on T of dry plants. Regardless of the chamber temperature, progressive drought caused stomatal closure, which in turn prevented stem water potential from reaching low levels (all SWP values were >-1.45 MPa). This, in turn, caused an increase of canopy temperature, which in the heated chambers reached a peak of around 45°C (up to 5°C warmer than in the watered plants). The heat stress decreased Fv/Fm both in watered and in dry plants, an effect that was stronger in the afternoon, but also occurred in the morning if the exposure to heat stress was prolonged. Other fluorescence parameters, like Y(NPQ) and qN, were affected by heat stress, while qP and qL were affected mainly by drought. At the end of the HW, chlorophyll fluorescence parameters recovered in watered plants, but not in the dry ones. Grapevines of the cv. Sauvignon b. were able to cope with a 6-day heat wave provided enough soil water was available to support the significantly enhanced transpiration rate. Drought had adverse effects on Pn regardless of the air temperature, but the drought-stressed plants that were also exposed to the heat wave experienced excess canopy temperatures. Leaf chlorophyll fluorescence proved to be a reliable indicator of heat stress.

How to cite: Asensio, D., Shtai, W., Kadison, A., Schwarz, M., Hoellrigl, J., Steiner, M., Raifer, B., Andreotti, C., Hammerle, A., Zanotelli, D., Hass, F., Niedrist, G., Wohlfahrt, G., and Tagliavini, M.: Interactive effects of high temperatures and drought on grapevine physiology during a simulated heat wave, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3171, https://doi.org/10.5194/egusphere-egu22-3171, 2022.

Limitation of photosynthesis by soil moisture variability is increasingly understood to be a central factor in the land carbon balance. Soil moisture dynamics are thought to be responsible for the bulk of interannual variability in the land carbon sink as well as the uncertainty in predicting it.

A key challenge in understanding this process is the mismatch between the spatial scales of cause and effect. Sharp and localised gradients in soil moisture availability can develop during drought, presenting a difficulty both for plants and researchers. Soil moisture heterogeneity triggers nonlinearities in flow in both the soil and plant, whose effects are not captured by current earth system models (ESM).

Most descriptions of soil moisture limitation of plants at earth system scales rely on a heuristic macro-scale ‘stress factor’ formulation that fails to adequately reflect current understanding of the inherently small-scale process. Recently, some models have adopted a linearised formulation of bulk flows in response to water potential gradients. While this step theoretically improves mechanistic representation of the process, increased prediction errors persist when vertical differences in soil moisture emerge while horizontal heterogeneity is commonly not represented at all.

This presentation introduces the upscaling approach to addressing this knowledge gap, which seeks new formulations of soil-plant hydrodynamics that bridge the scales of cause and effect. This approach relies on describing soil-plant hydrodynamics from first principles at small scale and mathematically scaling up the resulting formulations: deriving simple bulk scale forms while introducing as few approximations or errors as possible. Recent advances in this line of research include a fully general algorithm for upscaling the nonlinear equations describing flows in the root system without introducing discretisation error or increasing computational cost of finding solutions. Ongoing work aims to explore opportunities for addressing nonlinearities in the soil that arise from conceptual advances achieved in upscaling the plant flows. Applying the outputs of this promising theoretical approach in earth system models will require future empirical work to constrain the parameters of the new models at multiple scales.

How to cite: Bouda, M., Vandergborght, J., Couvreur, V., and Javaux, M.: Predicting plant water limitation in heterogeneously drying soils: the upscaling approach to improving soil-plant hydrodynamics in ESMs, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12601, https://doi.org/10.5194/egusphere-egu22-12601, 2022.

The water-use efficiency (WUE, carbon assimilation per unit of water-use) describes a major axis of variability of ecosystems and identifies how these are coping with environmental changes. However, the response of WUE to climate change and hydrological extremes between different ecosystems remains poorly understood. 

Here we investigated how the WUE of ecosystems in Europe varied from 1995 - 2018, as long-term trends and in response to precipitation (P) and soil moisture (SM) droughts. We aggregated data from remote-sensing and reanalysis to calculate three different WUE indices, conducted Mann-Kendall trend analyses and determined WUE anomalies for different hydro-climates and plant functional types during P and SM deficits. Finally, we applied the Peter & Clark Momentary Conditional Independence (PCMCI) algorithm to identify causative networks of environmental variables and WUE and differences among ecosystems.

We found extensive, negative long-term WUE trends in Eastern Europe, where WUE is predominantly controlled by carbon assimilation (GPP). Further, we identified soil moisture and transpiration control of GPP as drivers for the positive WUE response to droughts in arid ecosystems. In contrast, negative trends in humid ecosystems were driven mostly by temperature, which governed GPP variability.

In addition, outputs from a state-of-the-art land-surface and carbon model (CLM5-BGC) will be used to compare trends, drought response and the causative relationships with the ones from satellite and reanalysis data in order to evaluate the model representation of ecosystem variability.

How to cite: Poppe Teran, C., Naz, B., Baatz, R., Hendricks-Franssen, H.-J., and Vereecken, H.: Causes of Water-Use Efficiency Variability in Europe and Their Representation in the Community Land Model v5, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4956, https://doi.org/10.5194/egusphere-egu22-4956, 2022.

Efficient leaf-scale temperature control is instrumental to vegetation functioning and ecosystem-scale resilience to a drying and warming climate. When evaporative cooling is suppressed during drought, leaf thermal regulation requires modulation of the leaf energy budget and the balance between sensible heat (H) and latent heat (LE) fluxes.

We obtained rare leaf energy budgets under field conditions by combining measurements using a new methodology and theoretical estimates in naturally droughted and artificially irrigated plots of a dry Mediterranean pine forest, with low and high evapotranspiration rates, respectively.

The measurements revealed that under the same radiative load, leaf cooling shifted from equal contributions to heat dissipation of H and LE in irrigated trees to almost exclusively through H in droughted ones while maintaining comparable leaf-to-air temperature differences.

The results demonstrate that an assessment of the leaf energy budget in the field provides the means to identify effective leaf temperature control in pine trees under drought, enhancing their resilience to current drying trends. The shift from LE to H provides an ‘air cooling’ mechanism that equals the efficiency of evaporative cooling. It also provides a leaf-scale basis for the large ecosystem-scale ‘convector effect’ identified in semi-arid forests.

How to cite: Muller, J. D., Rotenberg, E., Tatarinov, F., and Yakir, D.: Shift to efficient leaf cooling through sensible heat revealed by detailed energy budget in mature pine trees in drought manipulation field experiment, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13247, https://doi.org/10.5194/egusphere-egu22-13247, 2022.

The ecological impacts of climate change encompass significant consequences on the interactions of soil-plant-atmosphere-continuum and flux dynamics and will thus affect forest ecosystems. Much work needs to be done to understand the distribution of ecosystem specific flow pathways and the characteristic timescales of water movement. Understanding the linkages and interactions between water use strategies, water storage and competition effects can thereby provide valuable knowledge about drought resilience of different tree stands.

 We assessed water fluxes and their water stable isotopy at high temporal and spatial resolution to evaluate ecohydrological processes and competition effects on water use strategies in a mixed forest in south-west Germany. Measurements in pure and mixed tree stands with two temperate tree species, European beech (Fagus sylvatica, n=18) and Norway spruce (Picea abies, n=18), include sap flow, stem water content, in-situ water isotopy, radial stem growth and climate conditions. Furthermore, a terrestrial lidar scan provided tree anatomical characteristics.  Our central hypothesis is that species identity and water competition between tree species is a major driver for ecohydrological flux dynamics. Thus, we aim to gain a comprehensive knowledge of water use strategies of the two dominating tree species and their competitive balance.

First results from the wet summer of 2021 indicated that, spruce trees showed lower sap flux densities in mixed stands compared to pure stands. Inversely, beech trees in mixture with spruce had higher sap flux densities than in pure stands. Although we only observed small species-specific differences in stem water content, a competitive impact could be shown on spruce trees by a reduced leaf area density in mixed stand trees. Dynamics in water isotopy of beech trees provided further insight in water use strategies between different stands. Future work will focus on exploring ecohydrological feedback processes and water transit times with high temporal resolution in situ isotope and sap flow measurements and labelling campaigns.

How to cite: Kinzinger, L., Mach, J., Haberstroh, S., Dumberger, S., Dubbert, M., Frey, J., Seeger, S., Seifert, T., Weiler, M., Orlowski, N., and Werner, C.: Quantifying intra- and interspecific competition effects on water use of different tree stands using sap flow, terrestrial lidar scan and advances in stable water isotopy, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8274, https://doi.org/10.5194/egusphere-egu22-8274, 2022.

Recent research highlights the ability of large trees to maintain transpiration during tree water stress, for example in summer dry periods, by using stored plant water held in the relaxed xylem. Use of soil water at different depths by shallow and deep roots according to soil water content may also vary by species and seasonal climatic conditions to maintain transpiration demands. Here we present daylight tree water usage measurements of mature oaks (Quercus robur L.) under future-forest CO₂ conditions, derived from compensation heat pulse (HPC) xylem sap flux transducers deployed at Birmingham Institute of Forest Research (BIFoR) Free-Air CO₂ Enrichment (FACE) forest in Staffordshire UK. We experienced longer dry periods in summers 2018 and 2019 and variable annual summer precipitation overall.  Xylem sap flux data is collected half-hourly from eighteen oak trees and a smaller number of subdominant trees (Acer pseudoplatanus and Crataegus monogyna) in nine experimental patches: 3 patches with elevated CO₂ infrastructure (eCO₂); 3 with infrastructure but ambient CO₂ (aCO₂); and 3 ‘ghosts’ (no treatment, no infrastructure).  Each tree has two probesets, E and W facing; long (7 cm 4-sensor) probes in oak and short (4cm 2-sensor) probes, to accommodate the smaller diameter subdominant trees, are deployed. We compare individual tree responses under the three treatments across the leaf-on seasons for early years of the FACE project 2017–2021. Between-individual within-species variability of summertime monthly average of daily daylight water usage in oak is linearly proportional to tree stem radius (ca. 2.9 litres per millimetre radius, range; 274mm ≥ radius ≤ 465 mm) at the point of probeset insertion ca. 1.1–1.3 m above ground level and oak responds sub-daily to solar radiation reduction events during cloud cover. Diurnal stem sap flux responses to canopy photosynthetic demand typically exhibit increased sap flux from dawn to around midday (UTC) and symmetrical decrease to dusk. We describe our continuing investigation of soil-plant-atmosphere flows, monitoring root-xylem-stomatal changes, and discuss how these results, from our tree-centred forest view, provide valuable new perspectives and help to improve our understanding of future-forest-water use at larger scales, contributing to development of more realistic ecohydrological vegetation, soil and landscape models.

How to cite: Quick, S., Curioni, G., Krause, S., and MacKenzie, A. Rob.: Xylem sap dynamics of 175-year-old Quercus robur under elevated CO2 at BIFoR FACE, UK, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7255, https://doi.org/10.5194/egusphere-egu22-7255, 2022.

In the past two decades, plant functional traits have become an important tool in ecology and biogeography for the study of consequences of changing plant composition on ecosystem functioning and the effect of environment factors on plant composition. Recently, considerable interest in the trait-based approach has arisen in palaeoecology as well. Applying the trait-based approach could offer a new way of interpreting pollen data, but the intricacies of using this method in palaeoecology are underexplored. Here we test the validity of the use of pollen records for plant functional composition reconstruction using modern pollen samples. Using a Bayesian approach for reconstructing plant trait composition from pollen records, we provide a robust method that can account for trait variability within pollen types. We apply this method to assess changes in plant functional composition over 10 000 years for 79 European sites with an agricultural history. We evaluate how agriculture and climate affect plant functional composition. We reveal selection of common traits across agricultural landscapes, with resource-acquisitive communities of low stature and seed mass dominating after the arrival of agriculture. Understanding these selection processes and trade-offs between traits will advance our understanding of the legacy of human impact on ecosystems functions.

How to cite: Veeken, A., Schrodt, F., McGowan, S., and Santos, M. J.: Pollen-based reconstruction reveals the impact of the onset of agriculture on plant functional trait composition, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2825, https://doi.org/10.5194/egusphere-egu22-2825, 2022.

Plants communicate information about their status, intra- and inter- plant, and with other ecosystem members, through the release of volatile organic compounds (VOCs). The effects of rising CO₂ in conjunction with ozone (O₃) on plant VOC emissions is not yet fully understood, but research suggests that some herbivore-induced VOCs are degraded by O₃, potentially reducing their signalling function. Furthermore, elevated CO₂ has been shown to attenuate induced VOC responses to herbivory in Brassica oleracea. 

We are using two tri-trophic model systems; black poplar [Populus nigra betulifolia], winter moth [Operophtera brumata] and a tachinid fly parasitoid of winter moth, Cyzenis albicans; and oil seed rape [Brassica napus], diamond back moth [Plutella xylostella] (DBM), and a parasitoid of DBM, braconid wasp, Cotesia plutella. Additionally, we are working within two ground-breaking facilities; the Birmingham Institute of Forest Research (BIFoR)’s free-air carbon enrichment (FACE) experiment, and University of Reading’s free-air diesel and ozone enrichment experiment. We will also collect some data from lab-based experiments.

Our project seeks to characterise the volatile organic compound (VOC) profiles emitted for both plants under herbivory, examine how these VOC profiles differ under combined elevated CO₂ and O₃, and explore whether changes to VOC profiles impact key ecological relationships, e.g, the ability of plants to signal to herbivore enemies.

How to cite: James, L., Pfrang, C., Girling, R., Hayward, S., and MacKenzie, R.: Characterising volatile organic compound emission changes in native black poplar under elevated carbon-dioxide (CO2), elevated ozone (O3) and herbivory, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4487, https://doi.org/10.5194/egusphere-egu22-4487, 2022.