In this session, we broadly explore the roles of temperature extremes, evaporative demand, and soil moisture in carbon, water, and energy relations, along with plant mortality across various spatial and temporal scales. We encourage submissions dealing with novel approaches for measuring and modeling plant and soil water status, responses to extreme conditions, and their impacts on ecosystem function. We invite contributions on these topics at scales ranging from individual plant tissues to entire ecosystems, applying experimental, observational, or modeling approaches and dealing with diverse disciplines such as plant physiology, community ecology, ecosystem ecology, land management, and biogeochemistry.
Forests are the primary terrestrial carbon sinks on Earth due to trees' unique capacity to absorb and store atmospheric carbon dioxide (CO2) through photosynthesis. However, increasing extreme hot-dry events, significantly contribute to global forest mortality. Severe soil drought can lead to xylem embolism, while heat can increase leaf transpiration and impair trees' capacity to cool their leaves, possibly leading to large-scale canopy mortality. Both extreme heat and water scarcity can accelerate widespread tree dieback and shift forests from carbon sinks to carbon sources. On the other hand, the continual rise in atmospheric CO2 from human activities may enhance plant net photosynthesis while reducing tree transpiration. Under elevated CO2 (eCO2) conditions, water use efficiency improves, which could, in turn, reduce the sensitivity of trees growing under elevated CO2 to depleted soil moisture levels. While the individual effects of heat, drought, and eCO2 have been studied, there is still a lack of critical data on how mature forests respond to the combined stress of eCO2 and hot droughts (both atmospheric and soil droughts). In this talk, I will review our current understanding and aim of identifying key knowledge gaps on the individual and combined impacts of heat, drought, and elevated CO2 on tree physiological responses. A better understanding of these interactions will improve the accuracy of current climate-vegetation models in predicting forest carbon dynamics under climate change.
How to cite: Gauthey, A.: Forest dynamics under climate change: the dual impacts of elevated carbon dioxide and hot-dry events on tree carbon and water relations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10517, https://doi.org/10.5194/egusphere-egu25-10517, 2025.
Increasing drought intensity, duration and frequency worldwide challenges tree health. In addition to the importance of drought resistance, post-drought recovery capacity is a vital determinant in tree growth and survival. However, the capacity of trees to recover from different types of drought stress remains largely unquantified. In this study, we applied three different drought treatments (short-term drought, long-term drought, and repeated drought) and a well-watered control to 3-year-old silver birch (Betula pendula) saplings growing in a greenhouse. Ecophysiological traits regarding key water- and carbon-relation processes such as sap flow, stem radial variation, leaf gas exchange, water potential, and leaf phenology were continuously measured throughout the drought-recovery process. The preliminary results indicate that water consumption, photosynthesis and tree growth were greatly diminished in all drought treatments, though their recovery capabilities and timing differed. None of the stressed treatments recovered to pre-drought status after re-watering, which might be attributed to their limited ability to repair xylem embolism and restore leaf area. The adjustment of tree-level leaf area emerged as a key strategy to cope with the drought, shedding light on traits other than physiology requiring consideration when studying drought resistance and resilience.
How to cite: Zhang, Y., Wang, Y., Oren, R., and Salmon, Y.: Does physiology alone explain Betula pendula recovery from drought?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3924, https://doi.org/10.5194/egusphere-egu25-3924, 2025.
The combination of higher air temperatures and lower precipitation has become increasingly frequent under global warming, potentially exacerbating their individual effects. Higher air temperatures constrain photosynthesis while simultaneously accelerating respiration, and might decrease tree net C uptake. Thermal acclimation may mitigate this negative effect, but its capacity to do so under concurrent soil drought remains uncertain.
Using a five-year open-top chamber experiment, we determined acclimation of leaf-level photosynthesis (thermal optimum Topt and rate Aopt) and respiration (rate at 25°C R25 and thermal sensitivity Q10) to chronic +5°C warming, soil drought, and their combination in European beech (Fagus sylvatica L.) and downy oak (Quercus pubescens Willd.) saplings. Using a process-based model, we evaluated the impacts of acclimation on plant-level net C uptake (Atot).
Our study showed that both species acclimated to warmer conditions by shifting their Topt to higher temperatures, but to a lower extent when combined with drought, and slightly reducing R25 and Q10. In contrast, drought reduced Topt (in oak), Aopt, and, to a lower extent, R25 and Q10 (in beech). However, despite these acclimation processes, Atot decreased drastically under warming and drought, mainly due to reduced plant leaf area. Our results suggest that, while photosynthetic and respiratory acclimation might moderate the adverse impacts of warming and soil drought on leaf-level C exchange, plant-level net C uptake may still decline in a persistently hotter and drier climate because of structural adjustments toward sparser canopies.
How to cite: Deluigi, J., Didion-Gency, M., Gisler, J., Mas, E., Mekarni, L., Poretti, A., Schaub, M., Vitasse, Y., and Bachofen, C.: Photosynthetic and respiratory acclimation cannot compensate reduced plant-level carbon uptake in beech and oak saplings under prolonged warming and drought, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3569, https://doi.org/10.5194/egusphere-egu25-3569, 2025.
Sun-Induced chlorophyll Fluorescence (SIF) is the most promising optical indicator of Gross Primary Production (GPP) in terrestrial ecosystems. However, the interpretation of SIF as a proxy of GPP is challenged when plants experience abiotic stress, particularly during extreme climatic events whose frequency is projected to increase in the future. Recently, the feasibility of canopy-scale active chlorophyll fluorescence measurements (LED-induced chlorophyll fluorescence), which directly measure the apparent fluorescence yield (FyieldLIF), has provided new perspectives on detecting the physiological responses of plants to abiotic stress. This study was conducted during summer 2022 European heat waves in a mixed temperate deciduous broadleaf forest, located in the Fontainebleau-Barbeau station (Integrated Carbon Observation System FR-Fon site), about 50 km South-East of Paris, France. Continuous measurements of carbon dioxide (CO2) and energy exchanges, SIF, FyieldLIF, and ancillary environmental variables were acquired. We investigated how atmospheric dryness, measured as Vapor Pressure Deficit (VPD), affected canopy chlorophyll fluorescence (both SIF and FyieldLIF) and GPP, as well as their relationships. Our results indicated that high VPD has a negative impact on GPP and FyieldLIF at both half-hourly and daily scales. In contrast, SIF exhibits a positive response to high VPD at the half-hourly scale, but this relationship reverses, showing a negative response at the daily scale. At the half-hourly scale, our results revealed a decrease of the correlation between SIF and GPP (R² decreased from 0.49 to 0.17) as atmospheric dryness increased. In contrast, the correlation between FyieldLIF and GPP strengthened significantly under the same conditions (R² increased from 0.07 to 0.43). However, at the daily scale, the correlations between SIF and GPP and between FyieldLIF and GPP showed an overall increase, suggesting a time-scale-dependent response of these relationships to atmospheric dryness. This study also highlighted the advantages of FyieldLIF over SIF in detecting plant responses to high atmospheric dryness. This underlines the potential of canopy-level active chlorophyll fluorescence measurements for understanding and quantifying the nature of the relationship between canopy chlorophyll fluorescence and photosynthesis in ecosystems under extreme climatic conditions.
How to cite: Li, Z., Hmimina, G., Latouche, G., Berveiller, D., Ounis, A., Goulas, Y., and Soudani, K.: Atmospheric dryness effects on canopy chlorophyll fluorescence and GPP in a deciduous forest during heat waves, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5263, https://doi.org/10.5194/egusphere-egu25-5263, 2025.
The efficiency-safety tradeoff has been thoroughly investigated in plants, especially concerning their capacity to transport water and avoid embolism. Stomatal regulation is a vital plant behaviour to respond to soil and atmospheric water limitation. Recently, a stomatal efficiency-safety tradeoff was reported where plants with higher maximum stomatal conductance (gmax) exhibited greater sensitivity to stomatal closure during soil drying, that is, less negative leaf water potential at 50% gmax (ψgs50). However, the underlying mechanism of this gmax-ψgs50 tradeoff remains unknown. Here, we utilized a soil-plant hydraulic model, in which stomatal closure is triggered by nonlinearity in soil-plant hydraulics, to investigate such tradeoff. Our simulations show that increasing gmax is aligned with less negative ψgs50. Plants with higher gmax (also higher transpiration) require larger quantities of water to be moved across the rhizosphere, which results in a precipitous decrease in water potential at the soil-root interface, and therefore in the leaves. We demonstrated that the gmax-ψgs50 tradeoff can be predicted based on soil-plant hydraulics, and is impacted by plant hydraulic properties, such as plant hydraulic conductance, active root length and embolism resistance. We conclude that plants may therefore adjust their growth and/or their hydraulic properties to adapt to contrasting habitats and climate conditions.
How to cite: Cai, G., Carminati, A., Gleason, S., Javaux, M., and Ahmed, M.: Soil-plant hydraulics explain stomatal efficiency-safety tradeoff, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6066, https://doi.org/10.5194/egusphere-egu25-6066, 2025.
Hydraulic status plays a large role in leaf thermoregulation, which is important for maintaining leaves below thermal thresholds during heatwaves. Still, little is known about how acclimation to warming and drought may affect trees’ abilities to avoid critical temperature thresholds. Using a five-year open-top chamber experiment, we studied the single and interactive effects of heat and soil drought on leaf temperature regulation, heat tolerance, hydraulic status, and gas exchange in two temperate tree species with contrasting water management strategies: common beech (Fagus sylvatica) and pubescent oak (Quercus pubescens). Drought-exposed trees were less homeothermic than control and warmed trees, leading to larger leaf-to-air temperature differentials and greater leaf temperature maxima, especially in hot-drought conditions. During the peak summer heat (ambient temperature reaching > 40°C), gas exchange and hydraulic safety margins in drought-exposed trees (including with added warming) were strongly reduced, particularly in beech, compared to the control and heat exposure alone. Indeed, drought induced extreme hydraulic stress, which limited the trees' ability to preserve thermal safety margins. Consequently, despite acclimation of heat tolerance to leaf temperature maxima, drought, and especially hot drought, led to narrower (even negative) leaf thermal safety margins, widespread leaf scorching, and early senescence. Our results show that while thermoregulation acclimates to increased temperatures, drought remains the dominant driver of canopy damage, which may be exacerbated when combined with heat waves.
How to cite: Kullberg, A., Milano, A., Bergström, M., Juillard, T., Gisler, J., and Schaub, M.: Hydraulic stress diminishes acclimation of leaf thermoregulation in European trees exposed to hot-drought., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6182, https://doi.org/10.5194/egusphere-egu25-6182, 2025.
Tree seedlings have their leaves very close to the ground and their roots are very shallow. They therefore experience more severe heat and water stress during hot summers than mature trees. As droughts and heatwaves increase in severity and frequency, the growth and survival of tree seedlings thus become more difficult, impairing forest regeneration in many regions. In response, forest managers are increasingly shifting from thinning regimes that promote light availability and seedling growth to regimes that promote seedling survival and the buffering of climate extremes. However, the identification of such thinning regimes is not trivial because the mechanisms underpinning the impact of canopy cover on understory microclimate, although all well understood, can have opposite effects on climate extremes. In particular, wind attenuation and water consumption by the remaining adult trees can sometimes create conditions for seedlings in the understory hotter and drier than in an open field. This has led researchers to hypothesize the existence of thinning thresholds beyond which forest canopies transition from buffering to amplifying climate extremes. Metrics such as leaf area index (LAI), crown aggregation and canopy height have emerged as critical factors, as well as other local factors such as species composition or water availability. Here, we use a physics-based model of forest hydrology, physiology and microclimate (MuSICA), in combination with microclimate observations from a variety of forest types across Europe, to address the following questions. (1) Is the threshold in LAI and/or crown aggregation below which summertime temperature and evaporative demand become amplified in the understory generic, or is this threshold site-specific? (2) How does understory microclimate evolve during heatwaves depending on the structure of the canopy above and the duration of the heatwave? (3) How does this translate in terms of plant water and heat stress for understory species?
How to cite: Bouwen, K., Charru, M., Domec, J.-C., Lemaire-Patin, R., and Ogée, J.: Are microclimate extremes always buffered in the understory, irrespective of tree density? Consequences for tree seedling survival., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6648, https://doi.org/10.5194/egusphere-egu25-6648, 2025.
In Central Europe, re-occurring compound events (drought and heat) have caused substantial damage to forest ecosystems with significant changes in carbon and water fluxes. Here we investigate the impact of the 2018 compound event and following drought years on net ecosystem carbon exchange (NEE) and vegetation dynamics of a Scots Pine (Pinus sylvestris) forest in SW-Germany (ICOS Site DE-Har, Hartheim, Germany). The compound event of 2018 caused severe hydraulic damage to trees, which led to high mortality rates of Scots Pine trees (>60% until 2024). While the forest ecosystem was a strong annual carbon sink in the past (up to -603 g C m-2 year-1), the ecosystem shifted to almost carbon neutral in a cold and wet year (2021). All other years since 2018 were hotter (and drier) than the long-term average, which led, in combination with legacy effects of 2018, to an annual carbon release with maximum values of +298 ± 12 g C m-2 year-1 in 2022. These values correspond to a difference in NEE of up to +901 g C m-2 year-1compared to conditions before 2018.
Concurrently, the vegetation composition of the ecosystem is slowly shifting from an evergreen coniferous forest to a mixed/deciduous forest. Deciduous trees in the understory expressed a higher resilience (higher water potentials and sap flux density) towards compound events compared to Scots Pine, potentially due to microclimatic buffering effects. This vegetation shift was clearly visible in the enhanced vegetation index (EVI) of the site, which increased in summer and decreased in winter, indicating an ongoing shift in canopy type and greenness towards deciduous species since 2018.
In conclusion, the compound event of 2018 caused significant legacy effects at the ecosystem and community scales in the studied Scots Pine forest. These effects were further exacerbated by recurrent atmospheric and edaphic drought conditions after 2018, which led to a significant ecosystem carbon release since then. If climate extremes do occur with the same frequency as in 2018-2024, this could significantly delay or even prevent ecosystem recovery, putting more ecosystems in Central Europe at risk.
How to cite: Haberstroh, S., Sulzer, M., Scarpa, F., Plapp, T., Christen, A., and Werner, C.: Persistent drought legacy effects in a Scots Pine forest after years of concurrent drought and heat, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16453, https://doi.org/10.5194/egusphere-egu25-16453, 2025.
Drought-induced mortality is expected to cause substantial biomass loss in the Amazon Basin. However, responses by rain forest to prolonged drought remain largely unknown. Critically, how drought impacts individual trees over decades, whilst potential changes in forest structure alter competition for resources, remain unreported for any tropical forest globally. We demonstrate that an Amazonian rain forest subjected to more than two decades of drought at a throughfall-exclusion experiment reached long-term eco-hydrological stability. The stabilisation was largely driven by ecosystem-level structural changes that resulted in the remaining trees to no longer experiencing drought stress. The loss of the largest trees to drought-related mortality during the first 15 years of the experiment led to increasing water availability for the remaining trees, facilitating a stabilisation in biomass in the last seven years of the experiment. The elimination of water stress led to hydraulic variables commonly associated with physiological stress, such as leaf water potential, sap flow, and tissue water content to be equal to those in corresponding non-droughted control forest, indicating hydraulic homeostasis. This work reveals that significant resilience to persistent (multi-decadal) soil drought in tropical rain forest. The resilience emerges from structural feedbacks at ecosystem scale that prevent drought-induced collapse, whilst also resulting in a forest with reduced biomass and lower but positive net wood productivity.
How to cite: Sanchez Martinez, P., R. Martius, L., Bittencourt, P., Silva, M., Binks, O., Coughlin, I., Negrão-Rodrigues, V., Athaydes Silva Junior, J., Da Costa, A. C., Rowland, L., Mencuccini, M., and Meir, P.: The fate of Amazon rain forests under drought: collapse or stabilisation? , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1566, https://doi.org/10.5194/egusphere-egu25-1566, 2025.
Increasing drought extremes represent a major stress on ecosystem function through direct impacts on critical plant physiological processes such as transpiration and growth. While the immediate effects of drought are well-documented, vegetation recovery processes and associated time lags in ecosystem function remain poorly understood due to the scarcity of the requisite plant physiological data time series.
This study relates transpiration to evaporative demand, soil moisture, and tree growth during the recovery period following a multiyear drought cluster, centred on the extreme in 2018. High-resolution time-series data were collected with dendrometers, stem sap flow, and soil water potential sensors in a Fagus sylvatica stand located in Pruhonice, Czech Republic.
We found unexpected evidence that transpiration may be xylem-limited during drought recovery by examining the different patterns in transpiration between 2021 (the first major drought recover year at our site) and 2022. During the 2022 (control) season, canopy transpiration achieved a balance between potential evapotranspiration and soil moisture, independent of seasonal stem growth. By contrast, in 2021, we observed an unexpected gradual increase in transpiration over the growing season corresponding to stem incremental growth but independent of soil moisture or potential transpiration. These results indicate that post-drought recovery may involve a feedback loop between growth and transpiration until plants overcome xylem-limitation following drought.
In sum, this study observed surprisingly strong plant carbon-hydraulic feedback during drought recovery. If confirmed, this feedback may prove key to predicting the pathways of plant and soil water status following drought events and their impacts on ecosystem function. The findings also suggest a possibility that plants’ ability to break out of the feedback loop may be a key trait to track when choosing suitable species for future forest management.
How to cite: Liu, Q., Weger, J., Weiser, M., Sipek, V., and Bouda, M.: Life after drought: breaking out of the transpiration-assimilation feedback loop , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16659, https://doi.org/10.5194/egusphere-egu25-16659, 2025.
Drought events are threatening forest ecosystems worldwide and are also expected to increase in intensity and frequency in the future. Around Europe, multiple experimental sites have been set up to investigate the impacts of drought, for example, by excluding rainfall from trees. Over the years, these experiments have increasingly incorporated high-resolution temporal sensors. These sensors collect data on tree physiology—such as changes in diameter, sap flow, and stem water potential — at intervals as frequent as once every 30 minutes. While these experiments provide valuable insights into the impacts of drought on tree function, they are typically limited to the specific environmental conditions and species present at the study site.
Vegetation modelling offers a way to generalise from experiments. Multiple state-of-the-art vegetation models now incorporate plant hydraulics which (1) allows for simulating water movement throughout whole trees in detail and (2) introduces a hydraulic failure-based mortality process that describes how trees may succumb to extreme drought stress. However, representing plant hydraulic processes comes at the expense of introducing additional, often difficult to constrain, parameters to models.
Here, we show how new experimental data can be integrated into process-based vegetation modelling. More specifically, we use high-resolution sapflow and stem water potential data to effectively constrain the most crucial plant hydraulic parameters of the terrestrial biosphere model QUINCY: saturated xylem hydraulic conductivity, and stem and leaf water storage capacitance.
We further apply QUINCY to several FLUXNET sites and show that our parameterizations are consistent across different environmental conditions. We also discuss how the incorporation of hydraulic failure-based mortality mechanisms may alter modelled carbon dynamics and how future experiments could help reduce uncertainty in modelling drought-induced mortality.
How to cite: Papastefanou, P., Donfack Somenguem, L., Klosterhalfen, A., Magh, R.-K., Paligi, S. S., Sabot, M., Schellenberg, K., and Zaehle, S.: Drought stress in European forests: Integrating novel experimental data to improve vegetation modelling , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13514, https://doi.org/10.5194/egusphere-egu25-13514, 2025.
Despite their broad climatic and geographic ranges and dominant ecosystem roles across boreal, alpine and arctic vegetation zones; dwarf shrubs can be sensitive to climatic changes, in particular shifting thermal regimes. But questions remain as to the macro-or micro-climatic conditions we should focus upon when considering these changing plant-climate relationships. As a case study highlighting how microclimate insights may contribute at scales from the field through to land-surface models, in the DURIN project we explore how thermal microclimate measures at plant- and leaf-levels can be used to better inform models regarding the thermal tolerance limits of leaves. At high-latitudes, increasing summer heat extremes and aseasonal freezing events associated with changing snowpack dynamics, will expose dwarf-shrubs to potentially stressful conditions for photosynthesis and carbon gain. We explore thermal damage by quantifying temperature at which there is a 50% decline in the maximum quantum yield of photosystem II (FV/FM), for a range of dwarf shrub species growing across habitats and bioclimatic zones. We then compare the extent of photosystem damage to various estimates of temperature extremes as derived from plant-level climate sourced from TOMST loggers, FLIR imagery, and leaf-level thermocouples deployed in dwarf-shrub and non-dwarf-shrub plots. The time-series of fine-scale characterization of dwarf-shrub microclimates in-situ will be correlated with downscaled microclimate estimates from NicheMapR, along with nearby weather station data to highlight the discrepancies between macro- and microclimates. These comparisons allow us to additionally ask fundamental questions about the ways in which we should assess thermal tolerance taking into greater consideration methods for quantifying heat stress. Classical assays of photosynthetic thermal tolerance limits have focused on singular and short exposure times to temperature stress, but increasingly the field is moving towards providing more biologically meaningful insights into thermal tolerance exposures, enabling us to better define and experimentally impose thermal stress events. An emerging discussion surrounds the use of the thermal death time framework, where cumulative thermal stress is applied, e.g. a range of exposure times at varying temperatures. Our work will help develop protocols for the thermal death time framework which requires a more nuanced understanding of thermal stress events at plant and leaf-level, integrating our micro-and macro-climate insights.
How to cite: Geange, S., Torre, A., Sangha, S., Carteron, V., Oudina, Y., Touriere, M., Birkeli, K., Garen, J., Bison, N., Michlaetz, S., Tang, H., Egelkraut, D., Halbritter, A., and Vandvik, V.: Temperature extremes from a leaf perspective: Micro- vs macro-climate predictors of dwarf-shrub thermal tolerance limits, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14439, https://doi.org/10.5194/egusphere-egu25-14439, 2025.
By 2024, anthropogenic greenhouse gas emissions have increased global average surface temperatures 1.55 °C above pre-industrial levels. This has led to an increase in both the intensity and frequency of heat waves. In recent years, it has been shown that the strong relationship (e.g., the coupling) between net photosynthetic CO2 assimilation (Anet) and stomatal conductance (gs) is decreased or even lost at high temperatures (Diao et al., 2024; Marchin et al., 2023). However, the isolated effect of environmental drivers (e.g. air temperature, vapour pressure deficit; VPDair) and the underlying plant physiological mechanisms are not yet fully understood (Mills et al., 2024).
To improve our understanding why at high temperatures gs continues to increase while Anet decreases, we conducted a climate chamber experiment with 3 temperate (Alnus cordata, Acer platanoides, Phillyrea angustifolia) and 3 tropical (Terminalia microcarpa, Terma tomentosa, Syzygium jambos) tree species. In one chamber, we increased the air temperature (Tair) from 20 to 40 °C in 5 °C steps (2 days at every temperature) while keeping the VPDair at 1.2 kPa. In the second chamber, we increased Tair the same way, but simultaneously increased VPDair every step from 1.2 to 6 kPa. One subset per chamber was kept well-watered (e.g. at capacity; ~35 vol-%), while in the other subset the trees were exposed to soil drought (~8 vol-%). Every second day, we conducted leaf gas exchange measurements.
Across all species, we observed gs to continue to increase while Anet decreased (but never reached 0 or negative values) at high temperatures above 35 °C under constant VPD, while increasing VPD maintained the coupling between the two by decreasing gs. However, the transpiration rate (E) showed the same pattern of decoupling under both VPD regimes. Since E is directly driven by gs and VPD, plants need to upregulate gs in order to upregulate E if VPD is too low. While E is important for the regulation of leaf temperature, it is also crucial for other plant physiological processes. We speculate that another reason for increasing E may be that E drives sap flow, which is important for the internal transport and distribution of nutrients, O2 and CO2 in plants. Thus, an increased sap flow might be crucial to sustain tree functioning during high-temperature driven periods of accelerated metabolic activity. Future specifically designed experiments are needed to simultaneously investigate plant physiological responses in different tissues as well as at the whole plant level.
Diao, H., Cernusak, L.A., Saurer, M., Gessler, A., Siegwolf, R.T.W., Lehmann, M.M., 2024. Uncoupling of stomatal conductance and photosynthesis at high temperatures: mechanistic insights from online stable isotope techniques. New Phytologist 241, 2366–2378. https://doi.org/10.1111/nph.19558
Marchin, R.M., Medlyn, B.E., Tjoelker, M.G., Ellsworth, D.S., 2023. Decoupling between stomatal conductance and photosynthesis occurs under extreme heat in broadleaf tree species regardless of water access. Global Change Biology gcb.16929. https://doi.org/10.1111/gcb.16929
Mills, C., Bartlett, M.K., Buckley, T.N., 2024. The poorly‐explored stomatal response to temperature at constant evaporative.pdf. Plant, Cell & Environment.
How to cite: Schuler, P., Juillard, T., Hoch, G., Kahmen, A., and Didion-Gency, M.: Stomatal decoupling: New insights into environmental drivers and underlying physiological mechanisms during simulated heatwaves in temperate and tropical tree species, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6257, https://doi.org/10.5194/egusphere-egu25-6257, 2025.
A restricted transpiration rate (TR) response to rising vapor pressure deficit (VPD) is often considered a useful ‘trait’ for soil water conservation under circumstances of terminal drought. The plant hydraulic conductance (Kp) is debated to play a role in shaping water use under atmospheric drought. Here, we investigated whether a limited TR response to elevated VPD defines as a ‘trait’ (or whether it is rather the product of several interacting traits). Further, we aimed to identify which role limitations in Kp play for plant water use regulation during increasing VPD. To achieve that, we tested whether a reduction in Kp would lead to an altered TR-VPD response.
We exposed five individual maize (Zea mays L.) plants to rising VPD up to 2.3 kPa in a climate chamber in wet soil conditions while simultaneously and continuously monitoring whole plant TR with balances and stem water potential (Ψstem) dynamics using optical dendrometers. Kp was calculated from the slope between TR and Ψstem in the linear part of the relation. To achieve a reduction in Kp, we destructively cut the root system in several places. The TR-VPD profile was measured on: (1) intact plants, (2) plants of which the root system was cut, and (3) disturbed plants after five days of recovery.
In undisturbed conditions, plants transpired linearly until a VPD of 0.9 kPa, upon which TR increased to a lesser extent. In damaged plants, TR was restricted at comparable VPD but subsequently decreased with rising VPD beyond this threshold. Despite strong stomatal regulation, Ψstem declined further and became more negative compared to undisturbed plants. Kp was reduced by merely 5% due to root cutting. Upon five days of recovery, plants transpired at a relatively lower initial rate compared to undisturbed conditions, but linearly with rising VPD.
Root cutting and recovery created new phenotypes of the same plant in terms of water-use regulation during atmospheric drought, implying (i) the difficulty of phenotyping for, and (ii) the context-dependency of water use responses. Root cutting leading only to a minor decrease in Kp indicates that root length does not linearly correlate with Kp. Given the minimal decline in Kp, we suggest that the strong change in the TR-VPD response that was associated with root cutting might be attributed to changes in the plant’s capacitive storage, modulating short-term Ψstem dynamics and stomatal response.
How to cite: Köhler, T., Bourbia, I., Ahmed, M., and Brodribb, T.: The role of the plant hydraulic conductance for the transpiration rate response to increasing VPD, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16732, https://doi.org/10.5194/egusphere-egu25-16732, 2025.
Both native and invasive plants can adjust photosynthesis and respiration when exposed to warmer temperatures. However, it is uncertain if invasive plants are more plastic and exhibit higher acclimation to rising temperatures than native ones, a trait that could contribute to their invasive behavior in novel environments.
We compared the capacity of a highly invasive palm in central Europe (Trachycarpus fortunei) and two native co-occurring species (Ilex aquifolium and Tilia cordata) to acclimate photosynthesis and respiration to air temperature changes using a two-year-long transplant experiment across Europe (mean temperatures ranging from 8.4 to 21.8°C). We measured the optimal temperature of photosynthesis (Topt), the assimilation at optimal temperature (Aopt), the thermal breath of photosynthesis (T80), the respiration at 25°C (R25), the temperature sensitivity of respiration (Q10), and simulated the whole-plant carbon budget.
For all species, Topt, Aopt, and T80 increased with warming, while R25 decreased in the native species and Q10 decreased in the invasive species only. Consequently, acclimation enhanced the carbon budget of the invasive and native plants in the warm and hot sites. The invasive palm had a similar or lower acclimation capacity than other species and a lower but constant carbon budget across the European temperature gradient. Our work reveals that not all invasive plants exhibit greater photosynthetic plasticity than native ones, suggesting that temperature-driven enhancement of their carbon budget may play a limited role in future invasion processes.
How to cite: Juillard, T., Conedera, M., Dumont, M., Limousin, J.-M., Milano, A., Pezzatti, G. B., ViIagrosa, A., and Bachofen, C.: Thermal acclimation fails to confer a carbon budget advantage to invasive species over natives, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6279, https://doi.org/10.5194/egusphere-egu25-6279, 2025.
In this study, we examine tree resilience in response to compound atmospheric and soil droughts, using dendrometer-based stem diameter measurements in a semi-arid pine forest. Our main question is: what is the differential impact of atmospheric and soil drought on plant growth and resilience? By analyzing data from an irrigation experiment on mature pine trees, we developed new tools for characterizing heatwaves, and introduced novel resilience indices, especially designed for robust evaluation of recovery and resistance in trees experiencing short and intense heatwaves against the backdrop of a long soil drought. Our findings show that irrigation effectively shielded trees from the negative impacts of heatwaves. Non-irrigated trees exhibited a significant decline in resilience during the dry season, primarily during compound droughts, which irrigated trees did not experience. Following the beginning of the wet season, the resilience of non-irrigated trees increased rapidly, matching that of the irrigated trees, suggesting minimal compromise in hydraulic functioning. These findings have significant implications for understanding forest resilience in the face of escalating climate change and provide practical tools for real-time monitoring and assessment.
How to cite: Feuer, E., Preisler, Y., Rotenberg, E., Yakir, D., and Mau, Y.: Dry Heatwaves Alone Do Not Reduce Tree Resilience, but Their Compounding with Soil Drought Does, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15076, https://doi.org/10.5194/egusphere-egu25-15076, 2025.
Urban trees cool their environment by transpiration (latent heat flux, LE) and shading, modifying thereby the energy budget and alleviating urban heat. However, the cooling effect from LE may be critically reduced during heatwaves, when trees reduce stomatal conductance (gS) to prevent hydraulic dysfunctions. Recent advances in our understanding of stomatal behaviour under high temperatures indicate that gS may still be maintained during extreme heat to allow canopy cooling, but implications for urban heat stress mitigation remain elusive.
We continuously recorded sap flow on eight Platanus x acerifolia trees in Geneva to assess LE and canopy conductance (GAsw) during the summer of 2023, which was characterised by two record-breaking heatwaves. We further repeatedly assessed leaf water potentials at pre-dawn and midday (Ψpre, Ψmid), stomatal conductance (gS), and leaf, canopy, and ground surface temperatures in shaded and sunlit parts around the trees (Tleaf, Tcan, Tsurf). Using the ecohydrological model UT&C, we determined the total energy budget of the urban square, and assessed whether LE predictions match empirical measurements during heatwaves.
We found that despite prolonged heatwaves with air temperature (Tair) reaching 39.1 ºC, trees were only marginally water stressed, with Ψmid mostly above -1.7 MPa, and continued transpiring throughout the day up to 37.1 kg h-1 (i.e. LE of 25.3 kW). Despite reduced GAsw measured LE was similar during heatwaves (i.e., Tair> 30 ºC) as during cooler periods and accounted for approximately 34 % of the urban heating by incoming solar radiation (Q*) throughout the season. In contrast, LE model predictions showed a marked decrease of urban cooling during heatwaves, thereby underestimating actual tree transpiration cooling.
Despite unprecedentedly high Tair during two summer heatwaves, trees maintained high transpiration, and thereby efficiently cooled the urban environment. Measured LE at Tair above 30 ºC surpassed model estimations due to continued tree transpiration. Consequently, actual cooling effects of urban trees during heatwaves might be considerably underestimated by current model predictions. Cities with intermittent heatwaves may thus continue to rely on effective vegetation cooling by transpiration.
How to cite: Bachofen, C., Peillon, M., Meili, N., and Bourgeois, I.: High tree transpiration despite extreme summer heatwaves supports atmospheric cooling in urban systems, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5923, https://doi.org/10.5194/egusphere-egu25-5923, 2025.
In the face of climate change, droughts are becoming more frequent and intense, exposing trees to ever-increasing physiological stress. Despite extensive research, the effects of recurrent droughts on tree carbon and water relations remain poorly understood, particularly in mountain forests. At a subalpine Long-Term Ecological Research (LTER) site in the Austrian Alps, we investigated the impacts of eight years of recurrent summer droughts on two conifer species—larch (Larix decidua) and spruce (Picea abies). Using comprehensive dendrometer and xylem sap flow data from 2021–2024, encompassing three years of drought followed by one year of recovery, we tested the following hypotheses: (i) recurrent droughts amplify drought responses of radial tree growth dynamics and water use, (ii) drought history causes lagged responses on growth dynamics and water use during a recovery year, with larch exhibiting greater resilience. Our preliminary findings reveal significant drought-induced reductions in sap flow, as well as in mean and maximum growth rates for both species during the treatment years. Yet, contrary to our expectations, multiple recurrent droughts did not significantly amplify the growth and water use sensitivity of trees at this subalpine site. During the recovery year, sap flow did not show legacy effects for either species; however, growth rates remained consistently suppressed, most notably in larch. Thus, our results suggest that although recurrent summer droughts do not have any lagged effects on water use dynamics in a recovery year, legacies lead to major reductions in growth, particularly in larch, which may be less resilient than expected.
How to cite: Tuñas Corzón, Á., Hwang, B., Bär, A., Wieser, G., Oberhuber, W., Mayr, S., and Bahn, M.: Impact of Recurrent Droughts on the Water Use and Growth Dynamics of Larch and Spruce: insights from a long-term experiment in the Austrian Alps, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18776, https://doi.org/10.5194/egusphere-egu25-18776, 2025.
Forests play a vital role in the earth’s ecosystems by regulating global water and carbon cycles and carbon assimilation. While significant advancements have been made in understanding the impacts of drought on tree physiology and gas exchange, the extent to which alleviation of soil drought mitigates the impact of high vapor pressure deficit (VPD) on leaf net photosynthesis (Anet) remains unclear. In a six-year study of soil drought alleviation (SDA; using dry-season supplement irrigation), we investigated its mitigating effects on the response of branch-scale net photosynthesis (Anet) to high atmospheric drought in a mature pine forest (Pinus halepensis). We demonstrate that while SDA improves Anet response to VPD, VPD remains a major factor in shaping the daily Anet cycle and overall tree productivity. In fact, in summer, SDA trees show an early Anet peak, followed by a sharp decrease reflecting a relative sensitivity to VPD greater than that of the soil-droughted (SD; control) trees. Specifically, we show peak Anet of around 7 AM and 9 AM in the SD and SDA trees, compared to ~noon in winter. The data also indicate that increasing VPD above a threshold of ~4 kPa, the SDA trees show enhanced sensitivity to VPD, shaping the above-noted daily peak and limiting productivity. The analysis of the partial dependence of Anet on key microclimatic variables and soil moisture content, using generalized additive models (GAMs), confirmed that the branch-scale Anet in the SDA trees improved under VPD only up to 4 kPa compared to SD trees. Above this apparent threshold, Anet in SDA trees declined sharply, associated with reduced stomatal conductance, and with increased respiration due to the elevated temperatures at these times. Further analysis, across the entire observed VPD range, showed that SDA trees do, in fact, have a greater sensitivity of Anet to VPD (and in particular to extreme atmospheric drought). The results indicated that while SDA effectively buffers trees from moderate atmospheric drought, it does not provide efficient mitigation towards extreme conditions. Our findings underscore the complex interplay between soil and atmospheric drought impacts in shaping tree physiological responses in pine forests, offering a important basis for predicting their response to different climate change scenarios.
How to cite: Nemera, D.-B., Rotenberg, E., Markos, N., Preisler, Y., Oz, I., Muller, J., and Yakir, D.: Differential effects of Soil Drought Alleviation on mitigating the Effects of Atmospheric Drought in mature Pine Forest Trees, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20405, https://doi.org/10.5194/egusphere-egu25-20405, 2025.
Marine heatwaves (MHWs) are increasing in frequency and intensity as part of climate change, yet their impact on marine angiosperms is poorly known. The present work evaluated the effects of a simulated spring-like MHW on the physiology and morphology of the seagrass Cymodocea nodosa, a temperate-warm adapted species. C. nodosa shoots were transplanted into a mesocosm facility. Water temperature was raised gradually from 20 to 28 °C, kept for 7 days at 28 °C, cooled down back to 20 °C and then maintained at 20 °C during an 8-day recovery period. The photosynthetic performance, antioxidative stress level and area/dry weight ratio of the plant’s leaves were investigated at the end of the heatwave and after the recovery period. The effective quantum yield of photosystem II increased during the heatwave and was higher in the mature parts of the leaves, allowing the plants to maintain their photosynthetic activity at the control level. Negative effects on the photosynthetic performance and leaf biomass of C. nodosa were observed during the recovery period. No significant oxidative stress was observed throughout the experiment. Although C. nodosa showed a relative tolerance to MHWs compared to other species, results show that the negative effects of a MHW may only become evident in the aftermath of the heatwave peak. In the Ria Formosa coastal lagoon, southern Portugal, C. nodosa is likely to be negatively affected by recurrent MHWs in the forecasted climate change scenarios, threatening the perennity of seagrass meadows’ ecosystems.
How to cite: Deguette, A., Barrote, I., and Silva, J.: Marine heatwaves: physiological and morphological effects on the seagrass Cymodocea nodosa, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-92, https://doi.org/10.5194/egusphere-egu25-92, 2025.
Spring freezing is an unforgiving stress for young leaves, often leading to death, with consequences for tree productivity and survival. With an increasingly unpredictable climate leading to more spring freezing events, it is important the we understand how freezing damages young leaf tissue. While both the plant water transport system and living tissues are vulnerable to freezing, we do not know whether damage to one or both of these systems causes death in young leaves exposed to freezing and thawing. Whole saplings of Liriodendron tulipifera were exposed to freezing and thawing trajectories designed to mimic natural spring freezes. We visualised freezing damage to the water transport system (xylem embolism) and living tissues (mesophyll freezing, decline in chlorophyll fluorescence). We 1.) provide the first visualisation of freeze-thaw embolism in leaves and compare this to drought-embolism, 2.) reveal a predictable progression of ice formation within the mesophyll which is strongly influenced by leaf vein architecture, notably the presence or absence of bundle-sheath extensions, and 3.) show that freeze-thaw embolism occurs only in the largest vein orders where mean vessel diameter exceeds 30µm. With evidence of both freeze-thaw embolism and damage to photosynthetic tissue, we conclude that this dual-mode lethality may be common among other wide-vesseled angiosperm-leaves, potentially playing a role in limiting tree distributions, and show that bundle-sheath extensions may stall or even prevent freezing spread.
How to cite: Johnson, K., Scherer, M., Gerber, D., Style, R., Dufresne, E., and Brodersen, C.: Ice and air: Visualisation of freeze-thaw embolism and freezing spread in young L. tulipifera leaves , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3416, https://doi.org/10.5194/egusphere-egu25-3416, 2025.
The occurrence of flood events in wetlands has a significant impact on ecosystem dynamics, particularly regarding carbon cycling and storage. While there is a consensus that an understanding of the dynamics of restored wetlands is essential for the mitigation of climate change, the potential for carbon sequestration in floodplain grasslands remains understudied. Two floods in June and July 2024 in a restored floodplain grassland in Austria, exhibited distinct ecosystem responses, notably in gross primary productivity (GPP), net ecosystem exchange (NEE), and ecosystem respiration (Reco), as well as methane exchange, measured via the eddy covariance (EC) method with open path gas analysers (LI-7500 DS and LI-7700, LI-COR Biosciences, Lincoln, NE, USA).
Following the floods, carbon uptake significantly declined due to complete submersion of around 2m water levels above grounds continuing for six days for the first flood and 1m lasting three days for the second flood. However, ecosystem recovery varied, with slower recovery after the June flood. Sediment deposition during the first flood in June hindered photosynthesis in older plant parts, as evidenced by a brown sediment layer, while new growth remained green and photosynthetically active. This sediment layer contributed to reduced GPP during recovery. Conversely, the sediment-light July flood caused only a brief decline in NEE and GPP, with rapid recovery and no sediment deposition. The contrasting sediment loads stemmed from the floods’ origins: the first from sediment-heavy Danube backwater following upstream precipitation and the second from a sediment-light sluice opening on the Morava River. These differences explain the varying impacts on plant vitality, photosynthesis, and CO₂ exchange. One month after the first flood net CO₂ uptake remained below pre-flood levels, reflecting ecosystem stress and adaptation. In addition, climatic conditions also played a role in ecosystem responses. The favourable temperatures and abundant rainfall in May 2024 provided an environment conducive to plant growth. However, heat and the natural decline of the growing period in July led to an exacerbation of the flood impacts. These differing growth stages likely contributed to the varying plant sensitivities to the flood events, intensifying GPP reductions in response to sediment transport. The prolonged presence of water bodies during recovery, with slow withdrawal and evaporation, further influenced evapotranspiration dynamics during flooding.
These findings indicate that sediment transport dynamics during floods can significantly influence plant vitality and photosynthesis, with implications for ecosystem CO₂ exchange. The interplay between sediment deposition, plant recovery, and flood timing underscores the necessity for further research to elucidate these processes and their role in carbon cycling, as well as ecosystem resilience and vulnerability in floodplain grasslands. This study illuminates the intricate interactions between hydrology, plant processes, and carbon cycling in floodplain grasslands under climatic extremes by linking sediment dynamics with ecosystem recovery.
Funded by the European Union. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or CINEA. Neither the European Union nor the granting authority can be held responsible for them.
How to cite: Lindenberger, A., Rauch, H. P., Kasak, K., and von der Thannen, M.: The role of sediment transport in regulating ecosystem CO₂ fluxes during flood events in a restored floodplain grassland, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2787, https://doi.org/10.5194/egusphere-egu25-2787, 2025.
Climate change is leading to rising temperatures and erratic rainfall patterns. Higher temperatures in combination with changes in frequency and intensity of precipitation have a strong effect on physiological processes in trees. For central european forest ecosystems higher frequency of droughts is predicted, which could lead to increased forest decline and tree mortality rates particularly for drought-sensitive species such as european beech (Fagus sylvatica L.; Dolschak et al., 2019). To assess the factors affecting beech tree growth in a changing climate, a better understanding of the coping mechanisms of beech forest under drought conditions is needed. How the relationship between tree growth and the site water balance is altered due to changing climatic conditions remains unclear. To improve the knowledge on this relationship is particularly important as tree growth can contribute significantly to the site’s carbon balance.
In our research we aim at: 1) quantify tree growth along a natural gradient of dry, intermediate, and wet soil conditions in a near-natural beech forest on sandy-textured soils, and 2) determine the influence of meteorological and soil parameters and sapflow dynamics on stem growth of beech trees. The overall research question is: to what extent do sapflow, soil variables, and meteorological parameters explain the differences in stem growth along a soil moisture gradient from wet to dry conditions? Under the same climatic and stand management conditions we hypothesize: 1) there is a stronger correlation among weather variables, sapflow, and growth under wet soil moisture conditions, and 2) the correlation between soil matric potential, sapflow, and growth is more pronounced under dry soil moisture conditions.
The research is carried out under the framework of the Beneath Project, which focuses on understanding how soil moisture and water balance influence carbon dynamics in beech forests. For this purpose, three sites along a soil moisture gradient have been selected for the installation of three intensive monitoring plots (IMPs). The IMPs are located at a dominant beech tree of the respective site; the monitored trees present similar characteristics. At each IMP, the monitoring of different elements of the hydrological and C cycles is carried out. Soil moisture and matric potential, sap flow, stem growth, leaf area index (LAI) and meteorological variables were measured for two growing seasons (June-October 2022 and 2023) in the Dübener Heide Nature Park in Saxony, Germany. The expected differences in growth among sites would suggest that the consideration of interdisciplinary approaches i.e. including soil-plant factors is necessary to improve the knowledge of growth dynamics in beech forests under a changing climate.
Reference:
Dolschak et al., 2019 The impact of rising temperatures on water balance and phenology of European beech (Fagus sylvatica L.) stands. https://doi.org/10.1007/s40808-019-00602-1
How to cite: Fontenla-Razzetto, G., Petrik, P., Koller, A., Kniesel, B., Azekenova, A., Schäfferling, R., Wordell-Dietrich, P., Feger, K.-H., von Oheimb, G., Julich, S., and Kalbitz, K.: Do sapflow and soil parameters shape tree growth in a near-natural beech forest?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3170, https://doi.org/10.5194/egusphere-egu25-3170, 2025.
Amazônia is the largest tropical rainforest in the world storing between 150-200 petagrams of carbon in its vegetation and soils, and it contributes significantly to the stability of the Earth’s climate system. However, there is alarming evidence of a continuous decline of Amazônia’s capacity to absorb net CO2. Abnormal carbon losses and an overall weakening of Amazônia’s carbon sink have been related to land use and climate change, raising concerns about its long-term stability and the potential culmination of a related tipping point. The 2023/24 El Niño marked the worst drought in the Amazon on record, with large impacts on forest functioning. This suggests that Amazonian rainforest trees were exposed to immense hydraulic stress during that period. Insights into how the Amazon rainforest responds to recurrent droughts can be gained from results from a long-term throughfall exclusion experiment (TFE) based in the eastern Amazon in Caxiuanã, simulating the recurrence of an El Niño type drought for 23 consecutive years. However, how do trees that have been subjected to recurrent droughts respond to future extreme events? Critically, we need to understand whether recurrent drought exposure affects the long-term future drought responses of Amazonian trees: could Amazonian trees have drought memory?
We tested this idea during the historic 2023/24 El Niño drought, by quantifying resistance, resilience, plant water-use regulation and growth in Amazonian rainforest trees. We compared tree performance between trees that had experienced natural rainfall over the past 23 years (control trees, CTs), and those that had survived repeated drought (drought-exposed trees, DETs) by being subjected to 23 years of artificial El Niño-type soil moisture reduction within the droughted forest plot (TFE plot). At the onset of the El Niño drought, both the TFE and Control plots displayed similar values of available soil water per biomass, indicating that all trees in each plot had a similar water availability. Astonishingly, the DETs were significantly more resistant than CTs during the 2023/24 El Niño and were more resilient post-drought as far as hydraulic transport is concerned. In addition, DETs were much less conservative with regard to water loss during the dry season. However, at the peak of the drought, control trees had used significantly more soil water despite strong water use regulation. While CTs displayed apparent negative stem growth due to the loss of water stored in their trunks, some DETs displayed positive stem growth, indicating that these trees not only survived, but physiologically functioned and grew during the extreme drought of 2023/24, without depleting available soil water. These findings suggest that Amazonian trees that have been subjected to recurrent drought events may build a form of drought memory that enables functional responses to future drought that exceed those from short-term phenotypic plasticity. Such plant memory may underpin phenotypic acclimation to new environmental conditions, ensuring survival and competitive advantage even in a rapidly changing climate.
How to cite: Martius, L. R., Sanchez Martinez, P., Negrão-Rodrigues, V., Bittencourt, P., Da Costa, A. C., Mencuccini, M., and Meir, P.: Amazônia doesn’t forget: Tropical trees with drought memory resist El Niño, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3591, https://doi.org/10.5194/egusphere-egu25-3591, 2025.
Multi-year droughts (MYDs), droughts lasting more than a year, have devastating effects on vegetation. Due to climate change, MYDs are expected to become more frequent and intense, making it crucial to assess their impact on vegetation accurately.
In this study, we combined MODIS satellite observations, ERA5 meteorological reanalysis data, and the LPJmL dynamic vegetation model to evaluate the sensitivity of vegetation to droughts and to quantify the impact of MYDs on seven types of vegetation in six different regions across the globe during the 21st century. To measure the response of vegetation to drought, we used the standardized Enhanced Vegetation Index (EVI) and compared this to EVI climatology.
As anticipated, the overall impact of MYDs on vegetation was negative, but our findings revealed significant spatial and temporal variations with areas showing significant greening during MYDs (around 35% of the world). In general, shrublands experienced the largest decrease in greenness, while trees flourished. The natural water availability of a region is the primary factor influencing vegetation response to MYDs. Vegetation in water-limited areas tends to suffer during MYDs, whereas vegetation in energy-limited areas thrives as long as sufficient water is available. Compared to normal droughts (NDs), MYDs generally caused stronger negative EVI anomalies.
To address the limitations of the short observational record and the relatively low number of MYDs during the 21st century, we extended the analysis back to 1901 using the LPJmL-5 dynamic global vegetation model. Simulating vegetation dynamics over this 120-year period allowed us to increase the number of MYDs available for study, improving the statistical robustness of our results.
How to cite: Ruijsch, D., van Mourik, J., Biemans, H., Hauswirth, S., and Wanders, N.: Understanding the Impact of Multi-Year Droughts on Vegetation: An Observational and Model Approach, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4151, https://doi.org/10.5194/egusphere-egu25-4151, 2025.
A statistical crop yield model developed by the author, ABSOLUT (Conradt 2022, https://doi.org/10.1007/s00484-022-02356-5 ), is capable of identifying the time aggregates of meteorological variables or indices most relevant for agricultural yields. Using climate change scenarios as input to the model calibrated on recent weather and yield data future crop yields have been projected for the districts of Germany and for Europe's NUTS-2 regions.
Previous research has shown that while large parts of inter-annual crop yield variations can already be explained by aggregates of temperature, precipitation, and solar radiation only, model performances are regularly increased by also including drought indices representing water stress related to soil conditions (see also Eini et al. 2023, https://doi.org/10.1016/j.agwat.2022.108107 ). This holds especially for maize and root crops (e.g. potatoes, sugar beets) grown in mid-latitudes and harvested in autumn. Consequently, all crop yield scenarios presented here are obtained from predictors including Standardized Precipitation Evaporation Indices (SPEI, Vicente-Serrano et al. 2010, https://doi.org/10.1175/2009JCLI2909.1 ) on 3- or 12-month scales.
Results show strong weather effects on green maize (high coefficients of determination in leave-one-out validation) and a generally negative outlook for the future: The median scenario under CMIP6 SSP370 climate shows 5–15% declines in green maize yields for the years around 2050 compared to nowadays levels in most European regions. Southern France, Northern Italy, and Bulgaria are predicted to experience yield losses of even more than 20%, albeit with lower reliability. The Mediterranean countries however include also some regions with positive trends on low confidence levels. In a more distant future of the years around 2080 the spatial pattern remains unaltered, but the strength of the changes will have doubled.
For winter wheat the model performs better in the eastern parts of Europe. Only slight declines in yield of 0–10% are projected there for the 2050 time slice; for the years around 2080 losses of more than 25% have to be expected, though. Drastic losses of 20–50% and exceeding 50% in the more distant future threaten many Mediterranean regions. There is however also a stable outlook for Britain and Ireland, The Netherlands, Belgium, and the North-Western parts of France. Yield increases are projected for Southern Finland and the Baltic states. This regional exception to the general downward perspective is in good agreement with a map presented in the European Drought Risk Atlas (Rossi et al. 2023, https://doi.org/10.2760/608737 ).
How to cite: Conradt, T.: Mapping future crop yield trends across Europe by auto-adaptive regression modelling, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6574, https://doi.org/10.5194/egusphere-egu25-6574, 2025.
Dryness can negatively affect vegetation functioning, including both immediate effects as well as effects extending beyond the duration of the dryness. Such legacy effects are for example related to hydraulic damage of plants or a reduced amount of leaves. While the impact of singular dry events are relatively well-studied, the impact of consecutive events on ecosystem functioning is less understood. Our study hypothesis here is that legacy effects may be more likely after consecutive dry periods as the vegetation adaptation potential (e.g. through carbon reserves) is exhausted after a previous dry period. In particular, we follow a four-step approach: (1) identify dry events through dry soil moisture, using a reanalysis soil moisture dataset from 2000 to 2023; (2) determine single and consecutive events by examining the temporal proximity of other dry events to a primary event. A single event is defined as a dry period with no subsequent or preceding dry events occurring within a two-year window, whereas consecutive events are further classified based on when, during this two-year period, another event occurs; (3) assess vegetation responses to single versus consecutive dry events, and their recovery, using anomalies of the Normalised Difference Vegetation Index (NDVI); and (4) use generalized additive models (GAMs) to explore the aspects contributing to vegetation response and recovery. We consider dry event characteristics, including timing, magnitude, and hydrological conditions during the event, as well as static variables such as climate and vegetation type. Preliminary results indicate that severe previous dry events events can induce legacy effects on vegetation, with impacts comparable in magnitude to those driven by dry event characteristics and static environmental variables. Notably, these legacy effects manifest as both positive and negative responses. By understanding how ecosystems are shaped by recurring climatic extremes, this research aims to provide insights for ecosystem response and management in a changing climate with more frequent dry periods.
How to cite: Müller, P. M., Orth, R., and Cheng, X.: Comparing vegetation impacts of single vs. consecutive dry events, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10071, https://doi.org/10.5194/egusphere-egu25-10071, 2025.
A meteorological drought occurs when high atmospheric water demand and low water supply lead to water scarcity. This can develop into an ecological drought, affecting ecosystems through hydraulic failure, carbon limitation, and ultimately plant mortality. The transition from meteorological to ecological drought is a complex process, influenced by the interplay of three fundamental components that are often used in the field of disaster risk management: the characteristics of the drought event itself (hazard), the inherent susceptibility of the ecosystem to drought (vulnerability), and the environmental conditions of the ecosystem (exposure). Specifically, we investigate how multiple water-related variables shape the hazard, how plant functional traits determine ecosystem vulnerability, and how groundwater levels affect the exposure of ecosystems to drought.
To investigate the transition, we integrate eddy covariance observations, plant trait databases, and groundwater level data within the hazard-vulnerability-exposure framework. Using a data-driven approach, we assess the relative importance of these components in driving ecological drought. Our findings are then compared with model simulations to provide a comprehensive understanding of the underlying mechanisms.
How to cite: Zhan, C. and Orth, R.: Studying the transition from meteorological to ecological drought in the disaster risk framework, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11602, https://doi.org/10.5194/egusphere-egu25-11602, 2025.
Temperature extremes exert a significant influence on terrestrial ecosystems, but the precise levels at which these extremes trigger adverse shifts in vegetation productivity have remained elusive. In this study, we have derived two critical thresholds, using standard deviations (SDs) of growing-season temperature and satellite-based vegetation productivity as key indicators. Our findings reveal that, on average, vegetation productivity experiences rapid suppression when confronted with temperature anomalies exceeding 1.45 SD above the mean temperature during 2001-2018. Furthermore, at temperatures exceeding 2.98 SD above the mean, we observe the maximum level of suppression, particularly in response to the most extreme high-temperature events. When Earth System Models are driven by a future medium emission scenario, they project that mean temperatures will routinely surpass both of these critical thresholds by approximately the years 2050 and 2070, respectively. However, it's important to note that the timing of these threshold crossings exhibits spatial variation and will appear much earlier in tropical regions. Our finding highlights that restricting global warming to just 1.5°C can increase safe areas for vegetation growth by 13% compared to allowing warming to reach 2°C above preindustrial levels. This mitigation strategy helps avoid exposure to detrimental extreme temperatures that breach these thresholds. Our study underscores the pivotal role of climate mitigation policies in fostering the sustainable development of terrestrial ecosystems in a warming world.
How to cite: Li, X., Huntingford, C., Wang, K., Cui, J., Xu, H., Anniwaer, N., Yang, H., Peñuelas, J., and Piao, S.: Increased crossing of thermal stress thresholds of vegetation under global warming, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13833, https://doi.org/10.5194/egusphere-egu25-13833, 2025.
Blue light-dependent photosynthesis and stomatal opening have been intensively studied in herbaceous crops but less so in tree species, where forests face more complex light environments compared with crops in agricultural fields. The light spectral environment in forests is influenced by factors such as the multi-layered canopy structure, dynamic light availability and shading through canopy gaps, and the occurrence of sun flecks. These factors result in dynamic variability in blue-to-red light ratio perceived by trees. Therefore, we conducted leaf gas exchange measurements, combined with online isotope discrimination, photorespiration and chlorophyll fluorescence on two contrasting tree species grey alder (Alnus incana) and holm oak (Quercus ilex) across a full gradient of blue light fraction, with the remaining fraction supplied as red light to maintain a constant total light intensity. Photosynthetic and stomatal responses to increasing blue light differed markedly between the two species but led to a consistently decreasing water-use efficiency (WUE). For grey alder, the decrease in WUE was primarily due to blue light-induced photosynthesis reduction, which is associated with light stress on the photosynthetic apparatus as detected by chlorophyll fluorescence; whereas for holm oak, blue light-stimulated stomatal opening played the major role in reducing WUE. Although isotope-based estimates of mesophyll conductance were lower at higher blue light levels, especially in grey alder, the changes in mesophyll conductance did not result in a CO2 shortage at the site of Rubisco under higher compared with lower blue light levels. However, across species, the component responding to blue light differed: the chloroplast membrane in grey alder and the cell wall and plasma membrane in holm oak. We suggest that, in tree species, blue light decreases WUE through distinct coordination between photosynthesis and stomata and species-specific blue light sensitivities of underlying mechanisms influencing the CO2 diffusion pathway.
How to cite: Diao, H., Lehmann, M. M., Holloway-Phillips, M., Gessler, A., Siegwolf, R. T. W., and Saurer, M.: Blue light reduces leaf-level water-use efficiency via contrasting physiological mechanisms in two tree species, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15346, https://doi.org/10.5194/egusphere-egu25-15346, 2025.
Plants are able to acclimatize to cold climates by developing physiological strategies to withstand periods of cold but nonfreezing temperatures during the growing season. In this study we investigated, if a long-term acclimation to low temperatures enables seedlings of temperate tree species to reduce the negative effects of low root temperatures on root water uptake and transport that has been observed in previous experimental studies. We investigated 7 common European tree species that differ largely in their natural elevational distribution ranges. To acclimatize the plants to different temperatures, newly germinated seedlings were raised at two different air temperatures (warm 22°C day/18°C night; cold, 12°C day/8°C night) for several months, and then were exposed to three different root temperatures (15, 7 and 2°C) in hydroponic systems while maintaining the same warm aboveground temperatures (20-25 °C) for a two-day period. We used stable isotope labelling with 2H-H2O source water to quantify the water uptake and transport from roots to leaves by the amount of 2H-label in leaf water after labelling. The species-specific sensitivity of root water uptake to low root temperatures was indentified by the relative change of 2H labels in leaf water at low root temperatures relative to 15°C. We found cold acclimation treatment did not improve the cold sensitivity of root water uptake, except for the two species with the lowest elevational distribution limits. In contrast, the majority of the investigated species showed a generally enhanced capacity of water uptake and transport after cold acclimation, regardless of the applied root temperatures. This results suggested a limited ability for physiological adjustemnts to overcome the cold limitation of water transport in roots, that is generally associated with decraesing efficiency of water diffusion across plasma membranes. The tendency towards an overall enhanced capacity for water movements in cold acclimatized seedlings might be due to the modified root anatomy and morphology that would help to improve root hydraulic conductance of trees in cold environments.
How to cite: Li, Y. and Hoch, G.: The cold acclimatization of root water uptake in temperate tree species, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18068, https://doi.org/10.5194/egusphere-egu25-18068, 2025.
Despite only covering ~3% of the land area, peatlands store more carbon (650 gigatons (Gt) of C) than global terrestrial vegetation (409 GtC). However, this C is vulnerable to climate warming and drainage. Plants mediate land-atmosphere C exchange and its coupling with water by regulating stomatal opening and root water intake during droughts. Stomatal regulation and photosynthesis are dependent on soil water content (SWC), air temperature (Tair), and vapour pressure deficit (VPD). However, the role of SWC on gross primary productivity (GPP) is still not straightforward, as evidenced by several contradictory literature. Considering this, we asked whether there is a threshold at which SWC drawback starts to regulate GPP in a peat bog in Vancouver, Canada. We used weekly time step eddy covariance data spanning five years (2016-2020). Our analysis suggests that stomatal regulation in response to increased VPD caused a reduction in GPP during the 2016 drought (~2.5gC m-2 day-1). On the other hand, an absence of stomatal regulation in 2017 and 2018 (to maximise C assimilation) following the initial drought caused the peat surface to dry out in 2019. This resulted in SWC regulating GPP more than VPD by 2019. We report a SWC threshold of 82.5% (-8 cm water table depth), below which it starts to regulate GPP at this site. The interaction between energy and water limitations on GPP is expected to intensify with the projected increase in the frequency of drought events across the northern hemisphere.
How to cite: Thayamkottu, S., Masta, M., Skeeter, J., Pärn, J., H Knox, S., Smallman, T. L., and Mander, Ü.: Atmospheric aridity and soil moisture fluctuations regulate GPP in a temperate peat bog, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15754, https://doi.org/10.5194/egusphere-egu25-15754, 2025.
The terrestrial biosphere plays a crucial role in Earth's climate system, influencing energy, water, and carbon cycles, while supporting biodiversity and ecosystem services essential for human sustainability. Predicting how ecosystems will respond to climate change and their feedback effects remains a significant challenge [1]. One major difficulty arises from the complexity of the systems involved, with non-linear processes and interactions occurring across varying timescales. Vegetative systems, particularly forests, exhibit processes that span from seconds to decades, indicating their persistence [2]. Many of these dynamics are driven by weather patterns, which are short-term processes. Soil moisture (SM), a key ecohydrological variable, has been shown to exhibit long-term persistence and plays a critical role in these interactions [3]. Modeling studies have demonstrated that SM can improve seasonal climate predictions [4]. Therefore, investigating the persistence of soil moisture-plant interactions is crucial for understanding long-term changes in the terrestrial biosphere.
In this study, we examine the coupled non-linear persistence between remote sensing derived SM and vegetation greenness using kernel Detrended Fluctuation Analysis (kDFA), a novel multivariate non-linear extension of DFA [5]. This method allows us to explore the non-linear interactions between these variables across multiple scales within a persistence space. By doing so, we can quantitatively assess how moisture influences persistence in vegetative systems. Additionally, we investigate the relationships between SM-vegetation persistence, forest greenness, and other eco-physiological proxies using a non-linear regression algorithm. First, we perform a spatial analysis, followed by a temporal analysis using a moving window approach. Our goal is not only to assess how SM-forest interactions evolve over time, but also to link this persistence to ecosystem stress and identify areas vulnerable to hot and dry conditions as drought-heat events increase in frequency and intensity [6].
References:
1. Overpeck, J., Whitlock, C., Huntley, B. (2003). Terrestrial Biosphere Dynamics in the Climate System: Past and Future. In: Alverson, K.D., Pedersen, T.F., Bradley, R.S. (eds) Paleoclimate, Global Change and the Future. Global Change — The IGBP Series. Springer, Berlin, Heidelberg.
2. Williams, et al. Sub-Seasonal Forest Carbon Dynamics Lose Persistence Under Extremes. Submitted.
3. Orth, R., and Seneviratne, S.I. (2012). Analysis of soil moisture memory from observations in Europe, J. Geophys. Res., 117, D15115, doi:10.1029/2011JD017366.
4. Besnard, S., et al. (2019). Memory effects of climate and vegetation affecting net ecosystem CO2 fluxes in global forests. PloS one 14.2.
5. Williams et al. Kernel Detrended Fluctuation Analysis. Submitted.
6. Seneviratne, S.I., et al. (2021). Weather and Climate Extreme Events in a Changing Climate. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the IPCC. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 1513–1766.
How to cite: Williams, T., Mahecha, M. D., and Camps-Valls, G.: Exploring Non-Linear Memory Between Soil Moisture and Forest Greenness Dynamics, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17801, https://doi.org/10.5194/egusphere-egu25-17801, 2025.
Snow plays a critical role in ecosystems by providing a reliable water supply in spring as winter snow melts. However, “snow droughts” are emerging as a growing threat to terrestrial ecosystem functioning. Snow droughts, characterized by reduced or even absent winter snow accumulation, arise either from warmer conditions where precipitation falls as rain instead of snow (warm snow drought) or insufficient winter precipitation (dry snow drought). Consequently, the amount of water stored in the snow pack (“snow water equivalent”, -(SWE)) is reduced, altering snow melt and thus impacting streamflow discharge and groundwater recharge. Even more directly, soil water storage is reduced, which can diminish late-season water availability, potentially reducing plant productivity. Despite their critical implications, the links between snow droughts and their broader effects on the carbon cycle remain poorly understood and quantified.
Using the LPJmL dynamic global vegetation model, we attribute spring gross primary productivity (GPP) anomalies in the Northern Hemisphere over the past decades to snow drought-induced soil moisture deficits. By tracking the seasonal progression of soil moisture and GPP, we quantify these linkages between snow droughts and subsequent soil moisture deficits, as well as their direct impact of GPP anomalies and their related “legacy or memory effects” on summer GPP anomalies.
This research highlights the feedback mechanisms between snow droughts, soil moisture, and vegetation, providing novel insights on drought impacts across seasons.
How to cite: Varela, M., Gampe, D., and Fader, M.: Understanding the Snow Drought–Soil Moisture–Vegetation Feedback: The impacts of seasonal Memory Effects on GPP Anomalies , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18514, https://doi.org/10.5194/egusphere-egu25-18514, 2025.
Forests are increasingly subjected to drought stress and heatwaves, with recent years showing widespread tree mortality, particularly in monocultures. To improve the climate resilience of forests, a transformation towards mixed-species and more adaptive stands is essential. Thinning is one effective measure to enhance water and light availability for the (natural or artificial) regeneration of diverse tree species. To better understand how microclimatic factors such as light, vapor pressure deficit, and soil moisture influence drought resistance in the main tree species Abies alba, Fagus sylvatica, Quercus robur, and Pseudotsuga menziesii, seedlings were planted under controlled greenhouse conditions, with either full light or shaded conditions provided by shading nets. A simulated drought period was then applied to observe the time until stomatal closure and critical hydraulic failure, defined as the point at which trees reached the water potential at 88% loss of hydraulic conductivity. Predawn water potential and gas exchange measurements were combined and stem diameter change continuously monitored.
Light-exposed seedlings generally exhibited greater height and stem growth than shaded seedlings. However, predawn water potential measurements indicated that these seedlings suffered from drought earlier than shaded trees and consistently died earlier. This was attributed to higher transpiration rates of light-exposed trees, resulting from a higher evaporative demand, larger plant size and total leaf area compared to the shaded trees. Under light conditions, differences in the sequence of drought-induced mortality were more pronounced (Q. robur died first, followed by F. sylvatica and P. menziesii), whereas under shaded conditions, mortality times were more uniform across these species. Overall, A. alba demonstrated the highest drought resistance.
These results emphasize the critical role of total leaf area in determining drought resistance among tree species. While light generally promotes CO₂ uptake and growth in most tree species, it can also exacerbate drought stress under certain conditions. Considering species-specific drought tolerance and implementing adaptive forest management practices, such as promoting mixed-species stands and adjusting thinning regimes, will be key to balancing the benefits of light with the need for drought resistance during the juvenile stage for mitigating the impact of future heatwaves and droughts on European forests.
How to cite: Rehschuh, R., Kocum, J., Skibbe, K., Schuldt, B., and von Oheimb, G.: Increased light exposure reduces drought survival in tree seedlings, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18471, https://doi.org/10.5194/egusphere-egu25-18471, 2025.
Turgor loss point (φTLP) serves as an indicator of plant drought acclimation strategies and is closely linked to water loss. While leaf φTLP has been extensively studied, little is known about flower φTLP, particularly its role in the drought acclimation strategies of flowers or its integration into whole-plant responses. In a semi-arid to semi-humid grassland on the Inner Mongolia Plateau, precipitation was reduced by 0%, 20%, 50%, and 80% using shelters. Hydraulic traits, gas exchange, temperature and morphological traits of a native perennial herb, Bupleurum smithii, were measured in both flowers and leaves. Results revealed that: (1) Both flowers and leaves employ drought avoidance strategies, with increased φTLP under drought. (2) Unlike leaves, higher flower φTLP does not effectively reduce water loss. (3) Drought avoidance in flowers is accomplished through easier decaying, and less dry mass is allocated into flowers to reduce carbon loss. (4) Morphological adaptations enable flowers to maintain attractiveness and reproductive function despite water stress. This highlights the functional specialization of plant organs, with flowers prioritizing reproduction under resource constraints.
How to cite: Li, Y., Zhang, Z., Li, Y., and Tang, Y.: Drought Avoidance: A Shared Strategy for Perennial Herbaceous Flowers and Leaves through Distinct Mechanisms, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8174, https://doi.org/10.5194/egusphere-egu25-8174, 2025.
In recent decades, the intensity and frequency of extreme climatic events have increased markedly, causing remarkable effects on terrestrial ecosystems. Understanding how these climate extreme climatic events affect vegetation growth is important for the global change ecology. We used vegetation index with satellite observations, including the normalised difference vegetation index (NDVI), enhanced vegetation index (EVI), and solar-induced chlorophyll fluorescence (SIF) to assess vegetation growth, and applied event coincidence analysis and sensitivity analysis to study how the climate extreme (extreme heat, cold, wet and drought) lead to abnormal vegetation growth in different areas. First, taking Northeast Asia as an example, our results show that extreme heat promotes vegetation growth, while extreme cold adversely affects vegetation growth. The beneficial effect of extreme heat on vegetation growth weakens with increasing temperature gradients, but amplifies with rising humidity gradients. This indicated that extreme heat is beneficial for vegetation growth in cold and humid regions. To further verify the above conclusions, we extended the study area to the Northern Hemisphere. In the Northern Hemisphere, extreme heat and cold are important climatic factors affecting the abnormal vegetation growth in the cold and humid ecosystems. Water-related extreme events were less influential to abnormal vegetation growth, mainly affecting relatively warm and arid ecosystems. In summary, our results emphasise the crucial role of background hydrothermal conditions in the attribution of vegetation growth extremes to diverse climate extremes.
How to cite: Liu, D., Cui, G., and Xu, Z.: Multiscale response of vegetation growth to climate extremes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14250, https://doi.org/10.5194/egusphere-egu25-14250, 2025.
Since the early 2000s, the western U.S has been entrenched in a historic 20 year "megadrought," ranked one of the worst in 1200 years. In addition to decreases in precipitation, atmospheric aridity, or vapor pressure deficit (VPD) has been also increasing. The important role that VPD plays in regulating plant growth has recently been implicated in a handful of studies, but primarily from the ecosystem perspective. The possibility of direct effects of VPD on cellular-to-organismic growth processes has not been adequately explored, beyond the obvious influences on stomatal conductance and photosynthesis. We hypothesize that from a biophysical perspective, persistent seasonal trends of high VPD may also play an important role at the whole tree scale. In a Pinus ponderosa forest in southern Arizona, USA, we collected bi-weekly micro-core samples and found that although earlywood cambial cell expansion occurs during spring (under high soil moisture and low VPD), the carbon and oxygen isotope ratios (13C/12C, 18O/16O ) of early-wood cellulose reflect the mid-summer climate of low soil moisture and high VPD. Using cellular modeling of xylogenesis, we have shown that the reason for these contrasting observations is a multi-week offset between cell expansion and cell-wall thickening. Thus, the deposition of isotopically-enriched sugars (revealed by 18O/16O and 13C/12C) occurs as a "backfilling" process during the weeks following cell expansion. The cellular modeling of xylogenesis also revealed that between our two sampling years, 2018 (drier winter, wetter summer) and 2019 (wetter winter, drier summer), there were differences in the timing and duration of cell enlargement and cell wall thickening, which correlated with VPD. Our next step is to conduct quantitative wood anatomy measurements to characterize the seasonal rates of carbon accumulation and to assess how this highly variable seasonal climate fluctuation influences seasonal rates of carbon accumulation.
How to cite: Hu, J. and Morino, K.: The influence of precipitation and vapor pressure deficit on xylem phenology: a case study in a semi-arid conifer forest, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14863, https://doi.org/10.5194/egusphere-egu25-14863, 2025.
Frequent and severe drought events in the Amazon threaten the carbon sink capacity of the world’s largest tropical rainforest. However, the extent to which trees can buffer the impacts of drought by adjusting their physiology to sustain growth remains uncertain. This is particularly relevant for the understory strata, which comprise trees that will mature over decades to centuries, shaping the future forest structure and function. Here, we leveraged 22 years of experimental throughfall exclusion in a 1-ha plot in Eastern Amazon, paired with a similarly sized control plot, to investigate how prolonged drought affects the growth of understory saplings. Given that drought has reduced the above-ground biomass by half and increased canopy openness, we hypothesized that (1) saplings grow faster under long-term drought conditions, (2) traits which correlate with sapling growth rate differ under drought and control conditions, and (3) trait plasticity under drought conditions increased sapling growth during drought. Our findings suggest that two decades of imposed drought increased sapling growth rates relative to saplings in control conditions. Despite sapling density being 51% lower in the droughted plot, droughted saplings grew on average three times faster than their control counterparts. In the droughted plot, growth rates increased with leaf-to-sapwood area ratio, leaf nitrogen content, stem conductivity, and leaf minimum conductance. Whereas growth rates increased with embolism vulnerability in the control plot. Within species, plasticity in the leaf-to-sapwood ratio emerged as the single driver of faster growth rates observed in droughted individuals relative to control individuals. In conclusion, we found that prolonged drought reduces understory sapling abundance, alleviating competition, and enabling the surviving individuals to maximize photosynthetic capacity and growth. This implies that drought reshapes the forest into a novel, low-density, fast-growing state which understory trees respond to by increasing their total leaf area.
How to cite: Silva, M., Bittencourt, P., Bartholomew, D., Giles, A., Sanchez Martinez, P., Martius, L., Rodrigues, V., Mencuccini, M., Meir, P., Junior, J., da Costa, A., and Rowland, L.: Plasticity in leaf-to-sapwood area ratio enables saplings to increase growth under long-term drought in Amazon, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1482, https://doi.org/10.5194/egusphere-egu25-1482, 2025.
Posters virtual: Wed, 30 Apr, 14:00–15:45 | vPoster spot A
EGU25-10635 | Posters virtual | VPS4
Drought sensitivity of gross primary productivity in willow: effects from physiological versus structural responsesWed, 30 Apr, 14:00–15:45 (CEST) | vPA.13
The increasing frequency and intensity of drought events make them a growing threat for plants that are sensitive to water scarcity. It is therefore important to understand how plants react to drought stress. The willow trees from the short-rotation coppices (SRC) on the DTU-Risø Campus in Denmark (DK-RCW) are particularly sensitive to water shortage as they are rainfed. We address the following question: how do extremely dry conditions affect the willows growth? We study the plants response mechanisms to periods of water scarcity and examine how these responses impact their gross primary productivity (GPP). There is a particular relevance to this in the current context of global warming, where the SRC are used to produce bioenergy and can store carbon to mitigate climate change.
Field measurements were carried out at the DK-RCW site to gather information on canopy structure (leaf area index). These results were integrated into a modelled relationship with carbon flux data from an eddy covariance flux tower located onsite and providing continuous CO2 and H2O flux data in more than 10 years. The simple empirical model was used to contrast the GPP’s sensitivity to stomatal and non-stomatal processes by comparison of extreme drought conditions (summer 2018 in Denmark) and wetter conditions (summers 2015 and 2021). These years represent the same stage of the rotational cycle.
This new model enables us to highlight two complementary responses to drought: the trees immediately react by adapting their physiology (stomatal resistance, increased sensitivity to vapour pressure deficit under drought), but also by changing the canopy structure as the drought increases (reduction of the leaf area index) and other responses on canopy photosynthetic capacity. High vapour pressure deficit and the reduction of the leaf area index both reduced the photosynthesis of willow trees under dry conditions. The simulated data imply limited drought recovery after the dry period had ended. For these reasons, the carbon uptake by the willow SRC is lower during droughts and thus limits the SRC productivity and carbon sink strength. We conclude from the very clear results from this case study that different drought response mechanisms must be considered when trying to understand and predict plant responses to extreme drought.
How to cite: Jaujay, M. and Ibrom, A.: Drought sensitivity of gross primary productivity in willow: effects from physiological versus structural responses, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10635, https://doi.org/10.5194/egusphere-egu25-10635, 2025.
