Comment on gmd-2022-17

Abstract. A widespread increase in tree mortality has been observed around the globe, and this trend is likely to continue because of ongoing climate-induced increases in drought frequency and intensity. This raises the need to identify regions and ecosystems that are likely to experience the most frequent and significant damage. We present SurEau-Ecos, a trait-based, plant hydraulic model designed to predict tree desiccation and mortality at scales from stand to region. SurEau-Ecos draws on the general principles of the SurEau model but introduces a simplified representation of plant architecture and alternative numerical schemes. Both additions were made to facilitate model parameterization and large-scale applications. In SurEau-Ecos, the water fluxes from the soil to the atmosphere are represented through two plant organs (a leaf and a stem, which includes the volume of the trunk, roots and branches) as the product of an interface conductance and the difference between water potentials. Each organ is described by its symplasmic and apoplasmic compartments. The dynamics of a plant's water status beyond the point of stomatal closure are explicitly represented via residual transpiration flow, plant cavitation and solicitation of plants' water reservoirs. In addition to the “explicit” numerical scheme of SurEau, we implemented a “semi-implicit” and “implicit” scheme. Both schemes led to a substantial gain in computing time compared to the explicit scheme (>10 000 times), and the implicit scheme was the most accurate. We also observed similar plant water dynamics between SurEau-Ecos and SurEau but slight disparities in infra-daily variations of plant water potentials, which we attributed to the differences in the representation of plant architecture between models. A global model's sensitivity analysis revealed that factors controlling plant desiccation rates differ depending on whether leaf water potential is below or above the point of stomatal closure. Total available water for the plant, leaf area index and the leaf water potential at 50 % stomatal closure mostly drove the time needed to reach stomatal closure. Once stomata are closed, resistance to cavitation, residual cuticular transpiration and plant water stocks mostly determined the time to hydraulic failure. Finally, we illustrated the potential of SurEau-Ecos to simulate regional drought-induced mortality over France. SurEau-Ecos is a promising tool to perform regional-scale predictions of drought-induced hydraulic failure, determine the most vulnerable areas and ecosystems to drying conditions, and assess the dynamics of forest flammability.

Towards an advanced assessment of the hydrological vulnerability of forests to climate change‐induced drought

As safety mechanism during droughts, plants close their stomata to reduce transpiration losses and prevent excessively negative water potentials. Although the coordination between stomatal closure, xylem vulnerability and leaf traits has been extensively investigated, the role of soil hydraulic limitation remains elusive. Here, we test the hypothesis that stomatal closure is triggered by the loss of soil hydraulic conductivity in the root zone. As the soil hydraulic conductivity is a function of soil texture, we further hypothesize that stomatal closure, and more precisely the relation between stomatal conductance and leaf water potential, are soil texture dependent. An alternative hypothesis is that the shoot to root ratio adapts to the specific soil conditions, and it is lower in soils with low hydraulic conductivity. We compared three field sites in an inner alpine valley in Switzerland (yearly rainfall of 600 mm) with the same oak tree species (Quercus pubescens) and varying soil textures. We used the model of Carminati and Javaux (2020), which predicts that the decline in transpiration rate is controlled by the soil hydraulic conductivity, and, consequently, it is more abrupt in 1) coarse textured soils compared to fine textured soils and 2) in plants with high shoot to root ratio.To test these working hypotheses, stem water status and soil matric potential were measured at various depths of three oak sites. The soil matric potentials were then linked to the hydraulic conductivity and soil water content using a new pedotransfer function (PTF) for forest soils, which overcomes the limits of existing PTFs that were trained for arable soils. A simple water balance model based on changes in water content (deduced from PTF and measured potentials) was used to calculate transpiration rates and was compared with sap flow measurements conducted on two trees per site. Leaf water potentials were estimated from dendrometers after calibration with pressure chamber measurements of leaves. The sap flow measurements correlate well with estimated transpiration rates (R2=0.7).Comparisons between sites show a similar decrease in stomatal conductance at all three sites during drought, regardless of soil texture. Rather, our data suggest the trees hydraulically adapt to the local soil texture by adjusting their leaf area index and root length density to the local water demand and supply. In coarse-textured soils, oak had a low leaf area index, reducing their water demand, and high fine root length density, increasing their supply. In conclusion, our study suggests a hydraulic adaptation of trees to their local soil texture by adapting their “shoot to root ratio” as quantified here by leaf area index and root density.

Drought and subsequent rewatering are common in agriculture, where recovery from mild droughts is easier than from severe ones. The specific drought threshold and factors limiting recovery are under-researched. This study subjected maize plants to varying drought degrees before rewatering, and measuring plant water status, gas exchange, hydraulic conductance, hormone levels, and cellular damage throughout. We discovered that stomatal reopening in plants was inhibited with leaf water potentials below about -1.7 MPa, hindering postdrought photosynthetic recovery. Neither hydraulic loss nor abscisic acid (ABA) content was the factor inhibited stomatal reopening on the second day following moderate drought stress and rewatering. But stomatal reopening was significantly correlated to the interaction between hydraulic signals and ABA content under severe drought. Extended drought led to leaf death at about -2.8 MPa or 57% relative water content, influenced by reduced rehydration capacity, not hydraulic failure. The lethal threshold remained relatively constant across leaf stages, but the recoverable safety margin (RSM), that is, the water potential difference between stomatal closure and recovery capacity loss, significantly decreased with leaf aging due to delayed stomatal closure during drought. Our findings indicate hydraulic failure alone does not cause maize leaf death, highlighting the importance of RSM in future research.

Rola zaburzeń w kształtowaniu struktury i dynamiki naturalnych lasów bukowo-jodłowo-świerkowych w Karpatach Zachodnich

Widespread climate-driven increases in background tree mortality rates have the potential to reduce the carbon storage of terrestrial ecosystems, challenging their effectiveness as natural buffers against atmospheric CO2 enrichment with major consequences for the global carbon budget. However, the global extent of trends in tree mortality and their drivers remains poorly quantified. The Australian continent experiences one of the most variable climates on Earth and is host to a diverse range of forest biomes that have evolved high resistance to disturbance, providing a valuable test case for the pervasiveness of tree mortality trends. Here we compile an 83-year tree dynamics database (1941-2023) from >2,700 forest plots across Australia covering tropical savanna and rainforest and warm and cool temperate forests, to explore spatiotemporal patterns of tree mortality and the associated drivers. Over the past eight decades, we found a consistent trend of increasing tree mortality across the four forest biomes. This temporal trend persisted after accounting for stand structure and was exacerbated in forests with low moisture index or a high competition index. Species with traits associated with high growth rate-low wood density, high specific leaf area and short maximum height-exhibited higher average mortality, but the rate of mortality increase was comparable across different functional groups. Increasing mortality was not associated with increasing growth, given that stand basal area increments either declined or remained unchanged over time, but it was associated with increasing temperature over time. Our findings suggest that ongoing climate change has driven pervasive shifts in forest dynamics beyond natural recovery in a range of forest biomes with high resilience to disturbance, threatening the enduring capacity of forests to sequester carbon under current and future climate scenarios.

Can vegetation optical depth reflect changes in leaf water potential during soil moisture dry-down events?

Massive and increasing tree mortality is currently observed in the two conifer species Norway spruce and Scots pine in Central Europe. Consecutive dry years are made responsible for this phenomenon. Leaf trait measurements, in specific leaf osmotic potential (πosm) and leaf water potential at turgor loss (πtlp), indicate that the underlying mechanisms for tree mortality are most likely different between the two species. πtlp of spruce was highly negative, revealing a potentially high drought tolerance of the species. πtlp of Scots pine was less negative, suggesting a higher susceptibility to drought stress. I conclude that the mortality of Norway spruce might be caused by rising temperatures and that the summer temperatures in the past years were beyond the species thermal tolerance threshold. Overall, I want to highlight and enhance the discussion that the search for suitable species for a climate change adapted forest should go in both directions, i.e., species should be chosen to make the forest fit for both increasing drought and heat stress.

Canada's boreal forests, which occupy approximately 30% of boreal forests worldwide, play an important role in the global carbon budget. However, there is little quantitative information available regarding the spatiotemporal changes in the drought-induced tree mortality of Canada's boreal forests overall and their associated impacts on biomass carbon dynamics. Here, we develop spatiotemporally explicit estimates of drought-induced tree mortality and corresponding biomass carbon sink capacity changes in Canada's boreal forests from 1970 to 2020. We show that the average annual tree mortality rate is approximately 2.7%. Approximately 43% of Canada's boreal forests have experienced significantly increasing tree mortality trends (71% of which are located in the western region of the country), and these trends have accelerated since 2002. This increase in tree mortality has resulted in significant biomass carbon losses at an approximate rate of 1.51 ± 0.29 MgC ha-1 year-1 (95% confidence interval) with an approximate total loss of 0.46 ± 0.09 PgC year-1 (95% confidence interval). Under the drought condition increases predicted for this century, the capacity of Canada's boreal forests to act as a carbon sink will be further reduced, potentially leading to a significant positive climate feedback effect.

PurposeAlthough the coordination between stomatal closure and aboveground hydraulics has extensively been studied, our understanding of the impact of belowground hydraulics on stomatal regulation remains incomplete. Here, we investigated whether and how the water use of maize (Zea mays L.) varied under hydraulically contrasting soil textures. Our hypothesis is that a textural-specific drop in the hydraulic conductivity is associated with a distinct decrease in transpiration during soil drying.MethodsMaize plants were grown in contrasting soil textures (sand, sandy loam, loam) and exposed to soil drying. We measured the relationships between transpiration rate, soil water content as well as soil and leaf water potential. We used a soil-plant hydraulic model to reproduce the experimental observations and infer the hydraulic conductance of the soil-plant system during soil drying.ResultsWe observed the impact of soil texture on plant response to soil drying in various relationships. The soil water potentials at which transpiration decreased were more than one order of magnitude more negative in loam than in sand. The soil-plant conductance decreased not only abruptly but also at less negative soil water potentials in sand than in sandy loam or loam. Stomata closed at less negative leaf water potentials in sand than in loam. The model predictions matched well the experimental observations.ConclusionThe results elucidated that the critical soil water content and potential at which plants close stomata depends on the soil texture. These findings support our plea to include soil properties for understanding and predicting stomatal regulation during soil drying.

The effects of a progressive increase in soil water deficit on the leaf conductance and gas exchange of lupin (Lupinus cosentinii) and wheat (Triticum aestivum) were investigated in pot experiments in a temperature-regulated glasshouse, using a coarse, sandy soil characteristic of the Western Australian wheatbelt. Transpiration rates decreased rapidly in both species after water was withheld, mainly as a result of stomatal closure. Photosynthesis declined also, but to a lesser extent than conductance. Leaf extension in lupin was equally as sensitive to a decrease in leaf water potential and soil water potential as stomatal conductance. Stomatal closure served to maintain the water potential of lupin leaves to within 0.1 MPa of that of control (watered) plants as the soil water content decreased from 0.14 to 0.06 m3 m-3 and as the leaf conductance and the relative transpiration rate fell to less than 50% of control values. Maintenance of leaf water potential with decreasing soil water content and stomatal conductance was less evident in wheat. In both lupin and wheat, leaf conductance decreased linearly with soil water content and curvilinearly with bulk soil matric potential, indicating that water uptake was restricted at similar water contents and matric potentials in both species. Diurnal measurements on lupin indicated a substantial reduction in stomatal conductance after water was withheld, even when the leaf water potential at midday was reduced by only 0.1 MPa and no change could be detected in the bulk leaf turgor pressure. Conductance in lupin was reduced even though the soil matric potential decreased in only part of the rooting zone. This, together with the absence of any significant change in the leaf water potential, turgor pressure, or relative water content in lupin during the initial stages of stomatal closure, suggests that a soil or root factor initiates the reduction in leaf conductance - and hence regulates the shoot water status - in response to soil drying.

Dryland forests are highly climate-sensitive are facing more frequent droughts and, consequently, increasing tree mortality, extreme wildfire events, and outbreaks of forest insects and pathogens. These changes, associated with climate change, are leading to biodiversity loss and the deterioration of related ecosystem services. Understanding the relationships between forest structure and function is essential for managing dryland forests to adapt to these changes. We studied the structure-function relationships in four dryland conifer forests distributed along a semiarid to sub-humid climatic aridity gradient. Forest structure was represented by leaf area index (LAI) and function by gross primary productivity (GPP), evapotranspiration (ET), and the derived efficiencies of water use (WUE= GPP/ET) and leaf area (LAE = GPP/LAI). The water and carbon fluxes at the ecosystem level were estimated by an empirical approach in which regression models were developed to relate multiple spectral data (VIs) derived from VENμS and Sentinel-2A satellites, combined with meteorological data, to local eddy covariance measurements from flux tower records available at three of the four study sites. The red-edge-based MERIS Terrestrial Chlorophyll Index (MTCI) from VENμS and Sentinel-2A showed strong correlations to flux tower GPP and ET measurements (R2cal >0.91, R2val >0.84). Using our approach, we showed that as LAI decreased with decreasing AI (dryer conditions), estimated GPP and ET decreased (R2>0.8 to LAI), while WUE (R2=0.68 to LAI) and LAE increased with decreasing AI. We propose that the higher WUE and LAE reflect an increased proportion of sun vs. shade leaves as LAI decreases. The results demonstrate the importance of high-resolution spectral and spatial data in low-density dry forests and the intricate structure-function interactions in the forests’ response to drying conditions.

Taller trees exhibit greater hydraulic vulnerability in southern Amazonian forests

Enhanced canopy growth precedes senescence in 2005 and 2010 Amazonian droughts

Increasing drought frequency and intensity driven by climate change have severely impacted tree vitality and contributed to rising mortality across Europe. To quantify and compare the intensity and regional extent of droughts, a variety of indices have been developed. Consequently, different methodological approaches are currently used in research to explain drought-induced tree mortality. The aim of this study was to evaluate the extent to which aridity (Climatic Water Balance, CWB; Site Water Balance, SWB) and drought indices (Standardized Precipitation Evapotranspiration Index , SPEI; CWB anomalies) were associated with tree mortality. The study used data from the German Crown Condition Survey (1990–2022) to derive mortality rates for four major tree species (Norway spruce, European beech, Scots pine, and oak). Using correlation analyses and generalized additive models (GAMs), we retrospectively quantified how aridity and drought indices, particularly the SPEI, explained tree mortality by comparing modelled mortality probabilities with observed rates. The results revealed distinct species-specific responses. For spruce, beech, and pine, SPEI and CWB anomalies showed the strongest links to mortality, with effects increasing over longer aggregation periods of up to five years. For oak, which exhibited lower mortality rates, the SPEI-based models did not adequately explain mortality, indicating the need for additional predictors. Overall, the results indicated that in Germany the mortality of the most common tree species was better described by relative deviations from long-term averages than by absolute aridity measures. The study shows that the SPEI is a robust indicator for predicting drought-related mortality across dominant tree species in Germany and contributes to improved tree species selection under climate change. • Tree mortality in Germany is better predicted by relative deviations from climatic long-term averages than by absolute aridity measures. • Standardized drought indices (SPEI) outperform aridity indices (CWB, SWB) in capturing mortality events. • SPEI robustly predicts drought-related mortality for Norway spruce and European beech. • Longer aggregations (up to 5 years) improve model performance. • Species-specific drought responses cause varying model performance across indices.