Thierry Améglio

Seasonally snow‐covered arctic and alpine ecosystems are among the fastest‐warming regions on Earth and are undergoing rapid ecological reorganization. Although satellite observations reveal widespread greening, increasing evidence highlights spatial heterogeneity, with instances of localized browning and biomass decline. These spectral trends reflect multiple ecological processes, among which shrub dynamics are an important component. The mechanisms underlying these divergent trajectories remain poorly understood. Snowpack serves a vital buffer, mitigating cold stress by insulating woody vegetation from freezing temperatures and regulating hydrological processes during the growth season. Declining snow depth, earlier melt, and more frequent winter thaw events compromise this buffer, exposing shrubs to intensified freeze–thaw cycles, frost drought, and hydraulic failure. These stresses can locally reduce xylem function, compromise cellular integrity, impair shoot survival, and trigger dieback, potentially pushing shrub communities toward thresholds of decline or reorganization. Interpreting the mismatch between greening trends and localized dieback requires a mechanistic, multi‐scale framework integrating dendrochronology, ecophysiology, and remote sensing. Linking physiological thresholds, such as frost tolerance and damage, to long‐term growth patterns preserved in stem rings, alongside broad‐scale trends captured via remote sensing, provides insight into how snowpack decline shapes woody vegetation dynamics. Such integration is essential to reconcile plant‐level stress responses with biome‐scale changes and improve predictions of vegetation dynamics under ongoing climate warming.

Branch water release and radial exchange influence drought responses. Although mechanistic models describe radial hydraulic and storage dynamics, experimental quantification of hydraulic capacitance (C) and radial resistance (Rr) across broad ranges of xylem water-potential (Ψx), particularly under severe dehydration, remains limited. We quantified how C and Rr vary with Ψx in Populus albaL., assessed the value of coupling controlled osmotic dehydration with high-resolution dendrometry, and evaluated cellular damage and bud viability across increasing dehydration levels. Branch segments were subjected to stepwise, experimentally induced dehydration by perfusion with D-sorbitol solutions (−1 to −4 MPa) to impose stable and controlled Ψx. Branch diameter dynamics and equilibration plateaus were used to estimate C and Rr, and diameter-derived pressure–volume curves were fitted using the Sureau model. Electrolyte leakage and bud development were monitored to assess injury and recovery capacity at the organ level. D-sorbitol equilibrated rapidly along the xylem with limited bark infiltration, and stepwise decreases in Ψx induced characteristic diameter contractions enabling robust extraction of C and Rr. Both traits varied with Ψx: C peaked at intermediate Ψx (−1 to −2 MPa) and declined under severe dehydration, while Rr increased steadily as Ψx decreased. Electrolyte leakage increased and buds failed to resume growth below −2 MPa. Controlled osmotic dehydration combined with high-resolution dendrometry enables precise quantification of C(Ψx) and Rr(Ψx) and reveals physiologically meaningful thresholds associated with cellular injury and loss of recovery capacity at the organ level, providing a useful framework for dehydration-tolerance phenotyping.

Summary: At high elevations, tree saplings and shrubs are usually protected by mid‐winter snow cover, although climate change is expected to extend the snow‐free (SF) period. Exposure to winter drought, freeze–thaw events and freezing temperatures will therefore increase, inducing damages to the hydraulic system and to living cells, resulting in reduced growth and increased mortality. A snow removal experiment was carried out at 1700 m. above sea level on saplings of five different species ( Acer pseudoplatanus , Juniperus communis , Larix decidua , Picea abies and Sorbus aucuparia ). Stem diameter was continuously monitored and compared with spring hydraulic conductivity (PLC _spring ), living cell mortality (PLD _spring ), nonstructural carbohydrates (NSCs), growth and survival rates. Under SF conditions, saplings had higher PLC _spring and higher PLD _spring , and thus experienced greater winter dehydration, resulting in lower growth compared with snow‐covered saplings. Summer mortality was strongly correlated with PLC _spring and PLD _spring . These two key ecophysiological parameters predicted the risk of mortality in all species, whereas only PLD _spring reduced growth. By monitoring stem diameter during winter, we have defined indices to quantify resistance and recovery of woody plants under increased frost pressure. Recovery strategies such as resprouting or embolism repair were critical for survival, highlighting the potential vulnerability of saplings to climate change at high elevations.

Dendrometry is the main non-invasive macroscopic technique commonly used in plant physiology and ecophysysiology studies. Over the years several types of dendrometric techniques have been developed, each with their respective strengths and drawbacks. Automatic and continuous monitoring solutions are being developed, but are still limited, particularly for non-invasive monitoring of large-diameter trunks. In this study, we propose a new type of automated dendrometer based on distributed fibre-optic sensing that continuously measures the change in stem circumference, is non-invasive and has no upper limit on the trunk diameter on which it can be installed. We performed a 3-month validation experiment during which we deployed a fibre-optic cable at three localities around the trunks of two specimens of Brachychiton. We verified the accuracy of this new method through comparison with a conventional point-dendrometer, and we observed a consistent time lag between the various measurement locations that varies with the meteorological conditions. Finally, we discuss the feasibility of the fibre-based dendrometer in the context of existing dendrometric techniques and practical experimental considerations.

Key message: Winter reddening of young Douglas-fir (Pseudotsuga menziesii Mirb. Franco), triggered by large thermal fluctuations in late winter, is a critical problem for European forestry. A literature review identified certain climatic conditions that are characteristic of ‘reddening’ years, including warm daily temperatures, high daily temperature amplitude, low relative humidity, moderate wind speeds, as well as the occurrence of freeze-thaw cycles with cold night temperatures. By describing the triggering environmental and stand factors, we propose three hypotheses for the physiological processes leading to winter reddening, namely (i) hydraulic failure due to winter drought stress, (ii) photo-oxidative stress in shade-acclimated trees, and (iii) early cold deacclimation during warm periods. i) Low soil temperature, by reducing root water uptake, combined with anticyclonic conditions, by increasing water losses, can induce hydraulic failure in the xylem. Hydraulic failure may be further accelerated by night frosts. ii) Winter reddening can occur when low temperature and high irradiance coincide, disrupting photostasis. Overwhelming of winter photo-protection may lead to photodamage and subsequent reddening. iii) Warm periods, by inducing cold deacclimation, make trees susceptible to frost damage. Finally, the three processes may interact under atypical anticyclonic conditions in late winter (e.g. cold or dry soils, warm days, high irradiance and/or freezing nights). Indeed, trees under water stress would develop a higher sensitivity to freezing night and photooxidative stress. We therefore proposed mitigation actions to avoid exposing trees to stressful conditions based on e.g. stand characteristics, understorey vegetation and planting.

Dendrometry is the main non-invasive macroscopic technique commonly used in plant physiology and ecophysysiology studies. Over the years several types of dendrometric techniques have been developed, each with their respective strengths and drawbacks. Automatic and continuous monitoring solutions are being developed, but are still limited, particularly for non-invasive monitoring of large-diameter trunks. In this study, we propose a new type of automated dendrometer based on distributed fibre-optic sensing that continuously measures the change in stem circumference, is non-invasive, and has no upper limit on the trunk diameter on which it can be installed. We perform a three-month validation experiment during which we deploy a fibre-optic cable at three localities around the trunks of two specimens of Brachychiton . We verify the accuracy of this new method through comparison against a conventional point-dendrometer, and we observe a consistent time lag between the various measurement locations that varies with the meteorological conditions. Finally, we discuss the feasibility of the fibre-based dendrometer in the context of existing dendrometric techniques and practical experimental considerations.

Summary: A specific, robust threshold for drought‐induced tree mortality is needed to improve the prediction of forest dieback. Here, we tested the relevance of continuous measurements of stem diameter variations for identifying such a threshold, their relationship with hydraulic and cellular damage mechanisms, and the influence of growth conditions on these relationships. Poplar saplings were grown under well‐watered, water‐limited, or light‐limited conditions and then submitted to a drought followed by rewatering. Stem diameter was continuously measured to investigate two parameters: the percentage loss of diameter (PLD) and the percentage of diameter recovery (DR) following rewatering. Water potentials, stomatal conductance, embolism, and electrolyte leakage were also measured, and light microscopy allowed investigating cell collapse induced by drought. The water release observed through loss of diameter occurred throughout the drought, regardless of growth conditions. Poplars did not recover from drought when PLD reached a threshold and this differed according to growth conditions but remained linked to cell resistance to damage and collapse. Our findings shed new light on the mechanisms of drought‐induced tree mortality and indicate that PLD could be a relevant indicator of drought‐induced tree mortality, regardless of the growth conditions.

Freeze-thaw-induced embolism is a key limiting factor for perennial plants in frost-exposed environments. Gas bubbles are formed during freezing and expand during thawing resulting in xylem embolism. However, when water freezes, its volume increases by 9%, generating local pressures, which can affect the formation of bubbles. To characterize local dynamic of pressure-tension and physical state of the sap during freeze-thaw cycles, we simultaneously used ultrasonic acoustic emissions analysis and synchrotron-based high resolution computed tomography on the diffuse-porous species Betula pendula. Visualization of individual air-filled vessels was performed to measure freeze-thaw induced embolism after successive freeze-thaw cycles down to -10°C or -20°C during the leafy and the leafless periods. We also measured the distribution of gas bubbles in frozen xylem of Betula pendula, and made additional continuous monitoring of embolism spreading during one freeze-thaw cycle using a dedicated cooling system that allowed X-ray scanning during freezing and thawing. Experiments confirmed that ultrasonic emissions occurred after the onset of ice formation, together with bubble formation, whereas the development of embolism took place after thawing in all cases. The pictures of frozen tissues indicated that the positive pressure induced by the volumetric increase of ice can provoke inward flow from the cell wall toward the lumen of the vessels. We found no evidence that wider vessels within a tissue were more prone to embolism although the occurrence of gas bubbles in larger conduits would make them prone to earlier embolism. These results highlight the need to monitor local pressure as well as ice and air distribution during xylem freezing to understand the mechanism leading to frost-induced embolism.

Climate change is having an increasingly rapid impact on ecosystems and particularly on the issue of water resources. The Internet of Things and communication technologies have now reached a level of maturity that allows sensors to be deployed more easily on sites to monitor them. The communicating node based on LoRaWAN technology presented in this article is open and allows the interfacing of numerous sensors for designing long-term environmental monitoring systems of isolated sites. The data integration in the cloud is ensured by a workflow driving the storage and indexing of data, allowing a simple and efficient use of the data for different users (scientists, administration, citizens) through specific dashboards and extractions. This article presents this infrastructure through environmental monitoring use cases related to water resources.

Trees participate in mitigating the urban heat island phenomenon thanks to their transpiration and shading. This cooling potential is highly dependent on leaf area. Nevertheless, leaf traits potentially vary across different land management practices in urban settings, thereby challenging the models used to estimate thermal budgets. The present study aims to investigate the variability of leaf area traits of linden (Tilia spp.) urban trees, and their effect on simulated tree transpiration. Reconstruction of the leaf area was undertaken at the tree scale at three different urban land management sites from three cities: London and Birmingham (UK) and Chantilly (France). The reconstruction combined allometric measurements at shoot and leaf scales, and a tree-scale 3D digitization with laboratory analysis using field data collected by citizen scientists. The management practices had a significant impact on leaf area, and on tree allometric relationships, which were propagated through the reconstruction process. Relative differences between the management practices ranged between 12% and 48% according to the city where the variable was considered (e.g., leaf area index, total leaf area, or tree transpiration). Trees in managed sites (i.e., individualized leaf crowns, frequent leaf litter removal, and standard thinning/pruning operations) develop a higher leaf area, thus promoting cooling potential. This study shows that the variability of leaf traits, and their responses to different land management, can be studied by comprehensive data collection through citizen science and lab-based modelling. It also highlights the importance of appropriate, well-designed urban planning, where landscaping using urban trees can play an even better role in climate proofing cities.