Katline Charra-Vaskou

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.

Bacterial canker remains a major constraint affecting apricot production in South-East of France. It is primarily caused by Pseudomonas syringae , a Gram-negative bacterium, many strains of which exhibit ice nucleation activity. By promoting ice formation at relatively high subzero temperatures, ice nucleation-active bacteria may facilitate tissue disruption and pathogen entry. Concurrently, climate-driven shifts toward warmer winter–spring periods have advanced flowering phenology, increasing exposure to late frost events. Despite breeders having developed less susceptible varieties to bacterial canker and early flowering varieties, the link between these traits and frost sensitivity, an emerging risk in this location, remains unresolved. Here, we have evaluated the links between canker susceptibility and frost sensitivity using three cultivar pairs contrasting in disease response and flowering time. Ice nucleation temperature was measured in excised buds under controlled conditions throughout the frost-risk period, alongside field-based diameter variation monitoring over two years. Disease susceptibility ( P < 0.001), phenology ( P = 0.003), varieties ( P < 0.001), locations ( P < 0.001), and sampling date ( P < 0.001) significantly affected nucleation temperature, whereas epiphytic bacterial abundance and xylem vessel diameter did not. Trees froze at higher temperatures in situ than in laboratory assays (1 to -2°C versus -3 to 4°C, respectively), indicating strong environmental modulation of freezing processes beyond Psy-like bacterial activity, which is reflected in contrasting disease susceptibilities ( P < 0.001) and precocities ( P < 0.001). These results shed light on the complexity of the freezing process in trees under natural conditions. We discuss the potential roles of microclimatic conditions and alternative ice nucleation sources beyond Psy-like bacteria in driving these physiological processes.

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.

Monitoring of freezing in trees via nondestructive methods revealed complex spatial and temporal freezing patterns that promote internal water shifts and cavita

Freeze–thaw cycles induce major hydraulic changes due to liquid-to-ice transition within tree stems. The very low water potential at the ice–liquid interface is crucial as it may cause lysis of living cells as well as water fluxes and embolism in sap conduits, which impacts whole tree–water relations. We investigated water fluxes induced by ice formation during freeze–thaw cycles in Juglans regia L. stems using four non-invasive and complementary approaches: a microdendrometer, magnetic resonance imaging, X-ray microtomography, and ultrasonic acoustic emissions analysis. When the temperature dropped, ice nucleation occurred, probably in the cambium or pith areas, inducing high water potential gradients within the stem. The water was therefore redistributed within the stem toward the ice front. We could thus observe dehydration of the bark's living cells leading to drastic shrinkage of this tissue, as well as high tension within wood conduits reaching the cavitation threshold in sap vessels. Ultrasonic emissions, which were strictly emitted only during freezing, indicated cavitation events (i.e. bubble formation) following ice formation in the xylem sap. However, embolism formation (i.e. bubble expansion) in stems was observed only on thawing via X-ray microtomography for the first time on the same sample. Ultrasonic emissions were detected during freezing and were not directly related to embolism formation. These results provide new insights into the complex process and dynamics of water movements and ice formation during freeze–thaw cycles in tree stems.

Plants can be highly segmented organisms with an independently redundant design of organs. In the context of plant hydraulics, leaves may be less embolism resistant than stems, allowing hydraulic failure to be restricted to distal organs that can be readily replaced.We quantified drought‐induced embolism in needles and stems of Pinus pinaster using high‐resolution computed tomography (HRCT). HRCT observations of needles were compared with the rehydration kinetics method to estimate the contribution of extra‐xylary pathways to declining hydraulic conductance.High‐resolution computed tomography images indicated that the pressure inducing 50% of embolized tracheids was similar between needle and stem xylem (P_{50 needle xylem} = −3.62 MPa, P_{50 stem xylem} = −3.88 MPa). Tracheids in both organs showed no difference in torus overlap of bordered pits. However, estimations of the pressure inducing 50% loss of hydraulic conductance at the whole needle level by the rehydration kinetics method were significantly higher (P_{50 needle} = −1.71 MPa) than P_{50 needle xylem} derived from HRCT.The vulnerability segmentation hypothesis appears to be valid only when considering hydraulic failure at the entire needle level, including extra‐xylary pathways. Our findings suggest that native embolism in needles is limited and highlight the importance of imaging techniques for vulnerability curves.

Summary: Ultrasonic acoustic emission analysis enables nondestructive monitoring of damage in dehydrating or freezing plant xylem. We studied acoustic emissions (AE) in freezing stems during ice nucleation and propagation, by combining acoustic and infrared thermography techniques and controlling the ice nucleation point. Ultrasonic activity in freezing samples of Picea abies showed two distinct phases: the first on ice nucleation and propagation (up to 50 AE s⁻¹; reversely proportional to the distance to ice nucleation point), and the second (up to 2.5 AE s⁻¹) after dissipation of the exothermal heat. Identical patterns were observed in other conifer and angiosperm species. The complex AE patterns are explained by the low water potential of ice at the ice–liquid interface, which induced numerous and strong signals. Ice propagation velocities were estimated via AE (during the first phase) and infrared thermography. Acoustic activity ceased before the second phase probably because the exothermal heating and the volume expansion of ice caused decreasing tensions. Results indicate cavitation events at the ice front leading to AE. Ultrasonic emission analysis enabled new insights into the complex process of xylem freezing and might be used to monitor ice propagation in natura.

Xylem embolism is a limiting factor for woody species worldwide. Conifers at the alpine timberline are exposed to drought and freeze-thaw stress during winter, which induce potentially lethal embolism. Previous studies indicated that timberline trees survive by xylem refilling. In this study on Picea abies, refilling was monitored during winter and spring seasons and analyzed in the laboratory and in situ experiments, based on hydraulic, anatomical, and histochemical methods. Refilling started in late winter, when the soil was frozen and soil water not available for the trees. Xylem embolism caused up to 86.2% ± 3.1% loss of conductivity and was correlated with the ratio of closed pits. Refilling of xylem as well as recovery in shoot conductance started in February and corresponded with starch accumulation in secondary phloem and in the mesophyll of needles, where we also observed increasing aquaporin densities in the phloem and endodermis. This indicates that active, cellular processes play a role for refilling even under winter conditions. As demonstrated by our experiments, water for refilling was thereby taken up via the branches, likely by foliar water uptake. Our results suggest that refilling is based on water shifts to embolized tracheids via intact xylem, phloem, and parenchyma, whereby aquaporins reduce resistances along the symplastic pathway and aspirated pits facilitate isolation of refilling tracheids. Refilling must be taken into account as a key process in plant hydraulics and in estimating future effects of climate change on forests and alpine tree ecosystems.

Freeze-thaw events can affect plant hydraulics by inducing embolism. This study analyzed the effect of temperature during the freezing process on hydraulic conductivity and ultrasonic emissions (UE). Stems of 10 angiosperms were dehydrated to a water potential at 12% percentage loss of hydraulic conductivity (PLC) and exposed to freeze-thaw cycles. The minimal temperature of the frost cycle correlated positively with induced PLC, whereby species with wider conduits (hydraulic diameter) showed higher freeze-thaw-induced PLC. Ultrasonic activity started with the onset of freezing and increased with decreasing subzero temperatures, whereas no UE were recorded during thawing. The temperature at which 50% of UE were reached varied between -9.1°C and -31.0°C across species. These findings indicate that temperatures during freezing are of relevance for bubble formation and air seeding. We suggest that species-specific cavitation thresholds are reached during freezing due to the temperature-dependent decrease of water potential in the ice, while bubble expansion and the resulting PLC occur during thawing. UE analysis can be used to monitor the cavitation process and estimate freeze-thaw-induced PLC.