Heat injuries sustained in a fire can initiate a cascade of complex mechanisms that affect the physiology of trees after fires. Uncovering the exact physiological mechanisms and relating specific injuries to whole-plant and ecosystem functioning is the focus of intense current research. Recent studies have made critical steps forward in our understanding of tree physiological processes after fires, and have suggested mechanisms by which fire injuries may interact with disturbances such as drought, insects and pathogens. We outline a conceptual framework that unifies the involved processes, their interconnections, and possible feedbacks, and contextualizes these responses with existing hypotheses for disturbance effects on plants and ecosystems. By focusing on carbon and water as currencies of plant functioning, we demonstrate fire-induced cambium/phloem necrosis and xylem damage to be main disturbance effects. The resulting carbon starvation and hydraulic dysfunction are linked with drought and insect impacts. Evaluating the precise process relationships will be crucial for fully understanding how fires can affect tree functionality, and will help improve fire risk assessment and mortality model predictions. Especially considering future climate-driven increases in fire frequency and intensity, knowledge of the physiological tree responses is important to better estimate postfire ecosystem dynamics and interactions with climate disturbances.

Summary The seedling stage is the most susceptible one during a tree′s life. Water relations may be crucial for seedlings due to their small roots, limited water buffers and the effects of drought on water transport. Despite obvious relevance, studies on seedling xylem hydraulics are scarce as respective methodical approaches are limited. Micro‐ CT scans of intact Acer pseudoplatanus and Fagus sylvatica seedlings dehydrated to different water potentials (Ψ) allowed the simultaneous observation of gas‐filled versus water‐filled conduits and the calculation of percentage loss of conductivity ( PLC ) in stems, roots and leaves (petioles or main veins). Additionally, anatomical analyses were performed and stem PLC measured with hydraulic techniques. In A. pseudoplatanus , petioles showed a higher Ψ at 50% PLC (Ψ 50 −1.13 MP a) than stems (−2.51 MP a) and roots (−1.78 MP a). The main leaf veins of F. sylvatica had similar Ψ 50 values (−2.26 MP a) to stems (−2.74 MP a) and roots (−2.75 MP a). In both species, no difference between root and stems was observed. Hydraulic measurements on stems closely matched the micro‐ CT based PLC calculations. Micro‐ CT analyses indicated a species‐specific hydraulic architecture. Vulnerability segmentation, enabling a disconnection of the hydraulic pathway upon drought, was observed in A. pseudoplatanus but not in the especially shade‐tolerant F. sylvatica . Hydraulic patterns could partly be related to xylem anatomical traits.

Heatwaves exert disproportionately strong and sometimes irreversible impacts on forest ecosystems. These impacts remain poorly understood at the tree and species level and across large spatial scales. Here, we investigate the effects of the record-breaking 2018 European heatwave on tree growth and tree water status using a collection of high-temporal resolution dendrometer data from 21 species across 53 sites. Relative to the two preceding years, annual stem growth was not consistently reduced by the 2018 heatwave but stems experienced twice the temporary shrinkage due to depletion of water reserves. Conifer species were less capable of rehydrating overnight than broadleaves across gradients of soil and atmospheric drought, suggesting less resilience toward transient stress. In particular, Norway spruce and Scots pine experienced extensive stem dehydration. Our high-resolution dendrometer network was suitable to disentangle the effects of a severe heatwave on tree growth and desiccation at large-spatial scales in situ, and provided insights on which species may be more vulnerable to climate extremes.

Recent studies on post-fire tree mortality suggest a role for heat-induced alterations of the hydraulic system. We analyzed heat effects on xylem hydraulics both in the laboratory and at a forest site hit by fire. Stem vulnerability to drought-induced embolism and hydraulic conductivity were measured in Picea abies, Pinus sylvestris and Fagus sylvatica. Control branches were compared with samples experimentally exposed to 90°C or damaged by a natural forest fire. In addition, xylem anatomical changes were examined microscopically. Experimental heating caused structural changes in the xylem and increased vulnerability in all species. The largest shifts in vulnerability thresholds (1.3 MPa) were observed in P. sylvestris. F. sylvatica also showed heat-induced reductions (49%) in hydraulic conductivity. At the field site, increased vulnerability was observed in damaged branches of P. sylvestris and F. sylvatica, and the xylem of F. sylvatica was 39% less conductive in damaged than in undamaged branches. These results provide evidence for heat-induced impairment of tree hydraulics after fire. The effects recorded at the forest fire site corresponded to those obtained in laboratory experiments, and revealed pronounced hydraulic risks in P. sylvestris and F. sylvatica. Knowledge of species-specific hydraulic impairments induced by fire and heat is a prerequisite for accurate estimation of post-fire mortality risks.

Synchrotron X-ray computed micro-tomography (microCT) has emerged as a promising noninvasive technique for in vivo monitoring of xylem function, including embolism build-up under drought and hydraulic recovery following re-irrigation. Yet, the possible harmful effects of ionizing radiation on plant tissues have never been quantified. We specifically investigated the eventual damage suffered by stem living cells of three different species exposed to repeated microCT scans. Stem samples exposed to one, two or three scans were used to measure cell membrane and RNA integrity, and compared to controls never exposed to X-rays. Samples exposed to microCT scans suffered serious alterations to cell membranes, as revealed by marked increase in relative electrolyte leakage, and also underwent severe damage to RNA integrity. The negative effects of X-rays were apparent in all species tested, but the magnitude of damage and the minimum number of scans inducing negative effects were species-specific. Our data show that multiple microCT scans lead to disruption of fundamental cellular functions and processes. Hence, microCT investigation of phenomena that depend on physiological activity of living cells may produce erroneous results and lead to incorrect conclusions.