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1. Using heat plumes to simulate post-fire effects on cambial viability and hydraulic performance in Sequoia sempervirens stems

   https://doi.org/10.1093/treephys/tpad006  

   Salladay, Ryan A; Pittermann, Jarmila (May 2023, Tree Physiology)

   Martinez-Vilalta, Jordi (Ed.)

   Abstract Injury to the xylem and vascular cambium is proposed to explain mortality following low severity fires. These tissues have been assessed independently, but the relative significance of the xylem and cambium is still uncertain. The goal of this study is to evaluate the xylem dysfunction hypothesis and cambium necrosis hypothesis simultaneously. The hot dry conditions of a low severity fire were simulated in a drying oven, exposing Sequoia sempervirens (Lamb. ex D. Don) shoots to 70 and 100 °C for 6–60 min. Cambial viability was measured with Neutral Red stain and water transport capacity was assessed by calculating the loss of hydraulic conductivity. Vulnerability curves were also constructed to determine susceptibility to drought-induced embolism following heat exposure. The vascular cambium died completely at 100 °C after only 6 min of heat exposure, while cells remained viable at 70 °C temperatures for up to 15 min. Sixty minutes of exposure to 70 °C reduced stem hydraulic conductivity by 40%, while 45 min at 100 °C caused complete loss of conductivity. The heat treatments dropped hydraulic conductivity irrecoverably but did not significantly impact post-fire vulnerability to embolism. Overall, the damaging effects of high temperature occurred more rapidly in the vascular cambium than xylem following heat exposure. Importantly, the xylem remained functional until the most extreme treatments, long after the vascular cambium had died. Our results suggest that the viability of the vascular cambium may be more critical to post-fire survival than xylem function in S. sempervirens. Given the complexity of fire, we recommend ground-truthing the cambial and xylem post-fire response on a diverse range of species. 

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2. Predicting immediate and delayed fire-induced mortality of Pinus monticola and Pseudotsuga menziesii saplings using a pyro-ecophysiology fire severity approach

   https://doi.org/10.1071/WF25003  

   Blanco, Alexander S; Wilson, David R; Hardman, Douglas D; Rainsford, Scott W; Sparks, Aaron M; Bhatta, Roshan P; Huang, Li; Adams, Henry D; Johnson, Daniel M; Schwilk, Dylan W; et al (November 2025, International Journal of Wildland Fire)

   BackgroundAlthough fires can cause tree mortality or reduce post-fire growth in trees of all ages, and models exist that predict fire-induced mortality in mature trees, the development of predictive models of how fires impact younger trees has received less attention. AimsTo assess whether inclusion of fire behaviour metrics alongside pre- and post-fire sapling morphological traits improve the prediction of fire-induced tree mortality as compared to existing models. MethodsIn this study, we subjected Pseudotsuga menziesii (Mirb.) Franco var. glauca (Beissn.) and Pinus monticola var. minima Lemmon saplings to increasing levels of fire intensity and evaluated models to predict immediate and delayed post-fire mortality. Key resultsFor Pinus monticola, the optimal model relied on the post-fire crown volume scorched, while for Pseudotsuga menziesii the optimal model used flame height and fire radiative energy. We show that while Pinus monticola saplings exhibit immediate fire-induced mortality, Pseudotsuga menziesii saplings are prone to delayed fire-induced mortality. ImplicationsEven in younger trees, crown volume scorched and related metrics remain consistent predictors of fire-induced tree mortality. Future studies should track mortality over extended periods to ensure that developed models better represent delayed fire-induced tree mortality. 

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3. Pre-existing embolism causes reverse vulnerability segmentation in Populus

   https://doi.org/10.1093/jxb/eraf523  

   Rimer, Ian M; Manandhar, Anju; McAdam, Scott_A M (November 2025, Journal of Experimental Botany)

   Steppe, Kathy (Ed.)

   Abstract Using the optical vulnerability method, we evaluated leaf and stem embolism resistance across three Populus species (P. trichocarpa, P. deltoides, and P. grandidentata) grown under field and glasshouse conditions to explore the mechanisms of vulnerability segmentation between organs. Classical vulnerability segmentation occurs when leaf xylem is more vulnerable to embolism than stem xylem, with leaves serving as hydraulic ‘fuses’ that protect the hydraulic integrity of the stem during drought. Recent evidence suggests that reverse vulnerability segmentation—when leaves have higher embolism resistance than stems—may occur in Populus species. We observed reverse segmentation exclusively in field-grown older stems, while no segmentation was found in glasshouse-grown or newly formed stems. X-ray micro-computed tomography and hydraulic measurements confirmed that the more vulnerable field-grown older stems had significant native embolism (>25% loss of conductivity). These findings support the hypothesis that reverse segmentation arises not from inherent xylem properties, but from the accumulation of native embolism, probably induced by winter freeze–thaw cycles or stem age. Our results provide a mechanistic explanation for reverse segmentation and suggest that its occurrence in Populus may be a byproduct of environment and life history rather than an adaptive trait of xylem architecture. 

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4. The relative area of vessels in xylem correlates with stem embolism resistance within and between genera

   https://doi.org/10.1093/treephys/tpac110  

   Avila, Rodrigo T; Kane, Cade N; Batz, Timothy A; Trabi, Christophe; Damatta, Fábio M; Jansen, Steven; McAdam, Scott A (September 2022, Tree Physiology)

   Martinez-Vilalta, Jordi (Ed.)

   Abstract The resistance of xylem conduits to embolism is a major factor defining drought tolerance and can set the distributional limits of species across rainfall gradients. Recent work suggests that the proximity of vessels to neighbors increases the vulnerability of a conduit. We therefore investigated whether the relative vessel area of xylem correlates with intra- and inter-generic variation in xylem embolism resistance in species pairs or triplets from the genera Acer, Cinnamomum, Ilex, Quercus and Persea, adapted to environments differing in aridity. We used the optical vulnerability method to assess embolism resistance in stems and conducted anatomical measurements on the xylem in which embolism resistance was quantified. Vessel lumen fraction (VLF) correlated with xylem embolism resistance across and within genera. A low VLF likely increases the resistance to gas movement between conduits, by diffusion or advection, whereas a high VLF enhances gas transport thorough increased conduit-to-conduit connectivity and reduced distances between conduits and therefore the likelihood of embolism propagation. We suggest that the rate of gas movement due to local pressure differences and xylem network connectivity is a central driver of embolism propagation in angiosperm vessels. 

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5. Seeing is believing: what visualising bubbles in the xylem has revealed about plant hydraulic function

   https://doi.org/10.1071/FP21326  

   Cardoso, Amanda A.; Kane, Cade N.; Rimer, Ian M.; McAdam, Scott A. (January 2022, Functional Plant Biology)

   Shabala, Sergey (Ed.)

   Maintaining water transport in the xylem is critical for vascular plants to grow and survive. The drought-induced accumulation of embolism, when gas enters xylem conduits, causes declines in hydraulic conductance (K) and is ultimately lethal. Several methods can be used to estimate the degree of embolism in xylem, from measuring K in tissues to directly visualising embolism in conduits. One method allowing a direct quantification of embolised xylem area is the optical vulnerability (OV) technique. This method has been used across different organs and has a high spatial and temporal resolution. Here, we review studies using the OV technique, discuss the main advantages and disadvantages of this method, and summarise key advances arising from its use. Vulnerability curves generated by the OV method are regularly comparable to other methods, including the centrifuge and X-ray microtomography. A major advantage of the OV technique over other methods is that it can be simultaneously used to determine in situ embolism formation in leaves, stems and roots, in species spanning the phylogeny of land plants. The OV method has been used to experimentally investigate the spreading of embolism through xylem networks, associate embolism with downstream tissue death, and observe embolism formation in the field. 

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