Water availability has been known to strongly modulate forest productivity and tree growth on an interannual basis (as revealed by numerous dendrochronological studies) and across biomes (Ellison et al, 2017). Recurrent episodes of severe drought lead to decreased soil water content and as a consequence to visible losses in annual growth increment, and in some cases even to tree death and forest decline. The occurrence of such drought events and of larger scale tree dieback, seem to be increasing over the last decades, albeit such processes are not new. The causes for drought-induced tree death are still disputed; in many cases, tree death occurs after the release of drought, and is caused by severe attacks by pests and pathogens. In other cases, tree death is caused by recurrent drought events over several years, leading to a depletion of stored carbohydrates, growth decline and ultimately death.
However, this understanding of drought-induced tree dieback, which applies to drought events that occurred in temperate climate biomes during the end of the 20th century, seems inadequate to explain the increasing occurrence of large scale dieback induces by recent drought episodes (Allen et al, 2015). In these recent cases a direct impairment of hydraulic functions seems responsible for tree death. Such impairments (cavitation and resulting massive embolism) have been well documented through extensive research that started in the 90s. Up to now, the consensus was that trees are fairly well protected against such potentially lethal dysfunctions: an efficient stomatal closure limits transpiration and the risk of runaway embolism. Many tree models based on the known hydraulic properties of trees (vulnerability of different organs to cavitation, hydraulic conductance of these organs, transpiration, stomatal conductance…) were developed since the seminal work of Tyree and Sperry (1989) and only seldom predicted the occurrence of runaway embolism (Cochard and Delzon, 2013).
These models considered the impact of drought through reduced soil water availability, which is indeed the central process during drought, but overlooked to some extent the fact that drought is frequently and increasingly associated to higher temperatures, which may change rather severely model parameters and result in a higher risk of runaway embolism.
The present preprint proposed by Cochard (2020) bases on such a new hydraulic model (the model SurEau, Martin StPaul et al, 2017; Cochard et al, 2020) integrating more explicitly the impact of temperature on different parameters. Two parameters appear particularly relevant and highly sensitive to temperature:
(i) the vapor pressure deficit of the air (VPD), which increases exponentially with temperature and results in increased transpiration and more rapid soil water depletion; this effect is well known and has been the matter of many research and modelling;
(ii) the cuticular conductance to water vapor, which becomes the most important limit to transpiration once stomata are closed, and which is much less well documented with respect to mean values and temperature sensitivity (mainly because this process is difficult to record). Recent advances (Schuster et al, 2016) provided some insight into the importance of this parameter and showed how it may rapidly increase with temperature (see references in the preprint).
The presented work bases on this new model to document more precisely how enhanced temperature may increase water loss through transpiration and consequently induce runaway embolism in trees more rapidly than usually expected. The hypothesis that the temperature response of cuticular conductance may play a central role in the sensitivity of trees to a combination of soil water depletion and enhanced air (and leaf) temperature was tested through numerical simulations with SurEau. The results are very clear: temperature-dependent increases in cuticular conductance may accelerate the onset of runaway embolism at a rate that was not expected before.
The demonstration is indeed very clear and convincing. It remains however a simulation (or an “in silico experiment”. Data providing real values of cuticular conductance remain scarce, and data documenting its response to enhanced temperatures even scarcer. This opens an avenue for new research and investigations, and Cochard (2020) provides some clues about which data and which experiments could confirm the central role of temperature induced changes in cuticular conductance with temperature (eg new measurements of Tp, the phase transition temperature that matches the range of temperatures known to trigger mortality during hot-droughts, Billon et al. (2020)).
I believe this preprint is an important contribution in this field, and the reviewers were of the same opinion (see their reviews attached to this recommendation). Indeed, this preprint illustrates how simulation exercises can help us identify some key processes that require further attention and documentation. I believe this is an important contribution to our understanding of the rapid, drought-induced tree death observed in different parts of the world at alarming rates.
As such, and combined with a detailed description of the model SurEau, this preprint is a very important addendum to the corpus of knowledge that is currently gathered around the hydraulic functioning of trees.

References

Allen, C. D., Breshears, D. D., and McDowell, N. G. (2015). On underestimation of global vulnerability to tree mortality and forest die‐off from hotter drought in the Anthropocene. Ecosphere, 6(8), 1-55. doi: https://doi.org/10.1890/ES15-00203.1
Billon et al. (2020). The DroughtBox: A new tool for phenotyping residual branch conductance and its temperature dependence during drought. Plant, Cell and Environment, 43, 1584-1594. doi https://doi.org/10.1111/pce.13750
Cochard, H. (2020) A new mechanism for tree mortality due to drought and heatwaves. bioRxiv, 531632, ver. 2 peer-reviewed and recommended by PCI Forest and Wood Sciences. doi: https://doi.org/10.1101/531632
Cochard, H., Martin-StPaul, N., Pimont, F., and Ruffault, J. (2020). SurEau.c: a mechanistic model of plant water relations under extreme drought. bioRxiv, 2020.05.10.086678. doi: https://doi.org/10.1101/2020.05.10.086678
Ellison et al. (2017). Trees, forests and water: Cool insights for a hot world. Global Environmental Change, 43, 51-61. doi: https://doi.org/10.1016/j.gloenvcha.2017.01.002
Martin‐StPaul, N., Delzon, S., and Cochard, H. (2017). Plant resistance to drought depends on timely stomatal closure. Ecology letters, 20(11), 1437-1447. doi: https://doi.org/10.1111/ele.12851
Schuster et al. (2016). Effectiveness of cuticular transpiration barriers in a desert plant at controlling water loss at high temperatures. AoB Plants, 8(1), plw027. doi: https://doi.org/10.1093/aobpla/plw027
Tyree, M. T., and Sperry, J. S. (1989). Vulnerability of xylem to cavitation and embolism. Annual review of plant biology, 40(1), 19-36. doi: https://doi.org/10.1146/annurev.pp.40.060189.000315