Freeze dehydration vs supercooling in tree stems: physical and physiological modelling.

Frost resistance is the major factor affecting the distribution of plant species at high latitude and elevation. The main effects of freeze-thaw cycles are damage to living cells and formation of gas embolism in tree xylem vessels. Lethal intracellular freezing can be prevented in living cells by two mechanisms, such as dehydration and deep supercooling. We developed a multiphysics numerical model coupling water flow, heat transfer and phase change, considering different cell types in plant tissues, to study the dynamics and extent of cell dehydration, xylem pressure changes and stem diameter changes in response to freezing and thawing. Results were validated using experimental data for stem diameter changes of walnut trees (Juglans regia). The effect of cell mechanical properties was found to be negligible as long as the intracellular tension developed during dehydration was sufficiently low compared with the ice-induced cryostatic suction. The model was finally used to explore the coupled effects of relevant physiological parameters (initial water and sugar content) and environmental conditions (air temperature variations) on the dynamics and extent of dehydration. It revealed configurations where cell dehydration could be sufficient to protect cells from intracellular freezing, and situations where supercooling was necessary. This model, freely available with this paper, could easily be extended to explore different anatomical structures, different species and more complex physical processes.