The low temperature limit of tree xylem

Trees are among the most cold-hardy plants. In midwinter, some boreal tree species can even survive immersion in liquid nitrogen (−196 °C), while many Central European trees are killed far earlier – a limit that shapes their latitudinal distribution.

The decisive tissue is the living wood. The parenchyma cells in the sapwood are what limits a tree: once killed, the tree is lost. How they survive freezing is still largely unknown, especially in European species. Extremely hardy cells appear to tolerate freezing by cellular dehydration, while less hardy cells avoid it by supercooling – cellular water stays liquid far below zero without cellular dehydration before it freezes spontaneously and the cell dies. However, there is growing evidence that supercooled cells dehydrate as well – the clean split between types may not hold. What decides, which way a cell goes is likely written into its architecture and chemistry.

Until now, the discrimination between freeze dehydration and supercooling was based on a single freezing signal: present or absent. A high-resolution calorimeter instead measures how much cell water is still liquid at every temperature. Cryo-microscopy reveals where and when ice forms within the xylem. Quantitative wood anatomy, Raman spectroscopy and atomic force microscopy then link freezing behaviour to cell structure, cell wall properties and chemical composition.

Cold hardiness of the wood is what draws the edges of many tree distributions on the map. Knowing which anatomical and chemical traits set that limit tells us which species still have reserves as the climate shifts, and which are already close to their edge – with consequences for forestry, fruit growing and the choice of ornamental and urban trees.