During the recent cold spell in the northern US, meteorologists issued warnings about exploding trees.
A tree’s first line of defence against freezing is its bark, which provides efficient insulation. In cold conditions, trees also enter a form of hibernation, with changes at a cellular level: cells dehydrate, harden and shrink, increasing their sugar concentration. This is the botanical equivalent of adding antifreeze, helping to prevent the formation of ice crystals.
But when temperatures drop suddenly, trees may not have fully acclimatised. The outer layer of wood may still contain water, which freezes and expands, putting pressure on the structure of the tree. In severe cold the tree may give way, sometimes with an explosive sound, producing deep vertical gaps known as frost cracks. Frost cracking is especially likely on sunny days when the temperature of the sun-warmed bark drops rapidly as night falls.
In his Encyclopaedia of Gardening from 1822, John Loudon described how a severe winter left many trees “miserably split and cleft”, literally breaking some in two, “attended with dreadful noises like the explosion of firearms”.
Frost cracks can harm trees as they may be exploited by insects and fungi, but exploding trees are not dangerous to bystanders. Witnesses do say, however, that the sudden percussive sounds from a dark forest can be unsettling.
