When temperatures drop suddenly, trapped water can freeze and expand, splitting tree trunks with a gunshot-like sound. This phenomenon, known as frost cracking, has been observed during recent cold spells in the northern United States, prompting warnings from meteorologists.
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 cellular changes that include dehydration, hardening, and shrinkage, increasing sugar concentration. This acts as a botanical antifreeze, helping to prevent ice crystal formation.
However, 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 tree's structure. In severe cold, the tree may give way, sometimes with an explosive sound, producing deep vertical gaps known as frost cracks. This is especially likely on sunny days when the temperature of sun-warmed bark drops rapidly at night.
Historical accounts, such as John Loudon's Encyclopaedia of Gardening from 1822, describe severe winters leaving trees “miserably split and cleft”, sometimes breaking them in two, “attended with dreadful noises like the explosion of firearms”.
While frost cracks can harm trees by allowing insects and fungi to enter, exploding trees are not dangerous to bystanders. However, witnesses report that the sudden percussive sounds from a dark forest can be unsettling.
