Exploding trees?

There have been some bitterly cold days this winter, with temperatures well into the minus teens and twenties Celsius. Clear, cold days are great times to be in the woods, but you may notice some unusual noises: the snap, crackle and pop of trees! Media have been having fun with sensational ‘exploding tree’ headlines, but let’s look beyond the clickbait.

A clear, bitterly cold day in the woods on PEI.

Trees do their best to prepare for winter. This includes getting rid of those moisture-sucking, wind-catching leaves; moving some water out of living cells and into the spaces around those cells; and concentrating sugars in living cells to act as antifreeze and lower the freezing point. Even with all of this preparation, some freezing will occur. Evidence of this can be both heard and seen.

You may have heard a loud ‘pop’ in the winter woodlands, not unlike the crack from a .22 calibre rifle. Inside each tree is an internal plumbing system comprised of tiny pipes (called xylem and phloem) that move water, nutrients and sugars throughout the tree.

Phloem is made up of living cells, which are the tree’s priority to protect; it does this by removing some water and concentrating sugar within those cells. Xylem is mostly dead cells (in trees, it forms wood) and it carries more water. As a result, xylem will sometimes freeze and burst, causing a loud, sharp sound. (This can happen to phloem too, it’s just less common). The xylem (and phloem) tubes are so tiny and so numerous that losing some to freezing doesn’t harm the tree.

The effects of freezing can also be seen on some trees in the form of frost cracks: long, vertical cracks in the bark. These occur more often in late winter, as we start getting more warmth from the sun.

Daytime sunshine warms and expands both the outer bark and the living cells underneath. After sundown, the sudden drop in temperature causes the outer bark to quickly cool and contract, while cells underneath cool and contract more slowly. This differential can cause the bark to stretch and crack, with a sound similar to exploding xylem. Frost cracks can also happen during sudden and severe temperature drops mid-winter, if the outer layers of the tree cool and contract more quickly than the inner wood. While frost cracks can’t be repaired, they don’t usually kill the tree.

We have lots of winter left to enjoy. If you hear the forest cracking and popping in the coldest weather, you’ll now know that this is a natural part of PEI Untamed!