The winter coats are out, as are the hats, scarves, and mittens. We humans are hoping for lots of sunny days this winter to help keep our spirits lifted through the darkest days of the calendar. All around us, however, the trees in the forest are hoping for just the opposite: the less sunshine, the better.
Winter sunshine can be a tree’s worst enemy. Deciduous trees, without protection from the needles that conifers have, absorb sunlight whenever the sun is out. The sun warms the wood, and as the wood heats up, it expands. As soon as that sun goes down, however, temperatures begin to drop, and the wood contracts quickly. If all the wood contracted at the same rate, this wouldn’t be a problem, but the outer wood, closer to the cooling air, contracts faster than the inner wood, which is better insulated. When the outer wood exceeds its maximum elasticity, you might hear a loud popping sound as the tree cracks open, creating what is called a frost crack.
These cracks usually occur where a tree was previously damaged, either from root or branch dieback or an accident of some kind. Scars from these injuries are weak, and the tree already has some inner decay associated with the injury, making it more susceptible to later damage. When a tree undergoes a fast cooling at sunset, the areas along the edges of existing injuries exist are most likely to crack.
Frost cracks can be many feet long and tend to recur where previous cracks have damaged the tree. Each time the outer wood and bark cool faster than the inner wood, a frost crack can expands a little more. In addition, water can seep directly into a crack, wedging it open with each freeze and thaw.
Frost cracks are usually found on the south side of the trunk, where the winter sun hits the tree directly. They tend to occur on the lower trunk, since that is an area prone to damage and uneven growth. A crack may heal over the summer but then reopen once the sun drops below the horizon on the next chilly winter evening. Especially prone to cracking are trees growing in openings, which do not have the surrounding trees to block some of the harsh sun.
Frost cracks do not themselves kill a tree. They do create an opening for fungi and insects, but the inner decay associated with the previous injury that began the frost crack in the first place is usually the real threat. This can weaken the structural integrity of the tree, making it a target for wind blowdowns.
Frost cracking is not the only winter injury that trees face. Another similar injury is called sunscald, and it occurs in thin-barked trees such as birch, aspen, and beech. Although caused by sudden temperature drops similar to frost cracking, sunscald damages the tree in an entirely different way. Sunshine warms the cells under the bark above the freezing point, causing water in the tree to melt and be absorbed into living cells. A sudden temperature drop can catch these cells off guard, freezing them solid before they can expel the liquid water back into the trunk and branches.
A tree that has suffered sunscald will most likely have discoloration of the bark, ranging from yellow to orange to a muted red. Peeling of the surrounding bark is also likely to occur. Although sunscald cannot kill a tree, it makes the tree more susceptible to other diseases or insects by creating an opening in its protective outer bark as well as disrupting the flow of water and nutrients between the roots and branches.
Our thin-barked trees in Vermont and New Hampshire attempt to escape this fate by having light-colored bark, which reflects some of the sun’s heat instead of absorbing it into the wood. But what about our other trees?
Trees with rough bark use the coarse texture to release the heat that builds up in the wood throughout the day. These fins or ridges of bark act much like a radiator, sending heat out into the air instead of retaining it in the wood. This adaptation does not always work, however, as maples, oaks, and ash, among others, are still susceptible to sunscald. This is especially true for young trees of these species, whose bark is much smoother than it is when these trees reach maturity.
Walking through the woods on a sunny winter’s day, it is hard to believe that the sunshine that makes winter bearable for us is a tree’s nemesis. For us, the danger of winter is the lack of heat, but for most trees, the danger is from too much sun.