ESCANABA — Thin-barked trees can be damaged during the winter, especially late winter, on sunny days when temperatures bounce around the freezing mark. However, the damage doesn’t become visible until spring.

For tree species such as maples and apples, particularly those with darker thin bark, dead tissue might be seen sloughing-off come late April or into May. This appears as some sort of disease. More commonly, it is the latent impact of sunscald. Frost-cracking is another term for this phenomenon.

Earlier, often in February and March, the tissues are killed. However, the damage is not noticeable at that time. Light sunny days, with a blanket of snow on the ground, solar insolation (not insulation!) can wreak a bit of havoc on the south and southwest sides of saplings with thin bark, and about three or four feet above the ground. The process is an unfortunate combination of tree physiology and physics.

The thin bark surface, especially darker bark, absorbs energy directly from the sun and that which is reflected off the snow. the ambient air temperatures are below the freezing mark. This microclimate warms the cambium, those living tissues under the bark.

The cells become “faked-out” by thinking spring is at-hand and absorb water which has been safely residing between the living cells as ice. Forcing water out of living cells and into those interstitial spaces is part of the strategy that trees use to survive cold winters. Warmed and re-hydrated, the cellular machinery begins to do what it’s designed to do.

occurs when the sun sets or shadows fall on the young tree trunks. The time frame of this rapid temperature drop is too short for the cell metabolism to force the water back into the safe interstitial areas. Without that warm microclimate, the temperatures quickly fall below the freezing mark and water trapped inside the living cells turns to ice. The expanding ice ruptures the cell walls, killing the fragile tissues.

Yet, under the right circumstances, large portions of the cambium can be killed. These conditions don’t happen every year. When they do, sunscald and frost-cracking occur.

For the remainder of the winter, the young stems appear unaffected. When spring arrives, the resource-rich tree juices flow and the cambium begins to construct a new transport system. However, if winter burn has killed patches of the cambium, this process does not happen. As time passes, the dead tissues dry-out and begin to separate from the interior wood. This is when people usually note the damage.

Nothing can be done to remedy sunscald. However, it can be easily prevented by shading young trees from the sun. A burlap wrap works just fine, or leaning a board on the south side of the tree. The ideas is to keep the trunk out of direct sunlight.

If trees do suffer sunscald, do not paint the wounds. Let the tissues dry-out and fall-off, and then hope that the tree is healthy enough to eventually grow-over the wound. Unfortunately, large patches of sunscald will disfigure the tree and invite germination of fungal spores, which can lead to heart rot. This, too, may not cause tree death but it will weaken the tree and leave it more vulnerable to a range of pests.

Conifers suffer from a somewhat similar malady called “winter burn”, which is a bit of a misnomer, not having little to do with temperature. Rather, on warm winter days, with temperatures above freezing, the needles slowly photosynthesize, which use liquid water. Normally, when nearby water sources are exhausted, the conifer will draw more water from the soil via the root system.

However, if frost remains a couple of feet into the ground, the frozen water is locked and not available to the tree. So, the needles dry-out and tissues begin to die, similar to what may happen during a drought. ar to sunscald, the needles don’t turn yellow, then brown, until spring. When the dead needles are noticed, once again, it’s common to incorrectly assume a disease agent at work. Winter burn also does not happen every winter. And, conifers, especially young conifers, that are more exposed are somewhat more vulnerable.

Winter burn usually does not affect the lifespan of a conifer, but it can cause major problems for Christmas tree growers. The appearance of the tree is all-important. Dead spots certainly detract from marketability.

As winter gives way to spring, especially after a winter with several warm periods when snow still covers the ground, evidence of sunscald and winter burn may become clear. Neither condition is usually fatal, but any damage to a tree can lead to other problems.