Injury of trees by subfreezing temperatures is complex. We generally distinguish between injury during the growing season (frost injury) vs. the winter (winter injury). The ability of trees to withstand cold (acclimation leading to cold hardiness) occurs in two stages .
Actively growing and succulent tissues can be killed by temperatures just a few degrees below freezing (frost). When growth ceases in the fall, susceptibility to frost is reduced by two changes. First is simple hardness of the tissues due to lignification.
Usually, and preferably, the term frost damage is restricted to damage due to subfreezing temperatures during the growing season, or while the tree is not dormant. This can be after bud break, due to a late spring frost. Spring frost damage is fairly common. Young tender shoots are killed, droop and discolor, then eventually fall off. The damage is soon difficult to detect. The featured image above shows a closeup of spring frost damage in Picea pungens (blue spruce). Frost killed the tender, expanding shoots, which lost their turgor and drooped.
Frost damage tends to occur most commonly and severely in low areas where draining of cold air downslope is largely blocked by the topography. These frost hollows or pockets may even be treeless because the frost-free season is often too short for trees to survive. In Colorado, such treeless drainages and valleys, locally termed “parks”, are common. Though not all are due to frost, it appears that most are. Often one can see that the lower treeline of the area corresponds to the elevation of the topographic feature that blocks air drainage at the lower end.
An overstory tends to protect seedlings and saplings, which are most susceptible to frost damage. This may account for regeneration failures after cutting in low areas, and should be considered before cutting in them .
It seems to be a paradox, but there is abundant evidence that climate warming has increased the risk of spring frost damage [1, 4]. Warmer, earlier springs induce premature bud break and foliage and flower development, while timing of late frosts has not changed appreciably.
Less commonly, frost may occur prior to hardening off of shoots and buds, due to an early autumn frost. An early winter in central Alaska led to an unusual situation where the winter came while green leaves were still on the deciduous trees. Leaves persisted through the winter and into the following summer. Apparently buds were not completely set so it probably led to some shoot damage.
Frost cracks are axial openings in a stem that may penetrate radially some distance into the wood. They may spiral somewhat, like lightning scars, presumably following the grain of the wood.
According to Boyce , frost cracks are formed during the dormant season, when there is a large, sudden drop in temperature. The outer wood contracts rapidly as it cools, while the inner wood remains warmer. Wood tends to shrink tangentially more than it does radially, so the result is a radial crack.
However, such cracks may have other origins as well. A stem that is twisted under the force of the wind will tend to have such tangential strain that may result in cracking, and internal decay makes this much more likely.
Damage due to low temperature during winter dormancy is a separate phenomenon, though related. It may actually be two phenomena. In one scenario, which may be called winter freezing, warm weather during dormancy induces partial deacclimation. If it is followed by a rapid and large drop in temperature, there may be freezing damage to shoots and/or buds. Repeated cycles of extreme temperature fluctuation make such trees much more susceptible to such injury. The result is reddening and browning of foliage in the spring (evergreens) and dieback of shoot tips.
Evidence suggests that acid deposition, such as acidity generated by SO2 and NOx in fog, can increase susceptility of trees to winter injury [3, 5].
In some cases, cambium and phloem are killed.
Sunscald is killing of cambium and phloem due to temperature changes associated with solar exposure. It is especially common when exposure of tree stems to the sun suddenly increases, such as by harvest, road construction, or a severe blowdown event.
Sunscald due to actual overheating and drying of bark during the summer is rare on forest trees . It is especially common in winter when solar exposure heats up the bark, which then freezes. The rapid and large temperature drop apparently kills the tissues.
Sunscald is recognized long after by long, narrow scars, mostly below the live crown, on the southern or southwestern side. There is usually evidence of a recent disturbance on that side that increased exposure, and it typically affects groups of trees. It is most severe on trees with thin bark.
Canker and decay fungi may enter through such lesions.