Frost and Winter Injury

Injury of trees by subfreezing temperatures is complex.  We generally distinguish between injury during the growing season (frost injury) vs. the winter (winter injury). 

Frost

Spring frost injury in Quercus gambelii (Gambel oak). The leaves that were expanding at the time of the frost are dead. The live leaves were produced after the frost.
Spring frost injury to flowers of Symphoricarpos oreophilus (mountain snowberry). Flowers and leaves of snowberry develop at about the same time in spring. Sometimes both are killed by a frost, but in this case the stage of development and severity of frost were such that the leaves survived. Such an event can lead to complete absence of fruit in the area, with significant implications to wildlife.

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 ​[2]​.

It seems to be a paradox, but there is abundant evidence that climate warming has increased the risk of spring frost damage ​[1, 4, 5]​.  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

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 ​[2]​, 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.

Winter Injury

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, 6, 7]​.

In some cases, cambium and phloem are killed.

Winter Sunscald

Another type of cold injury is sunscald. Sunscald is killing of cambium and phloem due to temperature changes associated with solar exposure, especially when solar exposure warms and deacclimates the bark, which then freezes.  The rapid and large temperature drop apparently kills the tissues. It is most common where exposure of tree stems to the sun suddenly increases, such as by harvest, road construction, or a severe blowdown event. See the page on Sunburn and Sunscald.

More general information is available on abiotic diseases and injury.

References

  1. 1.
    Augspurger CK. 2013. Reconstructing patterns of temperature, phenology, and frost damage over 124 years: Spring damage risk is increasing. Ecology 94(1):41–50 <10.1890/12-0200.1>.
  2. 2.
    Boyce JS. 1961. Forest Pathology. The American Forestry Series vol. New York: McGraw-Hill Book Company. 572 pp. 3rd ed.
  3. 3.
    Chappelka AH, Freer-Smith PH. 1995. Predisposition of trees by air pollutants to low temperatures and moisture stress. Environ Pollut 87(1):105–17 <10.1016/S0269-7491(99)80013-X>.
  4. 4.
    Gu L, Hanson PJ, Post WM, Kaiser DP, Yang B, Nemani R, Pallardy SG, Meyers T. 2008. The 2007 eastern US spring freeze: Increased cold damage in a warming world? Bioscience 58(3):253–262 <10.1641/B580311>.
  5. 5.
    Hänninen H. 2006. Climate warming and the risk of frost damage to boreal forest trees: identification of critical ecophysiological traits. Tree Physiology, pp. 889–898 <10.1093/treephys/26.7.889>.
  6. 6.
    Schaberg PG, Lazarus BE, Hawley GJ, Halman JM, Borer CH, Hansen CF. 2011. Assessment of weather-associated causes of red spruce winter injury and consequences to aboveground carbon sequestration. Can. J. For. Res.-Rev. Can. Rech. For. 41(2):359–369 <https://www.ncrs.fs.fed.us/pubs/jrnl/2011/nrs_2011_schaberg_001.pdf>.
  7. 7.
    Vann DR, Strimbeck GR, Johnson AH. 1992. Effects of ambient levels of airborne chemicals on freezing resistance of red spruce foliage. For. Ecol. Manage. 51(1–3):69–79 <10.1016/0378-1127(92)90473-m>.