Mechanical and Animal Injury

Mechanical and Animal Injury

Mechanical injury to forest trees is almost inevitable:

  • Wind, snow, and ice storms can break branches and even snap stems (see also Wind and Water.
  • Severe hailstorms can cause defoliation and abundant wounds on the upper side of branches.
  • Falling trees and branches can cause severe wounds to neighboring trees.
  • Lightning strokes cause moderate to severe damage.
  • Logging operations can cause many basal wounds in partial cutting.
  • Surface fires often cause basal scars but leave the tree alive.
  • Animals consume parts of trees (or whole seedlings or suckers) and cause additional mechanical damage.

Wounds that expose wood are usually the most consequential, primarily due to invasion of the wood by decay fungi.  The likelihood of infection and progress of decay increase with the size of the wound (Decays).

Trees are far from defenseless where wounds are involved.  One of the functions of bark is to protect the vital phloem, cambium, and sapwood from mechanical damage.  Trees with thick bark can withstand more violence than those with thin bark.  Trees are particularly vulnerable in the spring, when the cambium is actively dividing and sap is flowing, making separation at the cambium relatively easy.

Animal Injury

Severe damage due to elk feeding on a young aspen sapling. Smaller shoots were consumed as well as bark on older branches and stem.
Populus tremuloides (trembling aspen) with feeding damage by elk. Elk feed on aspen bark in their winter range. This photograph is from late May following the winter in which it occurred. Teeth marks are visible on the wood surface. Below the fresh wounds are darker, callused wounds from feeding in previous years.

Many animals feed on living plants or use them for other purposes. When trees are young, they can be severely affected by animals.  Among the common types of damage:

  • Beaver fell trees to build dams and harvest the small branches that they consume for food.
  • Elk consume young aspen suckers entirely, cause extensive feeding damage to larger suckers and young saplings, and inflict large wounds while feeding on bark of mature trees.
  • Male deer and elk often scrape off bark and break branches with their antlers while trying out their new headgear in autumn, and as a display to others in the area.
  • Woodpeckers create cavity nests in trees with stem decay.
  • Sapsuckers, a type of woodpecker, excavate fascinating sap wells in geometric patters on trees.  These excavations create a large surface area of severed sieve tubes in the phloem, which then leak nutritious phloem sap.  The birds feed on the sap as well as insects that are attracted to the wells.  Some tree species can be heavily damaged and killed by the cumulative effect [3].
Populus tremuloides (trembling aspen) with feeding damage by elk. Elk feed on aspen bark in their winter range. The fresh wounds are light colored with teeth marks. Older wounds are black spots, of which the small ones have mostly callused over.
Sapsucker damage on Pinus flexilis (limber pine).
Sapsucker damage on Abies concolor (white fir).

Girdling Roots

Sometimes roots develop in an unfortunate position, choking off other roots or the base of the stem as they expand. This is especially common on planted seedlings. It is made more likely by growing of seedlings in containers or close cultivation of the soil.

Lightning

Lightning causes significant injury to forest and shade trees. In a study of Pinus ponderosa forests in northern Arizona, lightning was responsible for 1/3 of the mortality and caused additional defect and cull in living trees [4]. In southern pines in Arkansas, it accounted for 70% of the mortality volume [2].

Lightning can cause causes mortality of tops, branches or whole trees, creates wounds for decay entry, and often leads to a slow death. The current usually goes along the cambium, but may go along the outer surface for a portion of the distance (discontinuous damage) or may even go inside the wood (shattering of stems).

Look for long strips of bark blown off the wood. There may be some outer sapwood blown off. It may even look like the tree, or a portion of it, exploded (it did!). Lightning scars often spiral around the stem, presumably following the grain of wood. Usually one can see that it goes all the way to soil. If you are visiting soon after the event, you may even see that soil was blown off the roots as the charge entered the soil.

Lightning may also damage and kill trees with no outward, visible lightning wounds [1]. Trees may be killed in groups in such a way, possibly due to current in the soil.

References

1.
Boyce JS. 1961. Forest Pathology. New York: McGraw-Hill Book Company. 572 pp. 3rd ed.
2.
Reynolds RR. 1940. Lightning as a cause of timber mortality. Southern Forestry Notes (Southern Forestry Research Station) 31:1.
3.
Rushmore FM. 1969. Sapsucker damage varies with tree species and seasons. Research Paper NE-136. USDA Forest Service, Northeastern Forest Experiment Station. [Source]
4.
Wadsworth FH. 1943. Lightning damage in ponderosa pine stands of northern Arizona. Journal of Forestry 41:684–685.