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Scars found on the bole of trees are caused by a number of agents; consequently, associated symptoms may be variable. Charring indicates a fire origin; extensive scars associated with broken and missing branches suggest damage caused by falling trees; and scars encompassing branches indicate animal or fungus attack. Logging activities frequently result in tree scars, which are usually irregular in outline, sometimes deeply gouged and most frequently near the base of the tree (Fig. 64a). Various diseases, notably Armillaria root rot, are also responsible for basal lesions. Direct loss from scarring is usually small; however, if the wood becomes infected with stain or wood decaying fungi, the indirect loss may be substantial (Fig. 64b).
Frost injury usually results from a sudden drop in air temperature before plant tissues have hardened-off. Localized lesions are formed when the bark and cambium are killed. Eventually, the dead tissues covering the lesion are sloughed-off exposing the sapwood. Several years may be required before the lesion is callused over. During this period the exposed wood acts as an entrance court for wood decay fungi capable of causing extensive damage to the heartwood. All tree species are susceptible to this damage.
Frost cracks (Fig. 64c) are formed when there is a pronounced drop in temperature during the dormant period of tree growth. The inner wood remains comparatively warm while the outer wood becomes cold and contracts rapidly causing cracks or splits in the trunk. Repeated opening of the cracks by cold temperatures or trunk movement results in considerable callus tissue growth that frequently appears as black raised lines on the stem. Frost cracks provide entrance courts for wood decay fungi and as such may result in significant damage.
Sunscald is caused by the intense heat of direct sunrays. Damaged areas on the main stem, usually on the southwest side of the tree, are initially copper to bright red, standing out in marked contrast to adjacent healthy bark. These colours are not persistent and it may be difficult after a period of time to recognize previously damaged tissues. In severe cases, the bark in affected areas dies and sloughs off. Rapidly growing trees and trees suddenly exposed to strong sunlight, as may occur following thinning or pruning, appear to be most susceptible. There is thought to be no permanent injury unless the bark is killed creating open scars through which wood decay fungi can gain entrance to susceptible tissues.
Although very low temperatures are required to kill trees, particularly if they are established on good sites, temperatures just below freezing are sufficient to damage foliage that has not hardened-off. Foliage may also be killed when exposed to warm, drying winds when the ground is still frozen because the trees are unable to replace the water lost through transpiration; the needles become desiccated, turn red and die (Fig. 64d). In mountainous areas injury may be confined to an altitudinal zone corresponding to the pathway of drying winds, hence the common name "red belt." Height and diameter growth are retarded and damaged trees are weakened and predisposed to attack by other agents.
Although not a disease, reference is made to cedar flagging because of interest in its cause and significance. Flagging is a normal condition in western redcedar; branchlets die during the dry, hot portion of the summer and toward the end of the growing season. It is recognized by the occurrence of isolated dying or dead, yellow or red branchlets (Fig. 64e). The condition is not believed to have a detrimental effect on tree growth. Cedar flagging could be confused with discoloured foliar symptoms caused by cedar leaf blight (Didymascella thujina). The fungal disease can be distinguished by its scattered foliar symptoms and the presence of fruiting bodies, when present.
Top killing of trees may be caused by fungi, insects, or adverse climatic disturbances. Killing by low temperatures is most likely to occur in exposed areas, such as ridges, or in frost pockets. Drought injury is generally prevalent on sites of low moisture-holding capacity; shoot dieback may be progressive if the moisture deficiency is severe and prolonged, appearing first in the upper crown, and generally extending downward. The first signs of drought damage to broadleaf foliage are wilting, followed by browning and drying from the margins inward (Fig. 64f). Fungi may attack the weakened and dead portions of a tree, extending damage to adjacent tissues.
A number of man-made chemicals in the form of gases (SO2, other atmospheric pollutants) or liquids (pesticide sprays, acid rain, road salt spray) can be damaging to trees. When trees are injured by noxious industrial fumes, the foliage absorbs the gases for a prolonged period before the injury becomes apparent. Symptoms and severity of damage vary among trees species, concentration and type of gas, duration of exposure, and distance from the source of the fumes. Generally, necrosis of conifer needles starts at the tips (Fig. 64g), while broadleaf foliage is affected first in the tissues between the veins, giving the leaf a mottled appearance (Fig. 64h). Foliage buds, branches, and entire trees may be killed, the damage sometimes occurring over extensive areas. Damage from salt spray is common on foliage in areas where salt is applied to roads in the winter (Fig. 64i).
In young trees, broken tops, attributable to snow, ice, and wind, and to injuries caused by animals and insects, result in double or multiple leaders (Fig. 64j). This is not usually of consequence for, unless a tree is affected repeatedly, one of the leaders will assume dominance and the reduction in height growth will be negligible. In older trees top breakage is usually attributable to the action of ice and snow. Breakage in stems greater than 10 cm diameter frequently serve as entrance courts for wood decay fungi and substantial volumes of timber can be lost.
Sapsuckers are only one of several groups of birds that cause tree damage. When feeding they puncture the bark in definite patterns, for example, partial rings around tree limbs or uniform vertical rows on the main stem (Fig. 64k). These punctures serve as entry points for stain or decay producing fungi.
When the soil surrounding tree roots becomes waterlogged, roots are often killed resulting in tree death (Fig. 64l). Trees damaged through flooding may be more susceptible to infection by fungal pathogens. Different tree species tolerate flooding to different degrees, for example, alder, willow, poplar, lodgepole pine, and black spruce can survive in soils that are periodically flooded, whereas white and Sitka spruce are more sensitive.
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Figure 64a: Large stem scar resulting from logging injury.
Figure 64b: Decay in western larch associated with a 10-year old injury.
Figure 64c: Frost crack on subalpine fir.
Figure 64d: Drought mortality in lodgepole pine and Douglas-fir.
Figure 64f: Foliar symptoms on black cottonwood due to moisture deficit.
Figure 64g: Air pollution damage to spruce.
Figure 64h: SO2 damage to red alder leaves.
Figure 64i: Roadside salt damage to Douglas-fir.
Figure 64j: Snow damage to young aspen.
Figure 64k: Sapsucker feeding scars.
Figure 64l: Trees killed by flooding.