Larix Laricina - Damaging Agents

Damaging Agents

The tamarack has thin bark and is therefore highly susceptible to fire damage, except perhaps in older, upland stands. However, the habitat of tamarack, especially south of the boreal forest, is normally wet enough to protect the tree from fire. The tamarack is also susceptible to high winds. Strong winds can uproot large tamarack trees growing in swamps or other wet-land sites where rooting is shallow. It has also been discovered that abnormally high water levels often kill tamarack stands. Those that survive under such conditions usually grow very slowly. Other effects of high water include dieback and the development of adventitious roots and shoots. Wetland road crossings and beaver damming are the primary causes of flooding.

Many insect species are known to be destructive to tamaracks. The larch sawfly is the most destructive. Epidemics occur periodically across Canada and the northern United States and have caused tremendous losses of merchantable tamarack throughout most of the tree's range. Indications are that radial increment declines markedly after 4 to 6 years of outbreak. After 6 to 9 years of moderate to heavy defoliation the trees die. In southeastern Manitoba and northern Minnesota, however, imported parasites of the sawfly have become established and should reduce the frequency and duration of future outbreaks. Another serious defoliator is the larch casebearer Coleophora laricella. The larch casebearer attacks tamarack of all ages, and several severe outbreaks have caused extensive mortality in some areas. Outbreak severity has lessened in recent years, however, probably due to imported parasites of the casebearer that have become widely established.

Only a few other insects and related organisms (such as mites) that feed on tamarack are known to sometimes cause serious injury. During an outbreak the spruce budworm (Choristoneura fumiferana) can severely damage tamarack. The larch bud moth (Zeiraphera improbana) has had occasional short epidemics, and the spruce spider mite (Oligonychus ununguis) is occasionally found in large numbers on tamarack. The larch shoot moth (Argyresthia laricella) is widely distributed but serious injury is unusual. One of the most common bark beetles attacking tamarack is the eastern larch beetle (Dendroctonus simplex), but it feeds mainly on weakened, dying, or dead trees.

Tamarack is a host to many pathogens, but none cause diseases serious enough to have an economic impact on its culture. The only common foliage diseases are rusts, such as the leaf rust in eastern and central North America. However, this rust, caused by the fungus Melampsora medusae, and other rusts do little damage to tamarack. The needle-cast fungus Hypodermella laricis has attacked tamarack in Ontario and has the potential for local damage.