Dwarf Mistletoe Management Guidebook Table of Contents]

Distribution of dwarf mistletoes

The following four maps (Figures 1 to 4) illustrate the geographic distribution of dwarf mistletoes in British Columbia.

Note: With the exception of a few areas in the Prince Rupert Forest Region, western hemlock dwarf mistletoe occurs only in coastal, not interior, hemlock stands.

Hosts and symptoms

Trees of all ages can be parasitized and affected by dwarf mistletoes. In British Columbia, there are four dwarf mistletoe species of concern (Table 1). For detailed information on their field identification, refer to the Field Guide to Pests of Managed Forests in British Columbia or other suitable field identification reference.

Table 1. Important dwarf mistletoes and host tree species in British Columbia

Other dwarf mistletoes in British Columbia include a subspecies on shore pine in coastal areas, and a subspecies on mountain hemlock in a few south-coastal, subalpine localities. Procedures outlined in this guide will also apply to these limited situations.

Secondary host tree species are often infected by dwarf mistletoe when growing near infected, major host tree species. Where such trees are infected, they should be treated the same as major tree species. Any species not listed in Table 1 as susceptible to dwarf mistletoe are potentially useful for regenerating infected stands or for retention as future crop trees, and for leave trees for wildlife or biodiversity purposes.

Dwarf mistletoes are readily identified in ground surveys during the data collection phase of forest development plans or any prescriptions. Symptoms of infection such as brooming and stem or branch swellings should be verified by identifying the dwarf mistletoe shoots on affected bark. Low-level aerial observations, although useful to indicate general areas of severe occurrence, must be verified by ground-level detection.

Dwarf mistletoes and stand dynamics

Dwarf mistletoe biology and behavior

Dwarf mistletoes grow in tree bark and wood, absorbing water and nutrients of the host tree that otherwise are used for growth. The parasite induces a localized swelling of bark and wood and, often, nearby buds and branches are stimulated to grow excessively, resulting in abnormal clumps of branches called “brooms” or “witches’ brooms.”

Parasitic and pathogenic effects of dwarf mistletoes include reduced growth rates and decreased strength and quality of infected wood. Individual small trees can be killed, and, in time, growth of infected, living trees can become completely stagnated. Statistically, growth losses caused by dwarf mistletoes become clearly evident (i.e., differences in growth rates are significantly different than those expected by chance variation alone) after 50% or more of tree branches become infected with mistletoe plants, generally when trees are 15 to 20 years of age or older. Very large stem swellings caused by hemlock dwarf mistletoe drastically affect wood quality. Severely infected trees are also more susceptible to other damaging agents.

Several features of dwarf mistletoe influence stand dynamics, and should be kept in mind when assessing pest risks or developing silvicultural or stand management presciptions:

Natural stands and dwarf mistletoe

Dwarf mistletoe survival depends on the continuous presence of host trees from one forest generation to the next, and is significantly affected by stand age, vertical and horizontal height structure, and species composition.

In the past, the age of most natural stands in British Columbia, and the relationship between dwarf mistletoe and its common host, were determined primarily by wildfire. Large, intense wildfires effectively eliminated host trees along with their dwarf mistletoes. These burned areas then regenerated with even-aged stands of dwarf-mistletoe-free seedlings. Less intense wildfires left infected, live overstorey trees, allowing dwarf mistletoe to rapidly establish and intensify in the understorey stand. Dwarf mistletoe spread, intensification and impact are greatest in these situations.

Tree species composition and succession influence the impact of dwarf mistletoe in natural stands. Species-diverse stands are less affected than single-species stands. In many instances, periodic wildfires have maintained single-species stands of lodgepole pine and western larch, thus ensuring long-term survival of dwarf mistletoe. In other stands, forest succession in the absence of fire results in stands of non-susceptible tree species, such as spruce replacing lodgepole pine, and western redcedar and western hemlock replacing western larch. On the other hand, western hemlock and Douglas-fir dwarf mistletoes are common in all-aged, climax stands of their respective hosts and, in these instances, cause severe damage.

Managed stands and dwarf mistletoe

Clearcut harvesting

Clearcut harvesting, coupled with eradication of all host-tree residual stems, successfully eradicates dwarf mistletoe from a stand. After, dwarf mistletoe may spread from adjacent infected trees along cutblock boundaries to infect newly regenerated trees.

Partial cut harvesting

Partial cut harvesting in stands infested with dwarf mistletoe can greatly enhance the impact of dwarf mistletoe because latent infections are activated by increased light in tree crowns. It is virtually impossible to ensure that all remaining overstorey trees are free of dwarf mistletoe unless all host species are cut. Scattered infected overstorey trees produce a barrage of dwarf mistletoe seed that can rapidly infect regeneration.

Single tree or group selection systems—usually considered for Douglas-fir, western larch, or western hemlock, but not lodgepole pine—will likely result in intensified spread and damage by dwarf mistletoe. Therefore, such systems are not recommended for infected stands. However, detrimental effects can be ameliorated by a cutting cycle of 10 to 15 years with removal of moderately to severely infested trees at each cutting entry (see the section on “Dwarf mistletoe infection rating” for definition of severity ratings). Non-host tree species must be favoured for regeneration or leave tree purposes. Cutting cycles of 20 years or more will result in severe damage. Where dwarf mistletoe is present, use of susceptible advanced regeneration is inadvisable.

Pre-commercial and commercial thinning

Both pre-commercial and commercial thinning increase the light available in stands and, therefore, can increase the activity of dwarf mistletoe. However, in commercial thinning, trees are of merchantable size and the time to final harvest usually does not allow substantial further impact. Fully-stocked stands have lower rates of spread and intensification of dwarf mistletoes. Dense stands suppress seed production of dwarf mistletoes, and shade out lower branches that are often the most heavily infected.

Intermediate cuts in stands infested with dwarf mistletoe should be undertaken with caution. It should be recognized that this type of disturbance can greatly exacerbate the spread and damage caused by dwarf mistletoe, often negating any potential benefits of thinning and other stand management treatments. It is virtually impossible to ensure that leave trees are free of dwarf mistletoe due to the three- to five-year life cycle of the parasite. With age, existing infections usually lose their aerial shoots, and become quiescent. However, a disturbance that increases available sunlight often reactivates these infections to produce new shoots. Thus, apparently disease-free trees may be infected, and quickly produce new aerial shoots and seed.

When thinning an infected stand, it is recommended that all infected stems should be removed, even if it creates a temporary void in the stand. Failing this, any overtopping diseased stems must be removed or girdled. If it is essential that infected trees be left, leave those with the least amount of infection. Any young trees with stem infections, particularly western hemlock, are not desirable because of the pronounced stem swelling that results.

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