[Dwarf Mistletoe Management Guidebook Table of Contents]
Management of dwarf mistletoes
Dwarf mistletoes significantly affect the health of forests and the success of silvicultural systems and treatments. Information is required in forest development plans for managing dwarf mistletoes at the landscape level, including occurrence and general levels of incidence. In stands infected by dwarf mistletoe, a risk assessment and appropriate treatments must be documented in any silvicultural or stand management prescription.
High hazard biogeoclimatic units for dwarf mistletoe
Dwarf mistletoes are widespread in British Columbia. Forest ecosystem biogeoclimatic units in British Columbia which have a high probability of dwarf mistletoe damage—for at least one forest region—are listed in Table 2. Stands within these units should only be considered at high hazard when a susceptible host forms a major component of the stand. Any high hazard area requires evaluation as part of the planning or prescriptive process.
For more specific information by region, see Forest Health Charts (Appendix 6) in the appropriate Establishment to Free Growing guidebook.
Table 2. Biogeoclimatic units with high hazard of dwarf mistletoe damage
Silviculture prescription walkthroughs
The presence of any dwarf mistletoe should be noted by host species.
Where a seed tree silviculture system is being considered, it is recommended that the percentage of potential seed trees infected and the severity of infection be recorded (see method in section on “Dwarf mistletoe infection rating”).
For the purpose of designing cutblocks that will reduce infection of regeneration, note distribution of infected trees and any natural barriers to spread such as rock outcrops, roads, patches of non-host tree species, and any other similar features which might be incorporated into the prescription.
Stand management assessments
Where dwarf mistletoe has been noted in the Integrated Silviculture Information System (ISIS), the Management Licensee Silviculture Information System (MLSIS) or found during a stand management assessment, or where there is a high probability of occurrence based on Table 2, record the following information:
- percentage of trees infected and severity of infection (see method in section on “Dwarf mistletoe infection rating”) in the current stand; or
- incidence and severity of infection on proposed residual crop trees (after treatment); and
- proportion of any non-host tree species.
Infection rating of potential residual crop trees is highly recommended.
Carefully consider the free growing criteria for dwarf mistletoes (see section on “Free growing criteria and assessment”) before making recommendations for stand management activities.
Dwarf mistletoe infection rating
For the purpose of describing the severity of dwarf mistletoe infection on a tree, use the Hawksworth six-class rating system as described on the following page.
Figure 5. The Hawksworth six-class dwarf mistletoe rating system.
Management and treatment considerations
Management of dwarf mistletoe is relatively simple where susceptible tree species grow in even-aged stands, and an even-aged stand is desired. Although control might be less certain or even problematic in other situations, some management or treatment options are available to reduce dwarf mistletoe impacts under almost any silviculture system (Table 3).
All cutblocks should be designed to minimize spread of dwarf mistletoe into the young stand by leaving residual non-host species as border trees, and incorporating natural barriers wherever possible. Non-host tree species should be planted or used for natural regeneration as much as possible.
Shelterwood and selection systems are not recommended in stands where susceptible tree species comprise more than 50% of the total stems, and more than 20% of the susceptible overstorey trees are visibly infected.
Table 3. Management options for dwarf mistletoes in silvicultural systems
Other resource management considerations
In certain instances, management of infected stands might be desirable to fulfill resource management objectives other than timber production. Infected trees with large witches brooms and some infected stands appear to be preferentially used by some animals and birds for nesting, cover and other habitat purposes. However, any prescriptions made where these other resource management objectives are paramount should explicitly consider expected impacts of dwarf mistletoes. Any strata managed under such regimes should be assessed, and expectations for growth and yield adjusted accordingly.
In recreation sites or other areas, dwarf mistletoe infested stands can be maintained or managed for certain features, such as wildlife habitat. However, hazards associated with infected trees should be considered. Dwarf mistletoe brooms can act as fuel ladders, increasing the fire hazard, and large brooms are prone to breakage. Pruning can be used to remove large brooms and maintain tree vigour.
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