FOREST HEALTH:
Monitoring Dwarf Mistletoe
by John A. Muir and Warren Warttig

Dwarf mistletoe is common in coastal western hemlock forests from northern California to southeastern Alaska. This parasite grows on live trees (Figure 1), causing swellings on branches and boles as well as proliferations of branches called “witches’ brooms” (Figure 2). These effects can persist for many decades, reducing wood quality and tree growth and becoming more pronounced as the tree ages. 

In coastal BC, current variable retention (VR) harvesting practices often leave mistletoe-infested trees in cutblocks. Previous clearcut harvesting and even-aged silvicultural regimes almost completely eradicated dwarf mistletoe-infested trees from second-growth hemlock forests, reducing or preventing impacts. The resulting low incidence of the parasite suggests that dwarf mistletoe may be ignored or disregarded in VR harvest systems. However, based on current findings, it is believed that infested trees in VR-harvested forests should produce more extensive and more severe infestations of dwarf mistletoe.

In the past, monitoring programs lacked detail on most disease infestations, including dwarf mistletoe. Therefore, starting in 2005, as part of International Forest Products Ltd. (Interfor) certification requirements to ensure sustainability of forest practices, several projects were initiated to develop techniques to monitor dwarf mistletoe infestations. 

A comprehensive monitoring program for dwarf mistletoe requires several pieces of data including: distribution of mistletoe-infested residual trees; mistletoe seed production on, and dispersal from, infected residual trees; and infection of young trees and development of pathogenic effects. Ideally, these components should be determined by forest-level sampling of infested trees and monitoring infestations for several decades in designed, replicated blocks. However, incidences and impacts of dwarf mistletoe have proven to be highly variable, and substantial long-term funding was unavailable for these types of studies. 
The initial approach was to monitor spread and infection of dwarf mistletoe by establishing a plot around the base of selected, infested residual trees in five to ten-year-old VR cutblocks. This does not provide an unbiased sample, but should provide valuable data from infested trees in a broad geographical area. To minimize costs, readily accessible areas were selected to serve as convenient demonstration sites and to facilitate re-measurements. Between 2005 and 2007, low elevation monitoring sites (less than 700 m) were monitored in the coastal western hemlock biogeoclimatic zone near Ucluelet, Port Alice, Port McNeill, and Courtenay. On each 11.28m radius plot the number of mistletoe-infested and non-infested trees was recorded, including residual and regenerating young trees, and the approximate location of each infested tree was mapped. Unexpectedly, in all but one of 33 monitoring plots established, dwarf mistletoe infected only 1% or less of the young hemlock trees. These plots will be re-assessed periodically to evaluate further trends.

In 2005, 2006, and 2007 dwarf mistletoe seed dispersal was monitored by establishing seed traps at approximately 4m from the base of two severely infested trees near Ucluelet. An average of 4.5 mistletoe seeds per m2 were dispersed from these trees, which was far less than expected, suggesting that mistletoe seed production could limit dwarf mistletoe infection of the seedlings. 

In 2007, the feasibility of using aerial surveys to detect dwarf mistletoe infestations was investigated. A slow speed, low-level helicopter flight was taken and a digital video camera recorder was used to document visual incidence, coupled with a hand-held GPS to record locations of infested trees. This kind of detailed survey appears promising for detecting and characterizing dwarf mistletoe infestations on residual trees in VR areas. Additional work is required to characterize tree condition, define severity ratings per tree, determine accuracy of aerial surveys, and develop methods for recording survey data. 

Further work is still required to determine the onset and degree of effects of dwarf mistletoe on growth and wood quality of thrifty, second-growth hemlock.

John A. Muir, PhD, RPF is a forest pathologist with John Muir Consulting in Victoria, BC and can be reached at johnmuir@consultant.com. Warren Warttig, RPBio is a planning biologist at International Forest Products Ltd. in Campbell River, BC.