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DWARF MISTLETOES
Dwarf mistletoes are small parasitic plants in the genus Arceuthobium. They grow exclusively on conifer stems and branches and are almost completely dependent on their host trees for food and water. Dwarf mistletoes are obligate parasites which means that they require a living host to survive. When the host tree or the branch on which a dwarf mistletoe plant is growing dies, the mistletoe dies too. Identification: There are two basic components to the dwarf mistletoe plant: the ectophytic system and the endophytic system.
The endophytic system is the root system of the mistletoe and is embedded within the host. It has two components; the cortical strands and the sinkers. The cortical strands are filaments that grow within the phloem and take nutrients from the host tree to feed the mistletoe plant. The sinkers are small, root-like structures that are embedded in the xylem where they extract needed water from the host.
Hosts in Southwest Oregon: The dwarf mistletoes in Southwest Oregon are fairly host specific, meaning that an individual species of mistletoe cannot grow on all conifers; it is usually restricted to a single genus or even subgenus. For example, western dwarf mistletoe (A. campylopodum) primarily colonizes ponderosa pine and Jeffrey pine. It cannot colonize Douglas fir but will occasionally colonize lodgepole pine. Table 1 provides a summary of hosts for the dwarf mistletoe species commonly found in Southwest Oregon.
Dwarf mistletoe disperses most effectively from overstory trees to smaller trees of the same species. Thus, spread tends to be most efficient in multi-aged stands of the same host tree species. Spread can also occur between nearby hosts of the same size, especially if they are within 13 meters (36 feet) of each other, but it is not nearly as rapid in this situation. Upward spread of dwarf mistletoe, especially within individual infected trees, is much less effective than downward spread. Susceptible branch tips are shielded by older needles when seeds are expelled from below. Upward spread of dwarf mistletoe is, on average, 30 cm (one ft) a year or less. If a tree is unable to put on that amount of growth annually, the mistletoe will be able to spread upward over time to the top of its crown. Though most infection is from nearby trees or from already established plants in the same tree, birds and other animals can spread dwarf mistletoe over long distances when seeds stick to their coats or feathers. Thus, when they do occur, new infection centers tend to be established in areas favored by birds and other animals (in or around small openings etc.). The life cycle of a dwarf mistletoe, from infection to seed production, takes an average of 4 to 5 years. Infection is the equivalent of seedling establishment among terrestrial flowering plants. Successful infection by a dwarf mistletoe requires penetration of the host branch (which it does at the base of the needle) and the development of cortical strands and sinkers. Once the mistletoe plant is established, an incubation period of 2 to 5 years elapses before the young shoots appear, although a swelling at the point of infection usually proceeds shoot production by a year or more. Dwarf mistletoe plants begin to flower one or two years after the initial shoots appear. Pollination is accomplished by insects and wind. Most temperate species of mistletoe require one or more years for fruit to mature. Effects: Dwarf mistletoes weaken trees by slowly robbing them of both nutrients and water. Heavy infections (many dwarf mistletoe plants distributed throughout the crown of the tree) can lead to severe growth loss and decreased survival. The growth reduction and mortality associated with dwarf mistletoe can lead to radically different forest structures, densities, and productivity levels in infected stands than in uninfected stands on similar sites. As examples, a small tree that is severely infected with mistletoe is unlikely to survive and grow into a large tree; stands of small trees that are severely infected will not grow into large tree dominated forests; a large tree that is severely infected has a significantly decreased life-span in comparison to an uninfected tree; and mature forests that are severely infected will not persist on the landscape nearly as long as those that are uninfected. The length of time it will take for mistletoe to actually kill a heavily infected tree will vary depending on a number of factors including the size of the tree, its vigor, the species involved, and whether insects, particularly bark beetles, are attracted to the tree. As a broad generalization though, trees are usually killed within 10-15 years once they become heavily infected throughout the crown.
The sizes and effects on host trees of witches' brooms vary depending on the species of dwarf mistletoe. For example, infection by A. douglasii (the cause of Douglas-fir dwarf mistletoe) often results in the production of very large witches' brooms. On the other hand, the witches' brooms produced by A. tsugense on hemlocks are usually fairly small and tightly attached. Branches with pronounced witches' brooms are more flammable than normal branches because they are larger, more resinous, and persist longer
Susceptibility to fire- Dwarf mistletoe-infested stands are generally more flammable than healthy stands due to the large amounts of fuels arising from the accumulation of dead witches' brooms, fallen trees, and live brooms in the lower crowns. Because of these fuels, normally nondestructive fires can become stand replacing fires in stands with dwarf mistletoe (Roth, 1966). Harrington and Hawksworth (1990) studied Effects of fire and A. vaginatum subsp. cryptopodum on mortality of old-growth ponderosa pine at Grand Canyon National Park. Dwarf mistletoe infested trees were influenced by fire in several ways. Because infested trees have highly flammable witches' brooms and lower live crowns, a larger proportion of the crown of an infested tree was likely to be scorched than the crown of a healthy tree. In addition, given equal amounts of scorch, the probability of survival of heavily infested trees was less than half that of healthy trees. DMR- The expected effects of dwarf mistletoe on tree growth and survival have been demonstrated to be strongly correlated with infection severity or the amount of the crown colonized by mistletoe plants. Because of this strong correlation between severity and effects, it is important to have a standardized rating system to describe severity and to use it consistently. The 6-class dwarf mistletoe rating system (DMR) developed by Hawksworth in the early 1950's and described in detail in Hawksworth (1977) has become the standard for quantifying the severity of dwarf mistletoe infections. In the 6 class system, the live crown is visually divided into thirds. Each third is then assigned either a 0 (there is no mistletoe in that third); a 1 (½ or less of the branches are infected); or a 2 (more than ½ of the branches are infected). The ratings for each third are then tallied to obtain a total for the tree. This total is termed a DMR for an individual tree. DMR can range from 0 (meaning the tree is uninfected) to 6 (the tree is very heavily infected). An average stand or plot rating may be computed by calculating the average DMR rating of all live trees (infected and non infected) by species. The terms "spread" and "intensification" are commonly used to describe changes in mistletoe occurrence and severity levels. When a tree progresses from an initial DMR of 0 to a higher number it means that dwarf mistletoe has "spread" to that tree and it has become infected. When there is an increase in a DMR that is greater than 0 (DMR changes from a 2 to a 5 for example) it means that the infection has "intensified" in severity. Ecological Role: Maintaining the balance- Dwarf mistletoes are believed to have co-evolved with their conifer hosts for over 5 million years. Obviously, neither has achieved the upper hand since both host and pathogen are presently thriving within many forest ecosystems. Historically, wildfire has been the most important single factor governing the abundance and distribution of dwarf mistletoes. Wildfires are frequently effective in limiting dwarf mistletoe populations because trees usually return to burned sites much faster than do dwarf mistletoes. Wildlife habitat- Within the natural balance between host, fire, and pathogen, dwarf mistletoe provides a source of vertical and horizontal diversity through gap creation, and production of snags, brooms and down woody material. Many species of mammals, birds, and arthropods can take advantage of the favorable structure mistletoe infection provides, while other species use mistletoe plants or host tissues associated with infection for food (Tinnin et al., 1982).
While dwarf mistletoe infection centers provide enhanced diversity and habitat for wildlife, it should be remembered that heavy infection of a stand, without the sanitizing effect of fire or use of cutting practices that reduce the mistletoe, can result in a decline in habitat quality over the long term. The decline would occur ultimately as a result of the negative effect of heavy mistletoe on the production of large woody structure (living and dead) required by many wildlife species. Conducive Habitat: Mistletoe is most abundant and its effects are most severe in situations that favor its rapid dispersal in relation to tree growth. Multi-aged stands with major host components on poor growing sites, for example, are ideal habitat for dwarf mistletoe proliferation. Management: Infestations of dwarf mistletoe not only affect timber values but also recreation, aesthetics, fire hazard, and wildlife habitat (both long and short term). Dwarf mistletoe lends itself to being managed silviculturally (Hawksworth and Scharpf 1978) because: · it is easy to identify and measure, Since the impacts of dwarf mistletoe are, in most cases, not significant until trees are heavily infected, the key to successfully avoiding serious effects of mistletoe on tree growth and survival as well as associated effects on stand structure is to prevent heavy infection. There are a variety of silvicultural options that can be used to control undesirable effects of dwarf mistletoe. These fall into 5 general groups: 1. Sanitation cuts- Infected trees are removed (or killed) to eliminate or reduce the spread of mistletoe to insignificant levels. The most drastic version of this practice would be a clear cut. 2. Thinning to outgrow the mistletoe- Trees are spaced to promote height growth that is greater than the upward spread of the mistletoe. Trees grow faster than the mistletoe can spread upward, proportionally less of their crowns are infected, and heavy infection levels are never achieved. This technique works well in single-storied stands, especially on good sites. 3. Favoring alternative species- By planting or featuring species that the dwarf mistletoe infesting the site cannot attack, the mistletoe is unable to spread into additional trees. 4. Converting to stand structures that are not conducive to dwarf mistletoe- Stands with more than one canopy level favor the rapid proliferation of mistletoe. By converting to a single canopy structure (at least for the species that is infected), the spread of mistletoe is greatly slowed because, once accomplished, the trees only have to deal with horizontal and vertical spread; both of which are much less rapid and effective than spread from an overstory source to a susceptible understory. 5. Individual tree treatments-broom removal and/or sanitation pruning. Techniques employed depend on individual situations. Management objective, stand age, structure, density, species composition, number of years to harvest, and the severity and distribution of the mistletoe need to be understood and considered in devising a management strategy. Predictive models like FVS (Forest Vegetation Simulator) are available to help resource managers evaluate the outcomes of various silvicultural treatments over time in stands with dwarf mistletoe. References: Bennetts, R.E., White, G.C., Hawksworth, F.G. 1992. Harrington, M.G; and Hawksworth, F.G. 1990. Hawksworth, F.G, and Wiens, D. 1996. Hawksworth, F.G., and Scharpf, R.F. 1978. Hawksworth, F.G. 1977. Hudler, G., Oshima, N., Hawksworth, F.G. 1979. Hull, R.O., and Leonard, O. 1964. Johnson, D.W., and Hawksworth, F.G. 1985. Lightle, P.C., and Hawksworth, F.G. 1973. Nicholls, T.H., Hawksworth, F.G., and Merrill, L.M. 1984. Roth, L.F. 1966. Scharpf, R.F., and Roth, L.F. 1992. Scharpf, R.F., Smith, R.S., and Vogler, D. 1987. Scharpf,R.F., Smith, R.S., and Vogler, D. 1988. Tinnin, R.O., Hawksworth, F.G., and Knutson, D.M. 1982. Links: The Dwarf Mistletoe Internet Site: An extensive body of literature exists for dwarf mistletoe. Topics range from the role of mistletoe as a disturbance agent on the ecology of the forest ecosystem to its effect on the outputs society expects from that ecosystem; from technical papers on taxonomy to ethnobotany of mistletoe. Access to this rich and varied body of literature is available to you on the web at: http://www.rms.nau.edu/mistletoe A Survey of Northern Spotted Owl Nests in Douglas-fir Dwarf Mistletoe Brooms in the Siskiyou Zone, Rogue River National Forest and Ashland Resource Area, Medford District, Bureau of Land Management (owlnestsurvey.pdf) Table 1. Summary Table of the Dwarf Mistletoes in Southwest Oregon. |
