Significance

Effects on trees

In many parts of the western United States, dwarf mistletoes are the most important group of diseases. In terms of timber volume, annual losses have been estimated at 11.3 million cubic meters of wood in the western U.S. and 3.8 million cubic meters in western Canada. The U.S. losses equate to about 3.3 billion board feet, which has a value of roughly $1.4 billion and is enough wood to build up to 1,000,000 homes per year!

The impact of dwarf mistletoe is greatest when trees are infected at a relatively young age. Data from intensive studies of lodgepole pine dwarf mistletoe in Colorado have shown that stands infected at an early age have only 12.4% the volume of healthy stands by age 75, and the difference does not decrease thereafter (Figure 30).

However, impacts only become substantial when half or more of the tree’s crown is affected. This is equivalent to DMR (dwarf mistletoe rating, see dwarf mistletoe rating) of 3 or greater. One should also consider, however, that on a stand basis when average DMR reaches about 2, over 80 percent of trees are probably infected, leaving few choices for retaining trees with good potential growth and survival.

Although mortality is generally a less important effect than growth reduction, it can be substantial. It is difficult to quantify because of indirect effects. Infested stands have smaller trees with thinner crowns, so there is often greater establishment of seedlings in the understory. Also, mortality due to suppression is reduced. Nevertheless, mortality due to dwarf mistletoe (based on standing dead trees, after subtracting the values for comparable healthy stands) can be 15-20% of stems in heavily infested stands. Mortality is greater on dry than on wet sites, and is greater during or following drought. In some situations, bark beetles preferentially attack trees with mistletoe.

An illustration of the effects of dwarf mistletoe, based on output from the Forest Vegetation Simulator and Dwarf Mistletoe Impact Model, is presented here. Numeric projections are transferred to the Stand Visualization System which produces a spatial, graphical animation (Figure 31). In a healthy stand, basal area (cross-sectional area of each tree at 4.5 feet, summed over an acre) increases greatly. In an infested stand, DMR rises and basal area fails to increase and may even decrease over time due to mortality and reduced growth (Figure 32). Clearly, dwarf mistletoe greatly impacts not only commercial value, but also stand conditions that influence other vegetation and animals.

A third effect of dwarf mistletoes is reduction in the number and viability of seeds produced by the host. Seed production is not much affected by light infection, but moderate infection of ponderosa pine reduced seed production to 42% of healthy trees (by weight); severe infection reduced it to 29% of healthy trees. When viability is considered, the effect is somewhat greater.

Figure 30

Figure 31 (Click on figure to see an animation of disease development)

Figure 32

Forest dynamics

Most dwarf mistletoes occur primarily on early-seral species (i.e., occurring in early stages of a successional sequence). By killing and suppressing the growth of their hosts, the pathogens can sometimes promote succession to later-seral species that may be present. For example, lodgepole pine stands may be invaded by a spruce understory, and the turnover would be hastened by debilitation of the overstory.

However, dwarf mistletoes tend to increase the intensity of fire and the likelihood of a surface fire becoming a stand-destroying crown fire. Although in the short term this kills the obligate parasite, such fires usually result in re-establishment of the early-seral species, ensuring more host material for future generations of dwarf mistletoe.

Infected trees often have large witches’ brooms in the lower crown, persisting after the live crown has been raised in healthy trees. These brooms, full of resin and dense accumulations of live and dead needles, act as fuel ladders that increase the opportunity for a surface fire to torch or become a crown fire (Figure 33). Numerous observers have noted selective torching of infected trees during a surface fire. In general, more heavily infested stands have higher total fuel loading than uninfested stands.

Figure 33

Kissing under the mistletoe

A Christmas tradition is to hang mistletoe over a doorway (Figure 34). Amorous individuals may then kiss someone they catch under the mistletoe. However, this tradition is based on true, or leafy, mistletoes rather than dwarf mistletoes. The most common types of these have relatively large, white berries and prominent leaves.

The tradition is ultimately based on the European mistletoe, Viscum album (Figure 35). As legend has it, the Druids, priests of the ancient Celtic culture of Europe, thought that mistletoe was sent to Earth by God because it occurred on the sacred oak trees. As a parasite, it represented human dependence on God. They recognized that the mistletoe thrush carried the mistletoe seeds and considered them to be messengers of God. Because it came as a gift from God, mistletoe had spiritual power. One way of using that power was to hang it over the doorway in winter to keep evil spirits from entering. This was especially helpful in winter because of the darkness and cold. No doubt the bright, evergreen foliage was a festive addition to winter décor as well.

The kissing part of the custom is probably related to the traditional association of mistletoe with fertility, but perhaps also to the fact that mistletoe was associated with the Norse goddess of love, Frigga. Part of the custom that is sometimes conveniently forgotten today limits its usage. Supposedly one should remove a berry from the plant for each kiss stolen from someone beneath it. When the berries are gone, so is the opportunity for kissing!

The similar American true mistletoes, most commonly used for this tradition here, are in the genus Phoradendron (Figure 36). Dendron in Greek means “tree” and phora means "to bear", or in this case "thief", thus “tree thief.” They are larger than Arceuthobium spp. and most have substantial leaves (though most that occur in deserts have greatly reduced leaves). They also differ in that the fruits are large, fleshy, and not explosive. They are dispersed by birds that feed on them. An APS Education Center article, Introduction to Parasitic Flowering Plants, has more information on the various types of mistletoes and other parasitic plants (http://www.apsnet.org/education/IntroPlantPath/PathogenGroups/parasiticPlants/default.htm).

More on this tradition, with lavish illustrations, can be found here:
What Does Mistletoe Have To Do With Christmas?
http://www.apsnet.org/online/feature/mistletoe/
Traditions and legends - Introduction http://www.mistletoe.org.uk/TraditionandLegend/TraditionLegendIntro.htm
Or search the annotated bibliography at the Mistletoe Center using the keyword folklore
http://www.rms.nau.edu/mistletoe/dyn/mtbib.shtml

Figure 34

Figure 35

Figure 36

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