Kissing Good-Bye to Some Old-Fashioned Views about Mistletoe
New research suggests that these parasitic plants are a benefit for forests and forest wildlife
Because it grows without soil, and because it often remains green during winter, people have long venerated mistletoe. Many cultures believe it has the power to cure disease, grant fertility and ward off bad luck--a tradition that lingers in the modern custom of holiday kissing. The shrub´s positive influences apparently extend to wildlife as well. Ecologists are discovering that various types of mistletoes are crucial to a wide array of creatures, from insects to owls. As a result, a bit of holiday goodwill is beginning to infuse our relationship with these plants--and to change the face of forest management.
There are two categories of mistletoe in the United States: leafy "Christmas" mistletoe and dwarf mistletoe. Foresters have long been most concerned about the latter, a group comprising 42 species in the genus Arceuthobium, found around the world. They parasitize conifers, notably such valuable species as ponderosa and lodgepole pine, Douglas fir and western larch. In contrast to the leafy varieties, which produce their own food through photosynthesis, dwarf mistletoes have little greenery--some species have cylindrical stems less than an inch long.
But this small plant’s impact is considerable. The stems are dwarfed by a large root system that penetrates deep into the host tree´s trunk or branches and collects water, minerals and nutrients. In response, the tree sprouts more branches in the area. After a few years of infection, the resulting clump of twigs and needles--called a "witches’ broom"--may be more than three feet wide. It accumulates fallen needles and other detritus and can grow nearly impenetrable. Large trees often support dozens of brooms.
Unlike leafy mistletoes, the dwarf versions don´t produce much of their own food. "When a tree becomes severely infected, almost all the food it produces is going to feed the dwarf mistletoe," says Robert Mathiasen, a biologist at Northern Arizona University. "They’re like leeches; one doesn’t harm you much, but a thousand of them do a lot of damage."
Over time, dwarf mistletoes often kill their hosts by depleting nutrients or by making them more susceptible to insects. Dense brooms are also intensely flammable and can increase the severity of forest fires. And infestations are contagious, as dwarf mistletoe fruits explosively eject sticky seeds that can fly to trees as far as 40 feet away. But in recent years biologists have learned that these unsightly brooms are integral to forest ecology.
"It’s very hard to find a broom with no signs of use," says Shaula Hedwall, a biologist who recently completed a study of dwarf mistletoes in the Southwest. "The amount of red squirrel use we found is outrageous. They’ ll build their nests on the broom platform, or between the broom and the trunk. They take mushrooms or pine nuts out to brooms and cache them or eat them. You’ll often find ten mushrooms cached in one broom. One red squirrel even hauled loads of elk pellets 22 meters up into a broom."
Animals like brooms because they provide protection from predators and weather. Large brooms are relatively warm in winter and cool in summer.
And some are large indeed. Hedwall found some brooms in eastern Arizona in which wood rats had built nests of sticks. "One was large enough that I could have lowered myself into it."
Raptors such as northern goshawks and great gray owls also make their homes there. In one Oregon study, 19 out of 20 long-eared owl nests found were in brooms. In the Southwest, Douglas fir brooms are popular nesting sites for threatened Mexican spotted owls.
Songbirds, too, are attracted to mistletoes, not only for shelter but also for food. The shrub’s foliage, flowers and fruits attract insects and other prey for these birds. Some prey species, such as the larvae of the thicket hairstreak butterfly, feed solely on dwarf mistletoes. Larger animals, such as grouse and porcupines, also favor the arboreal foliage and fruits, while heavy snows that pile on brooms and break them off provide the same meals to deer and elk.
Dwarf mistletoes often do exact the ultimate cost from their hosts. "There is perhaps from five to ten times as much tree mortality in infested stands as in uninfested ones," says Mathiasen. But even those dead trees serve a variety of forest animals, from carpenter ants to woodpeckers. And the space a tree’ s death opens up often sprouts brushy vegetation browsed by deer and elk.
This influx of new data is causing foresters to reconsider their practices. In the past, infested trees were often cut down or severely trimmed to prevent the spread of seeds to nearby trees. "We used to preach ‘sanitizing’ to save the stand," says Catherine Parks, a Forest Service plant pathologist in Oregon. "Then the biologists realized that there were wildlife values, and they wanted to keep all the broomed trees without acknowledging the damage being done to timber stands. I’m encouraged to see wildlife biologists and silviculturists starting to come together on this."
Managing mistletoes in the future will not be a simple matter of trying to maximize tree growth, or of sparing all broomed trees. "The issue calls for cooperation and collaboration between biologists and foresters, because neither one has the entire answer," says Bob Rainville, a Forest Service official in Oregon. That’s a matter as complex as the eons-long coevolution among trees, their parasites and forest animals--and as delicate as a holiday kiss.
This holiday season, journalist Peter Friederici plans to ski under mistletoes in the forests around his home in northern Arizona.