When Mistletoe Attacks: Investigating a Forest Parasite

John Roach
for National Geographic News
December 24, 2002
It's that time of the year when a white-berried, leafy green plant hangs
above doorways around the world and provokes lovers and serendipitous
strangers to share a kiss.

There are more than 1,300 species of mistletoe, including the two varieties popularly hung as a lure to sweethearts. All species can grow as parasites on trees and shrubs, stealing their food and water.

"They can, over time, stunt the growth of trees, even kill trees," said Robert Bennetts, a research scientist with the United States Geological Survey in Gainesville, Florida. Bennetts studies dwarf mistletoe, one of two common U.S. mistletoe varieties.

Most mistletoe varieties have green leaves that enable the plants to create some of their own energy via photosynthesis. They are what scientists call "hemi-parasites."

Forest Pest

Dwarf mistletoe is a nuisance species to the timber industry. It may reduce by half the annual timber harvest in Colorado alone, Bennetts and colleagues reported in the journal Ecological Applications in 1996.

The plant takes root in old, mature trees, weakening them by sucking their food and water. When dwarf mistletoe berries ripen, they explode, shooting out seeds to distances up to 50 feet (15 meters). Seeds that land on young trees and germinate will steal nutrients from the saplings.

Foresters and timber companies have tried for years to manage the spread of mistletoe in forests because of the plant's impact on their bottom line.

Detlev Vogler, a U.S. Forest Service plant pathologist in Davis, California, explained that dwarf mistletoe infestations are most prevalent in western forests where second generation trees grow in even-aged groupings.

The infested groupings, Vogler said, are seeded by older, mature trees that were not logged at the turn of the 20th century due to poor health caused by dwarf mistletoe infestations.

"Dwarf mistletoe is occasionally far more abundant than it might have been if the forests had matured in the presence of fire, which would have made the age structure more heterogeneous, thus reducing the threat of general mistletoe infestations," he said.

Popular campgrounds can also have serious mistletoe infestation for similar reasons. Because of frequent human disturbance and management that encourages retention of existing trees, natural regeneration or seedling plantings are seldom successful, said Vogler.

"Thus these sites are now often dominated by dwarf mistletoe-infected trees that are declining or dying, with the result that the campgrounds lose between-campsite cover and adequate tree cover, not to mention scenic values," he said.

Key to Diversity

Bennetts, the USGS biologist, appreciated forest managers' complaints against dwarf mistletoe, but wondered whether or not the cousin to the kissing plant had an upside.

In search of an answer, Bennetts and his colleagues examined four stands of timber in various stages of mistletoe infestation, studying the plant's effect on bird diversity and abundance.

"Among these stands we did see increasing numbers and birds with increasing levels of mistletoe," said Bennetts. He concluded that "if you have a diverse habit, there will be diversity in what uses it."

Because mistletoe stunts tree growth and eventually kills trees, it creates multiple layers in the forest structure, Bennetts explained. By eventually felling some host trees, Mistletoe creates patches of open space, allowing for grasses to grow and adding diversity to the age structure of the forest.

The word mistletoe derives from observations that that the plant would often appear in places where birds had left their droppings.

"Mistel" is an Anglo-Saxon word for "dung" and "tan" is the word for "twig," according to the USGS. Roughly translated, mistletoe means "dung on a twig."

The plant's seeds are thought to be distributed, in part, by bird beaks, feet, and digestive systems. The relationship is mutually beneficial: Many birds use mistletoe for their nests.

"You have to realize that mistletoes have been a part of the forest system for a long time; they are an integral part of the forest," said Bennetts.

In some parts of the world, however, human development and invasive species have caused some mistletoe varieties to become increasingly threatened with extinction, said Dave Kelly, a plant biologist at the University of Canterbury in Christchurch, New Zealand.

Kelly studies three mistletoe species in decline in New Zealand. The varieties are closely related and are so-called "bird-pollinated" mistletoes because they rely on birds to open their flowers for pollination.

"There are reduced numbers of pollinating birds in New Zealand now because of the introduced mammalian predators brought by humans, especially stoats, cats, and rats," said Kelly.

Other factors contributing to the mistletoe decline in New Zealand include the clearcutting of forests and the brushtail possum—an Australian mammal introduced in 1837 for the fur trade—which eats mistletoe.

Kissing Tradition

Despite the consternation mistletoe prompts in forest managers today, the plant has brought people closer together for thousands of years.

Mistletoe traditions date back to pagan ceremonies involving the winter solstice, explained Linda Neave, a horticulturist at Iowa State University Extension in Ames.

Since mistletoes have fruit during the winter, cultures have long associated them with fertility. "Several references that I have read make associations between different countries in regards to mistletoe and fertility one way or another," she said.

The exchange of a kiss under the mistletoe is linked back to the ancient times of the Druids. When enemies met under mistletoe in the forest, they had to lay down their arms and observe a truce until the next day.

From this tradition, Christmas historians believe, comes the custom of hanging mistletoe from the doorway or ceiling and sharing a kiss under it as a sign of friendship or goodwill.

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