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.

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