Hemlock forests are in a world of hurt. Across the eastern United States, an aphid-like pest is ravaging the trees, while booming populations of deer devour other native plants. Now, researchers have shown that the combination of these two threats adds up to even more trouble for the native ecosystem by favoring the invasion of weeds.
Researchers first noticed the hemlock woolly adelgid (Adelges tsugae), a 1.5-millimeter-long insect from Asia, in an arboretum near Richmond, Virginia, in 1951. The bugs feed on starch in new twigs and can kill trees in just 3 years. As the hemlocks die, exotic plants such as garlic mustard (Alliaria petiolata) have been spreading and altering the habitat that native species rely on. Anne Eschtruth, a graduate student in forest ecology at the University of California, Berkeley, wondered how the two phenomena were linked.
Two factors appear to be involved. First, by defoliating the forest canopy, the adelgids allow more light to reach the forest floor. That promotes the growth of native and exotic plants, Eschtruth and colleagues report online this week in Conservation Biology.
Second are white-tailed deer (Odocoileus virginianus). Following anecdotal reports that the deer sometimes prefer to eat one kind of plant over another, the researchers studied the animals' behavior in 10 forests in northeastern Pennsylvania and western New Jersey from 2003 to 2006. They fenced off 40 1-square-meter patches of forest so that deer could not feed there. In these enclosures, the invasive plants grew about as well as the native plants did. But where deer were present, the exotics did better than the natives. The more deer there were, the more the invaders thrived.
One reason could be that additional sunlight causes native and exotic plants to put more resources into growing stems and leaves rather than roots, which would make them more vulnerable to browsing. So savory natives would suffer more from large deer populations, while uneaten exotics would benefit.
"These effects are happening right around us and appear to be increasing with mounting deer densities and woolly adelgid expansion," says plant ecologist Don Waller of the University of Wisconsin, Madison. The findings suggest that land managers need to consider weed invasions, deer overpopulation, and tree health together rather than as separate issues. And, he says, reducing deer populations could be an effective way of combating exotic plants.
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