Eastern US: Hemlock demise from not just Adelgid but deer overpopulation as well

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.

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. 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.
http://sciencenow.sciencemag.org/cgi/content/full/2008/1219/3

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