Deer munching may not be sealing wildflowers' fates

University Park, Pa. -- Although Bill Sharpe is a hunter and an outdoorsman, he admits he never paid much attention to wildflowers until he kept hearing that they were disappearing because an overpopulation of white-tailed deer was eating them into oblivion.

So the professor of forest hydrology in Penn State's College of Agricultural Sciences worked with ecology graduate student Jacob Thompson to design a research project to learn if it was true that too many deer were eradicating wildflowers, or if the culprit in their demise was really acidifying soils from decades of acid rain, as Sharpe suspected.

Sharpe, who has been researching the effects of acid rain for more than three decades, was frustrated by the dearth of historical information about wildflowers in Pennsylvania, the lack of known specifics about the plants' biological preferences and needs, and what he views as an absence of science behind claims that deer are responsible for a disappearance of wildflowers.

"I've worked with forest soils, trees and acidic precipitation long enough to know that soil plays a critical role in the welfare of plants," he says. "You can't just assume that when a plant starts disappearing, it is caused by deer browsing. Deer have always eaten wildflowers."

Sharpe's goal was to answer, once and for all, whether deer are behind wildflower decline in Pennsylvania. But it didn't work out that way.

"Turns out it is not so simple," he says. "We found evidence that acidifying soils and deer browsing both have an impact, but in all honesty, since there doesn't seem to be any baseline data about past wildflower prevalence in Pennsylvania, I can't even be sure they are becoming scarce. All the information we could find was anecdotal."

For his research, Sharpe selected three of the 35 species of trillium growing in eastern United States forests. Members of the lily family, purple (Trillium erectum), white (Trillium grandiflorum) and painted (Trillium undulatum) trilliums all are fairly common in Pennsylvania. Thirty-seven patches of trillium in Centre, Clearfield, Somerset and Westmoreland counties, and in West Virginia, were used as research plots.

What Sharpe and colleagues discovered was that each kind of trillium was affected differently by soil acidity and deer browsing. Sharpe speculates that is true of all wildflower species. "You can't generalize about the trilliums at least," he explains. "They have different soil preferences and it appears that deer may prefer some over others."

Deer populations in the study areas were gauged by counting pellets (fecal matter) in a specified distance around the trillium patches. The method may seem primitive, but it is an accepted method of estimating deer numbers in a given area, Sharpe says. "With plants that were obviously eaten, we assumed deer ate them," Sharpe explained. "But there was really no way to be sure, except for a few deer tracks."

"We did learn a lot," says Sharpe. "We now know, for example, the range of soil acidity factors that painted trillium actually prefers. We found a strong correlation, however, between purple trillium and better soils with lots of calcium. White trillium only grew on sites with relatively high calcium and pH as well. They are truly calcium dependent. I believe the gradual acidification of soils may have a negative impact on purple and possibly white trilliums, but not as much on painted trillium."

Deer snacking on trillium also was not as straightforward as you might expect. Purple trillium didn't seem to be much affected by deer browsing, but painted trillium definitely was and white trillium also was, but to a lesser degree. And where painted trillium was affected, the browsing seemed to have the greatest impact only on the number of plants with flowers, not the density of plants in the patch. Which brings up the obvious question: Do deer like the taste or appearance of white flowers more? "I can't answer that," Sharpe says. "I am going to take the researcher's cop-out and say, 'More research needs to be done.'"

So what conclusions can be drawn from Sharpe's small, sample-size study of trillium? Maybe none, he admits, but more is known about these wildflowers than before. "The mixed results tell me that we must be cautious when we make claims about an animal making a plant disappear," he says. "The wildflower guides we saw didn't even have any basic soil or leaf chemistry information about trillium. Now at least we know two of three we studied would be negatively impacted by increased soil acidity. And we do know deer are having some effect.

"All trilliums take five to 10 years to flower," he adds, "so some of these patches are quite old. And because trillium sends out shallow shoots or stems under the soil called rhizomes, all the flowers in a patch might be the same plant."

They are a beautiful, interesting species that have many admirers, Sharpe points out. "The patches of wildflowers we studied in the research were located with information provided by wildflower enthusiasts," he says. "Some people were reluctant to tell us where the patches were because they didn't want the flowers picked or the patches damaged. I can see how people become attached to them."

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Last Updated March 19, 2009