Sapped Sugar Maples

By Dana Bauer

For the early settlers of Pennsylvania, the sugar maple was an indicator of rich soil — dark, stone-free loams ideal for raising grains and grasses.

Over the past 20 years, sugar maples across northern Pennsylvania have shown symptoms of decline: thinning canopies, small discolored leaves, broken branches, and withering buds. Nutrient-poor soils may be a key factor. Other suspects? Insect defoliation, drought stress, atmospheric deposition, and extreme weather. “There are so many factors working together, it’s hard to isolate one,” says Patrick Drohan.

Drohan, who recently earned a doctorate in soil science, is part of a Penn State team working with the United States Forest Service to examine the causes of sugar maple decline. Field foresters first observed a problem in the late 1970s, and soon afterwards the Forest Service began a series of studies, according to Stephen Horsley, a plant pathologist at the Northeastern Research Station of the Forest Service, who leads the effort.

The Forest Service’s first method of treatment was to increase the nutrients in the soil by adding pulverized limestone. At first, the only noticeable changes were in the soil, but after six years, “growth was significantly better on the limed plots,” says Susan Stout, a Forest Service silviculturist and a member of Drohan’s dissertation committee.

Stout continues, “There was an astonishing wave of stress in the early 1990s.” The elm span-worm outbreak, for example, defoliated a million acres of forest. “Trees on limed plots were more resilient,” she says.

Drohan conducted a broad study of all the different variables that might cause sugar maple decline. As part of his doctoral dissertation, he examined 28 sugar maple plots — both healthy and unhealthy — that were part of a Forest Service monitoring program from 1979 to 1989. Plots with more than 20 percent dead or dying sugar maples were classified as unhealthy, or declining.

With a team of undergraduates, Drohan looked at topography, tree health, and the physical and chemical properties of the soil. He then looked for a statistical correlation among the variables.

Drohan found that plots of declining trees tended to occur at higher elevations, to have sandstone geologies and poor soil chemistry, and to experience frequent severe defoliations. However, both unhealthy and healthy trees had experienced defoliation and drought. “The real difference was in the soil chemistry,” says Drohan.

Leaves of the unhealthy sugar maples — and the soils they grew on — had low levels of magnesium and calcium. Also, soils on the declining plots were rockier or sandier than the soils on healthy plots. “Sandy and rocky soils hold less water than dark, rich soils,” he says. “And a lack of moisture deprives the trees of nutrients.

“So is the problem a lack of moisture, or poor nutrition, or both?” Drohan asks. “The goal here was to look at a wide area, to see relationships.”

Patrick Drohan received his Ph.D. in soil science in August 2000. He was one of three first prize winners in the Health and Life Sciences division of the 2000 Graduate Exhibition. Drohan now teaches environmental science at Shepherd College in Shepherdstown, WV; pdrohan@shepherd.edu. His adviser was Gary W. Petersen, Ph.D., professor of soil and land resources in the College of Agricultural Sciences, 116 ASI Building, University Park, PA 16802; 814-865-1540; gwp2@psu.edu. Stephen Horsley, Ph.D., leads the Forest Service’s sugar maple decline project; shorsley@fs.fed.us. Susan Stout, Ph.D, a silviculturist at the U.S. Forest Service, was on Drohan’s doctoral committee; sstout@fs.fed.us. This project is funded by the U.S. Forest Service.