sealPurdue News

July 2001

Purdue biologists study factors in dogwood decline

WEST LAFAYETTE, Ind. – One of the first signs of spring is the blooming of flowering dogwood, a small tree with bright white flowers.

But you now may have to look more carefully for this telltale sign of spring. For the past 20 years, scientists have documented a decline in this forest species, a trend reflected throughout the Midwest.

Though many dogwoods have been lost to an infection called anthracnose, Purdue University researchers say changes in forest composition also may be a contributing factor.

"Since 1981, flowering dogwoods have declined by 43 percent," says Aaron Pierce, a graduate student in Purdue's Department of Biological Sciences. Pierce studies forest growth and succession in the Ross Biological Reserve, a 67-acre forest preserve owned by Purdue.

A popular ornamental tree, flowering dogwood occurs naturally in many forested regions. Purdue biologists have conducted a census of the reserve every 10 years since 1949, identifying and tallying each adult tree within the forest boundaries. The data is used to compare forest composition from year to year and to study long-term trends, Pierce says.

"Throughout the 1970s and '80s, the dogwood population at the reserve appeared to be relatively stable," he says. "By the late 1980s, however, the population began a noticeable decline."

Since the early 1980s, dogwoods throughout the country have been in decline, says Kerry Rabenold, professor of biology at Purdue.

"This is a pattern in many places – what used to be groves of beautiful trees along the highways and throughout the woods are now just skeletons," he says.

In some regions, an infection called dogwood anthracnose has been linked to the drop in dogwood numbers. The infection is caused by a fungus believed to have been imported in nursery stocks from Asia.

But Rabenold and Pierce, working with Gail Ruhl of Purdue's Plant and Pest Diagnostic Lab, say trees at Purdue's Ross Biological Reserve have shown no signs of anthracnose. They attribute the decline to a change in forest composition.

"A consistent trend we've seen in forests across the Midwest is a shift in the composition of the dominant tree species," says Pierce.

Indiana, like much of the Midwest, was once dominated by fire-dependent, oak-hickory forest. This type of forest relied on frequent, small fires to clear out the understory and allow new trees to grow. A recent history of fire suppression, however, has allowed other trees to slowly take over.

Sugar maple, a fast-growing shade tree, is one such species. Census data at the reserve and other locations show a significant increase in the proportion of sugar maples during the past 20 years.

"Sugar maples produce a lot of shade and reach higher into the canopy than dogwoods," Pierce says. "Dogwoods are an understory tree. They don't do well in full sunlight, and they also don't do well under maples, which form a complete canopy, allowing little sunlight to penetrate. Dogwoods are more suited to forests where the canopy is not as dense."

The dogwoods' decline has wide-reaching implications for forest dynamics, Pierce says. "Losing these trees could seriously impact songbird populations. Dogwoods produce nutritious fruits in the fall, which serve as a food source for forest songbirds. Birds need high-energy resources they can easily find, especially when they are migrating."

Rabenold says there are no plans to change the management of the Ross Reserve to encourage more dogwoods to grow. "Forests are dynamic systems; they are constantly changing," he says. "It's possible that this decline may be a natural phenomenon, and, as such, we shouldn't interfere with it."

Rabenold also says that managing the reserve for one species, such as dogwoods, may impact other organisms.

"Managing the forest by cutting down sugar maples, for example, could have negative consequences for other species living in the area," Rabenold says. "Basically, we don't know how to turn the clock back, and we don't know how effective we'd be."

Homeowners with woodlots are less likely to see a decline in their dogwood populations. "Most home woodlots do not pose a problem in terms of shade," Ruhl says. "Usually the degree of canopy closure is much less in a homesite than in a maple forest."

She suggests homeowners occasionally check their dogwood trees for signs of infection and have a trained eye look at trees or leaves that show signs of infection such as spots, blotches or blight.

"Anthracnose is usually characterized by blotches or spots on the leaves and sprouting of new shoots from the trunk of the tree," she says. "Different environmental conditions can lead to different symptoms, though, so you usually can't tell just by looking at a tree whether it has anthracnose. Environmental stress also can generate symptoms that look like a disease."

Sources: Kerry Rabenold, (765) 494-8120,

Aaron Pierce, (765) 494-8142,

Writer: Jenny Cutraro, (765) 494-2096,

Intern coordinator: Susan Gaidos, (765) 494-2081;

Purdue News Service: (765) 494-2096;

* To the Purdue News and Photos Page