A devastating drought in the summer of 1988, one of the worst since the Dust Bowl, prompted a long-term study on tree growth and survival at Cedar Creek Ecosystem Science Reserve in Minnesota. Retired Macalester College ecology professor Mark Davis was surveying prairie plants in the area at the time.
"I began to wonder what effects the extreme weather was going to have on the growth and survival of the trees in the oak savanna and woodland habitat in which we were working," Davis said.
"There was a fire and tree census project going on at Cedar Creek for some time, but with censuses conducted every five years," Davis said. He hoped to capture a more accurate picture of how year-to-year environmental changes, such as the 1988 drought, might affect individual trees and, ultimately, the broader oak savanna ecosystem.
That summer, Davis and his students set up a 16-hectare experimental grid in Cedar Creek’s oak woodland to survey tree growth and survival. In 1995, they tagged all live trees with a trunk diameter of 10 centimeters or greater and recorded their location in the grid. By 1996, they tagged trees at least 2 centimeters in diameter.
The project, known as GLADES (Grid for Landscape Analysis and DEmographic Study), was funded for several years by the National Science Foundation and examines how tree growth and survival are associated with variables like tree diameter, soil nitrogen, elevation and the spatial distribution of trees.
Surprisingly, even as Minnesota climate has become warmer during the past 30 years, Davis has not noted a significant change in tree growth and survival that he could attribute to climate conditions. His study provides a clearer picture of how the tagged trees responded to the prescribed burns used to manage the oak savannas at Cedar Creek.
The 16-hectare grid is nested in a larger Cedar Creek burn experiment that examines the effects of fire frequency. When Davis's study began, a third of the grid was burned every three years, another third was burned every five years, and the rest was left unburned. Since 2000, however, all areas are burned frequently.
Today, less than a third of the trees that were there in 1995 are still alive. "No surprise there, I suppose, that fire kills trees," Davis said.
The restoration of an open canopy with standing dead trees, known as snags, provides a home for several unique and disappearing savanna species, including the Red-headed Woodpecker. This species thrives in Cedar Creek's oak savannas despite a population decline of more than 90% in the rest of Minnesota. Davis is concerned, however, that the high frequency of burns in his study area will eventually reduce most woody species, converting the open woodland to more of a treeless, prairie-like, habitat.
The most fire-resistant trees in the grid are bur oaks. They are so resilient the results showed that they can even help protect neighboring trees during a fire. Because bur oaks don't catch fire easily, nearby trees are exposed to less heat than if they were next to a less resilient species.
His data also shows that trees located at lower elevations grow faster, and in the case of pin oaks, survive better than trees at higher elevation. Davis believes these findings are likely due to the fact that lower elevation trees are nearer to the water table. Besides having better water access, these trees also grow in soils higher in nitrogen. This could also contribute to their increased growth. Both factors likely contribute to the trees’ ability to stay resilient during prescribed burns.
More than 30 years after securing funding, Davis is now retired and plans to transition the experiment to College of Biological Sciences faculty member Peter Kennedy and Cedar Creek staff member Kally Worm. Having developed a fondness for the habitat, Davis is no less passionate about the project than when he began.
"For the last 15 years, I've just been doing the censusing by myself," said Davis, who explained that it takes nearly a week to take all the basic measurements for a single year. "And it's in the spring, usually mid-May. And I really like birds, and that's peak migration season. It's quite beautiful out there."
His motivation to keep the project going has only compounded through the years. “The longer an experiment goes, the more potentially interesting your findings can be," he says. – Adara Taylor