An Ongoing Story of Growth, Change and Adaptation
Norm Christensen's Professional Path to Ecology Followed Many Twists and Turns p.2
Working with students and faculty from Duke, the University of North Carolina at Chapel Hill and other universities, he established a network of 193 new experimental research plots, representing a wide range of forest types, forest ages and soil conditions. Each plot measured one-tenth of a hectare, or about a quarter of an acre. Many were located in Duke Forest.
Being able to re-sample permanent plots originally established in the 1930s by Duke Forest's founding director Clarence Korstian was vital to the project's success, Christensen says. "Having access to 75 years of detailed records on the history of research in that forest created a whole set of opportunities for us to document the effects of change that occur over decades or even longer. That's a rare thing to be able to do in forest ecology."
By 1977 the network was complete, and team members began measuring the growth of individual trees and herbaceous plants on each plot, meticulously documenting changes in size, leaf area, extent of cover, population density and species diversity, as well as sampling and recording any changes in the plots' soils.
"It sounds complicated, but it's not that different from measuring a child's height from year to year on the back of a closet door," Christensen says. "Individual measurements aren't that significant. What matters is the rate of growth—how they fare from one measurement to the next."
Christensen's goals were to document the variability that existed in southeastern forests; to measure the speed and extent to which changes were occurring in the forests; and—if possible—to determine which changes were attributable to the process of succession that resulted from past human disturbance, and which changes were occurring independently of them.
As the volume of sampling data grew, some surprising answers began to emerge.
Forest productivity was found to be remarkably similar between most old-field forests and natural ones, although the hardwood species most commonly found in old-field forests were quite different from the hickory or oak found in natural stands. "These differences have both economic and ecological consequences," Christensen says.
Differences documented in forest populations and ecosystem health were more significant. A rampant deer population and the suppression of wildfires were having profound impacts on the spread of invasive species such as European privet. The consequences and extent of these disturbances varied from site to site, but they underscored a near-universal maxim: once disturbed, a forest ecosystem tends to be more susceptible to further disturbances as it matures. Change begets change.
"A hurricane, for instance, could be viewed as a random, independent disturbance, but the effects it will have on an ecosystem very much depend on disturbances that have taken place in that ecosystem in the past," Christensen says. "These will affect how the ecosystem changes and recovers after the winds and waters recede."
The impact of climate change may have similarly far-reaching repercussions, he warns. "Summers have been warmer and growing seasons have been longer. We expect these changes to have cumulative effects on the forest over time."
Fellow ecologists credit Christensen's work with reshaping how they view the complex and often contentious issue of human impacts on forests.
"The importance of history and human impact on ecosystems is underappreciated by most ecologists. Given the complexity and diversity of human impacts, it is tempting to avoid coming to grips with the topic at all. Norm recognized this problem and the singular importance of looking at the ecological pattern on the landscape as the results of historical contingencies and past events," says Christensen's longtime research colleague Robert K. Peet, professor of ecology at UNC-Chapel Hill. "He has been a central figure in bringing the ecological community together to contribute to major ecological issues of importance to society."
In particular, Peet says, Christensen's work on the impact of human disturbance in forests of the southeastern coastal plain "is the first and still the clearest and most complete assessment of this understudied but critical eco-region."
photo captions: David Gilluly MF'87 measures loblolly pine in Duke Forest; Norm Christensen measuring tree diameter at a permanent sample plot; Preparation of field area in the Blackwood Division of the forest; Bark of shortleaf pine; Clarence Korstian, the first forest director, examines a permanent sample plot in the early 30s.