Ecologist Carol Wessman: how much can a forest stand?
Student Focus:
Brian Buma
Carol Wessman’s graduate student Brian Buma is studying ecological diversity patterns following major disturbances in forests.
"We suspect more diversity or less diversity is a function of how much disturbance a site has accrued," said Buma. "How do communities reorganize after disturbances? What sort of plants come in?"
To gain a large-scale perspective on those questions, Buma analyzes remote-sensing data obtained from aerial surveys by the U.S. Forest Service, to determine the relationships between the different disturbances across the landscape.
"It’s good to get concrete numbers in a spatial sense over the whole landscape," said Buma. "Then you can draw these large-scale interpretations which is really hard to do when you are just standing on the ground."
growth for existing seedlings was reduced. It appeared that while the forest could recover from a single disturbance, multiple disturbances proved more challenging.
After the fire, however, some interesting and contradictory results began to emerge. Wessman found seedlings and new growth in the sites where the salvage logging had taken place, but not in the areas where just the blowdown had occurred—a reversal of previous results.
“The research has provided a really good example of the cumulative and interactive effects of the disturbances. The various permutations of the disturbances interact quite differently and produce quite different measurable results,” said Wessman.
Predicting the impacts of beetle kill
While Wessman’s besieged experimental plots were struggling to recover from the effects of multiple disturbances, the Routt National Forest—with many other forests in the Western United States—was hit with another catastrophe: beetle kill. The mountain pine beetle has devastated large regions in the Routt National Forest and District Rangers estimate 600 acres of the forest land are logged a month to remove affected trees. Moreover, the beetles have impacted both Wessman’s “control” forest and regions where the fire was less severe, effectively burrowing their way into her research.
Wessman, however, believes the latest disturbance simply adds another layer to her experiment. “The question is how will the beetle kill interact with the other recent disturbances as it hits along the margins and in the remaining areas of green forest? The whole area is undergoing change,” she said.
Wessman also hopes that the insights gleaned so far will provide useful information in predicting how these subalpine ecosystems will recover from the beetle kill. “Forest regeneration and nitrogen cycling in beetle-killed stands may parallel the blowdown areas in some ways, because in both cases the overstory is dead,” she said. Ultimately, however, she hopes that her research will inform some of the questions faced in a climate of unpredictable change brought about by warming temperatures.
“How resilient are our forests to more frequent, and perhaps more extreme, disturbances? And how do we (humans) have to adjust to accommodate those changes?” 
When fire decimates the site of an experiment, some researchers might throw their hands up in dismay. Not so for CIRES Fellow and profesor of ecology and evolutionary biology Carol Wessman.
"My first thought was, ‘This is cool,’" said Wessman. "Now things are getting really interesting."
Wessman’s research is focused on the impacts of multiple disturbances on forest ecosystems. Until 2003, she had been studying a region in the Routt National Forest of northwestern Colorado that had suffered a catastrophic blowdown in 1997, and had then been partially salvage logged in 1999. Wessman was comparing metrics of forest recovery between the two. Then came the fire.
The 2002 Mt. Zirkel Complex Fire, which consumed more than 12,500 hectares, may have destroyed Wessman’s experiment. But she simply took it as an opportunity to investigate a key ecological question: just how much can a forest stand?
"I am interested to see if there is a limit to the number of severe disturbances that a forest can experience before its ability to regenerate breaks down," said Wessman. "Do the effects of different disturbances accumulate? Do the disturbances interact?"
Wessman’s research is critical for those who want to understand the future of forests and the services they provide, from biodiversity to clean water and carbon storage. Climate change and other human-forest interactions are expected to disturb forests with greater frequency, extent, and intensity. The effects of such disturbances are likely to have far reaching economic and social consequences. Better understanding of how forests will respond could help forest managers better plan for the potential stormy waters ahead.
Studying forest regeneration
Since the fire, Wessman and her students have been visiting the differently affected regions of the forest to meticulously log the fine details of forest regrowth. They measure the soil temperature, water content and nitrogen availability at 23 sites, and map the location, height, and species of every new seedling. Prior to the fire, Wessman had found that regrowth in the areas that had been logged after the blowdown was slower than that in the "blowdown only" regions; fewer new seedlings were observed and the rate of
