Forest ThreatNet

In 2001, when large numbers of red spruce trees began dying atop Mt. Mitchell in western North Carolina, Eastern Threat Center researchers stepped in to investigate. During the four years before their arrival, unusual drought and abnormally high air temperatures combined with acid rain pollution and a rare outbreak of southern pine beetles to wreak havoc in those forests covering the tallest peak in the eastern United States. Some red spruce trees survived through it all, providing a unique opportunity for the researchers to examine the differences between the live and the dead trees. As the significance of these differences became clear, the researchers formulated an idea that could redefine forest health and management in a world with increasing climate variability.

The researchers measured physical, chemical, and atmospheric characteristics to uncover variations between Mt. Mitchell’s unhealthy or “chronically stressed” red spruce trees—the slow- growing ones on drier sites with poor soils—and the previously healthy or “non-chronically stressed” trees. Some of the chronically stressed trees had died during the onslaught of threats between 1996 and 2000, but, overall, their survival rates surpassed those of their non-chronically stressed counterparts.

Right: The rust-colored needles of dead red spruce trees are visible across Mt. Mitchell in 2001. Photo by Johnny Boggs.

The researchers believe that the loss of the weakest chronically stressed trees may have reduced competition for water and enabled other similar trees to survive the drought and beetle attacks, while non- chronically stressed trees continued to compete for resources before they died. Researchers used these results as a case study published in the journal New Forests that explores the concept of inverse resilience—the possibility that trees growing under conditions of chronic, or long-term, environmental stress might better withstand acute, or short term, environmental disturbances and threats. Researchers have coined this scenario “The Age of the Mediocre Forest” to describe the endurance of less productive, yet potentially more resilient, chronically stressed trees.

“Traditionally, forests comprised of large, full-crowned, fast-growing trees with minimal insect and disease damage were considered the model of forest health, and our expectations of how trees will respond to changing climatic conditions have been based on historic observations of individual tree and forest responses to stress,” says Steve McNulty, a research ecologist with the Eastern Threat Center and the article’s lead author. “Our paper raises the central question of whether or not trees and ecosystems will continue to respond to ongoing or temporary environmental stresses in the same manner when faced with more extreme climate variability.”

The Mt. Mitchell case study highlights an extreme example of a forest under stress, so additional research is needed to explore and further develop the “mediocre forest” hypothesis. However, forest managers should take note: “If the ‘mediocre forest’ scenario is correct, management strategies will need to evolve to reduce mortality in traditionally healthy forests during periods of acute stress,” says Eastern Threat Center biological scientist Johnny Boggs. “Regardless of how common or how severe future stress and disturbances become, forest science will need to play a prominent role to inform management practices for adapting to these changes.”