The death and resurrection of tree buds and the rapid wilting of new blossoms as a result of the easter freeze of April 7-9, 2007, shocked many long-time residents of Aalabama, Georgia, Kentucky and Tennessee. With temperatures dipping to 22°F for two consecutive mornings, the spring freeze was the worst in memory. What many locals referred to as a "cold snap" offered an excellent opportunity for regional scientists to understand better how potential extreme weather events brought about by climate change might influence growth of forests, grasses and other species of terrestrial ecosystems.
"We had an early warm spring in March that led to early bud break and leaf development," ORNL researcher Paul Hanson recalls. "Then in early april a cold air mass swooped down from Canada and froze the tree leaves and branches.
"Aircraft and satellite images revealed that the once-green forest canopy turned brown and stayed brown for more than three weeks. Then the trees, grasses and shrubs showed their resiliency and started to kick back. By mid-June all species had shown recovery."
The biggest surprise, Hanson notes, was that the yellow poplar was the last tree species to recover. While dogwood flowers got hammered, their tree foliage survived. Red maple leaves also survived because they were developed enough to have sufficient sugars to avoid being chilled to their freezing point.
"Freeze damage of non-native garden plants planted too far north is relatively common," Hanson says. "But a freeze event that affects natural systems like an entire forest is quite unusual."
ORNL researcher Lianhong Gu says his studies with Hanson of the easter freeze's after-effects demonstrate the importance of the heat storage capacity of large forests and lakes.
"We observed that plants inside large forests fared better and did not suffer as much damage as plants in open areas such as home gardens," Gu says. "The heat capacity of the Oak Ridge Reservation's wooded areas helped protect small trees, shrubs and grasses inside the forest from freezing. Another phenomenon we found is that plants along the edge of a large lake did not suffer as much damage as did plants farther from the shore and out in the open."—Carolyn Krause
Contact: Paul J. Hanson
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