May 4 2009
A first-of-its kind, long-term study of hurricane impact on U.S. trees shows that hurricane damage can diminish a forest’s ability to absorb carbon dioxide, a major contributor to global warming, from the atmosphere. Tulane University researchers from the Department of Ecology and Evolutionary Biology examined the impact of tropical cyclones on U.S. forests from 1851–2000 and found that changes in hurricane frequency might contribute to global warming. The results will be published in an upcoming issue of the Proceedings of the National Academy of Sciences.
Trees absorb carbon dioxide as they grow, and release it when they die -- either from old age or from trauma, such as hurricanes. The annual amount of carbon dioxide a forest removes from the atmosphere is determined by the ratio of tree growth to tree mortality each year.
When trees are destroyed en masse by hurricanes, not only will there be fewer trees in the forest to absorb greenhouse gases, but forests could eventually become emitters of carbon dioxide, warming the climate. And other studies, notes Tulane ecologist Jeff Chambers, indicate that hurricanes will intensify with a warming climate.
“If landfalling hurricanes become more intense or more frequent in the future, tree mortality and damage exceeding 50 million tons of tree biomass per year would result in a net carbon loss from U.S. forest ecosystems,” says Chambers.
The study, which was led by Tulane postdoctoral research associate Hongcheng Zeng, establishes an important baseline to evaluate changes in the frequency and intensity of future landfalling hurricanes.
Using field measurements, satellite image analyses, and empirical models to evaluate forest and carbon cycle impacts, the researchers established that an average of 97 million trees have been affected each year for the past 150 years over the entire United States, resulting in a 53-million ton annual biomass loss and an average carbon release of 25 million tons. Forest impacts were primarily located in Gulf Coast areas, particularly southern Texas and Louisiana and south Florida, while significant impacts also occurred in eastern North Carolina.
Chambers compares the data from this study to a 2007 study that showed that a single storm – Hurricane Katrina -- destroyed nearly 320 million trees with a total biomass loss equivalent to 50–140 percent of the net annual U.S. carbon sink in forest trees.
“The bottom line,” says Chambers, “is that any sustained increase in hurricane tree biomass loss above 50 million tons would potentially undermine our efforts to reduce human fossil fuel carbon emissions.”