Hurricanes Impact Carbon Sequestration By Forests

Global warming has been tied to increasing amounts of carbon dioxide released into the atmosphere from human activities ranging from clearing land to burning fossil fuels. Attention has been focused on US forests as possible sinks for carbon dioxide from various emissions. Estimates of the amount of carbon dioxide taken up by forests vary; predictions based on these estimates are an important factor in policy debates about global warming control.

In the March 2002 issue of the journal Environmental Pollution, Steven McNulty, USDA Forest Service Southern Global Change Program Leader, suggests that the effects of hurricanes must be taken into account in predicting the carbon storage capacity of US Forests along the southeastern seaboard.

At least one major hurricane hits the southeastern US coastline two out of every three years. Over 55 percent of the land in the southern U.S. is forested: timber damage from one hurricane can exceed $1 billion – and significantly reduce carbon stored. “A single hurricane can convert ten percent of the total annual carbon storage for the United States into dead and downed forest biomass,” said McNulty. “Hurricanes leave behind a lot of dead trees that decompose and return carbon to the atmosphere before it can be harvested.”

McNulty analyzed hurricane damage data collected between 1900 and 1996 to address three issues related to carbon sequestration. First, he looked at how much carbon is transferred from living to dead carbon pools when trees are broken or uprooted. Second, he explored what happened to the downed trees – whether they were salvage logged, burned, or consumed by insects. Finally, he examined the long-term impacts of hurricanes on forest regeneration and productivity.

McNulty found that even though hurricanes do not immediately change the state of carbon in a downed tree, a large amount of accumulated forest carbon is lost in the years following a major storm. For economic reasons, most of the wood from hurricanes is not salvaged. Not only is carbon lost as trees decompose, but the downed wood becomes fuel for wild fires that can kill surviving vegetation and release additional carbon dioxide. “If increased carbon sequestration is going to be one of the mechanisms used to reduce net emissions of carbon dioxide in the United States,” said McNulty, "incentives to increase post-hurricane timber salvage need to be addressed.”

Hurricanes remove the most mature trees, allowing the younger trees in the forest understory – more active in absorbing and converting carbon to biomass – to take over. Forests fill in the gaps, though former density may not return for generations. “Short-term increases in forest productivity do not compensate for the loss in numbers of trees and the 15 to 20 years needed to recover the leaf area of mature forests,” said McNulty. “Hurricanes must be considered a significant factor in reducing long-term carbon storage in US forests.”