Katrina's Forest Legacy

Newspapers have picked up this report from Science Magazine (AAAS): Hurricane Katrina's Carbon Footprint on U.S. Gulf Coast Forests. Here's the abstract. (Full text requires paid subscription):

Jeffrey Q. Chambers, Jeremy I. Fisher, Hongcheng Zeng, Elise L. Chapman, David B. Baker, George C. Hurtt

Hurricane Katrina's impact on U.S. Gulf Coast forests was quantified by linking ecological field studies, Landsat and Moderate Resolution Imaging Spectroradiometer (MODIS) image analyses, and empirically based models. Within areas affected by relatively constant wind speed, tree mortality and damage exhibited strong species-controlled gradients. Spatially explicit forest disturbance maps coupled with extrapolation models predicted mortality and severe structural damage to ~320 million large trees totaling 105 teragrams of carbon, representing 50 to 140% of the net annual U.S. forest tree carbon sink. Changes in disturbance regimes from increased storm activity expected under a warming climate will reduce forest biomass stocks, increase ecosystem respiration, and may represent an important positive feedback mechanism to elevated atmospheric carbon dioxide.

Obviously, extensive decomposition will release much carbon dioxide; from the abstract alone, it looks like the authors have hit on a way to get press attention (and continuing grant money)--talk about global warming. I was more impressed by the Washington Post November 16 article: Katrina, Rita Caused Forestry Disaster Die-Off Will Add To Buildup of Greenhouse Gases by Marc Kaufman. It gives a more meaningful context to the issue by tying together the scale of the environmental destruction, unsuccessful government attempts to address the problems, and ongoing reforestation problems.

New satellite imaging has revealed that hurricanes Katrina and Rita produced the largest single forestry disaster on record in the nation -- an essentially unreported ecological catastrophe that killed or severely damaged about 320 million trees in Mississippi and Louisiana.

The die-off, caused initially by wind and later by weeks-long pooling of stagnant water, was so massive that researchers say it will add significantly to the global greenhouse gas buildup -- ultimately putting as much carbon from dying vegetation into the air as the rest of the nation's forest takes out in a year of photosynthesis.

In addition, the downing of so many trees has opened vast and sometimes fragile tracts to several aggressive and fast-growing exotic species that are already squeezing out far more environmentally productive native species.

Efforts to limit the damage have been handicapped by the ineffectiveness of a $504 million federal program to help Gulf Coast landowners replant and fight the invasive species. Congress appropriated the money in 2005 and added to it in 2007, but officials acknowledge that the program got off to a slow start and that only about $70 million has been promised or dispensed so far. Local advocates said onerous bureaucratic hurdles and low compensation rates are major reasons.

"This is the worst environmental disaster in the United States since the Exxon Valdez accident . . . and the greatest forest destruction in modern times," said James Cummins, executive director of the conservation group Wildlife Mississippi and a board member of the Mississippi Forestry Commission. "It needs a really broad and aggressive response, and so far that just hasn't happened."

A large portion of the forest devastated by Katrina and Rita belongs to relatively small landowners, who use their property as an investment to be logged when they need some cash. The federal program designed in 2005 to address the destruction was an emergency add-on to the popular federal Conservation Reserve Program, which pays landowners "rent" for returning marginal or environmentally sensitive land to more natural conditions.

Larry Payne, director of cooperative forestry for the U.S. Forest Service, said that "Congress wanted to get money back into the hands of these people, and that was the top priority." But generally it has not worked out.

Native tree species are not recolonizing devastated areas on their own due to competition from exotic species, so human intervention is critical for forest restoration. The "Chinese tallow tree" mentioned below is the plant I know as "candleberry tree"--Triadica sebifera (L.), a tropical member of the spurge family, Euphorbiaceae. I'm a little confused about the exotic grass they mention. Cogon grass (Imperata cylindrica) is a troublesome weed, as is Japanese stilt grass (Microstegium vimineum), a plant that escaped after being used for packing material.

Hurricane Katrina came ashore along the Pearl River, which divides Mississippi and Louisiana and is ecologically very rich and diverse. The Chambers study, as well as the work of local conservationists including Cummins, found that such native species as longleaf pine, live oak and cypress survived the hurricane much better than species planted primarily for logging, such as loblolly and slash pine.

But some of the native deciduous forests were severely damaged, and the young, slow-growing oaks and maples are being squeezed out by Chinese tallow trees -- an ornamental plant imported more than a century ago. It thrives on disturbed land and is running wild in the damaged area, foresters said. The tree produces a milky, toxic sap that keeps insects away and makes an inhospitable habitat for birds and small mammals.

In pine forests, the suddenly open spaces are being taken over by other invasive species, especially cogon. The aggressive Japanese grass was initially imported as packing material for oranges, but it has gotten into the environment and pushes out more productive native species.

"People are very concerned about the invasives -- you hear that everywhere Katrina went," said Richard Martin, director of conservation services at the Nature Conservancy in Louisiana. "As the Chinese tallow and other invasives take over, they form a dense canopy that makes it hard for the oak and maple to grow well. Those trees will win out in the end, but it will take hundreds of years rather than a much quicker response if the invasives weren't there."

The slow pace of the reforestation has disappointed many conservationists, as has the government's failure to encourage the planting of longleaf pine -- which once dominated 40 million acres in the Southeast but is now down to 1 million acres.