Raining Forests Dummies' Guide

Since Hurricanes Katrina and Rita left more than 320 million trees either blown down or poisoned to death by salt water, Mr. Hodges chose the Mississippi Delta as a canvas for this experiment. If all goes as planned, a mangrove forest would flourish in this area within the next 10 years.

The Impact of Hurricanes Katrina and Rita

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

The die-off, caused initially by the huge winds 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 forests take out in a year of photosynthesis — a staggering 105 million tons.

Scientists found that native species such as longleaf pine, live oak and cypress survived the hurricanes much better than species planted primarily for logging, such as loblolly and slash pine. 

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.  The tree produces a milky, toxic sap that keeps insects away and makes an inhospitable habitat for birds and small mammals.

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

According to Edward Macie, a regional urban forester for the Forest Service's Southern Region, about 75 percent of the trees in New Orleans died because of Katrina.

The Louisiana Wetlands

Louisiana's 3 million acres of wetlands are lost at the rate about 47 square miles (75 square kilometers) annually, but reducing these losses is proving to be difficult and costly.

Approximately half of American original wetland habitats have been lost over the past 200 years.  In part, this has been a result of natural evolutionary processes, but human activities, such as dredging wetlands for canals or draining and filling for agriculture, grazing or development, shares a large part of the responsibility for marsh habitat alteration and destruction.

Louisiana's wetlands today represent about 40 percent of the wetlands of the continental United States, but about 80 percent of the losses. The state's wetlands extend as much as 81 miles (130 kilometers) inland and along the coast for about 186 miles (300 kilometers).

Not all the wetlands are receding; in fact, some wetlands are stable, and others are growing.  But, at the present net rate of wetlands loss, Louisiana will have lost this crucial habitat in another 200 years.

U.S. Geological Survey (USGS) scientists and other studies indicate that major shifts in the course of the Mississippi River have contributed significantly to the demise of the wetlands.  The delta plain and its associated wetlands and barrier shorelines are the product of the continuous accumulation of sediments deposited by the river and its distributaries during the past 7,000 years. 

Regular shifts in the river's course have resulted in four ancestral and two active delta lobes, which accumulated as overlapping, stacked sequences of unconsolidated sands and mud.  As each delta lobe was abandoned by the river, its main source of sediment, the deltas experienced erosion and degradation due to compaction of loose sediment, rise in relative sea level and catastrophic storms.  Marine coastal processes eroded and reworked the seaward margins of the deltas forming sandy headlands and barrier beaches.  As erosion and degradation continued, segmented low-relief barrier islands formed and eventually were separated from the mainland by shallow bays and lagoons.

Barrier islands fronting the Mississippi River delta plain act as a buffer to reduce the effects of ocean waves and currents on associated estuaries and wetlands.  Louisiana's barrier islands are also eroding, however, at a rate of up to 66 feet (20 meters) per year. The erosion is happening so fast that, according to recent USGS estimates, several will disappear by the end of the century.

As the barrier islands disintegrate, the vast system of sheltered wetlands along Louisiana's delta plains are exposed to the full force and effects of open marine processes such as wave action, salinity intrusion, storm surge, tidal currents and sediment transport that combine to accelerate wetlands deterioration.

Human activities during the past century have drastically affected the wetlands.  The seasonal flooding that previously provided sediments critical to the healthy growth of wetlands has been virtually eliminated by construction of massive levees that channel the river for nearly 1,200 miles (2,000 kilometers); sediment carried by the river is now discharged far from the coast, thereby depriving wetlands of vital sediment.

In addition, throughout the wetlands, an extensive system of dredged canals and flood-control structures, constructed to facilitate hydrocarbon exploration and production as well as commercial and recreational boat traffic, has enabled salt water from the Gulf of Mexico to intrude brackish and freshwater wetlands. If that’s not bad enough, forced drainage of the wetlands to accommodate development and agriculture also contribute to wetlands deterioration and loss.