Coastal Retreat In Face Of Rising Sea Levels Found To Be Influenced By Wildfires

The researcher's findings about the impact of wildfires raisequestions about whether fire suppression or controlled periodic burningare the best strategies in areas being gradually inundated by risingseas resulting from global warming, said Benjamin Poulter. He is aresearch scientist at Duke's Nicholas School of the Environment andEarth Sciences as well as a visiting lecturer at the University ofNorth Carolina at Chapel Hill.

Poulter will describe his findings during a 5:45 p.m. talk onThursday, Oct. 20, 2005, during an Estuarine Research Federationconference at the Marriott Waterside Hotel and Convention Center inNorfolk, Va. His research was mostly funded by NASA.

In analyses for his doctoral degree, Poulter mapped how plantlife is responding to one of the world's highest rates of sea-levelrise -- about 30 to 40 centimeters per century -- along the westernshoreline of two interconnected North Carolina estuaries, the Pamlicoand Albemarle Sounds.

As a starting point, Poulter took advantage of newhigher-resolution digitized data that he said enable the firstsensitive analysis of elevations lower than 1 meter above sea level.Ocean heights are predicted to rise by 1 meter by the year 2100.

Overlaying those maps with aerial photographs made in the years 1932,1969 and 1998, Poulter could then assess visually how far marshlandshad penetrated into forested areas at various points within thiscritical inundation zone over a 66-year period.

"From looking at these photographs we found that there are someareas that haven't changed at all," Poulter said in an interview."There were other areas where there have been gradual changes of justseveral meters. But there were also areas where large expanses offorest have been converted into marsh."

Scientists initially hypothesized that since the sea level isrising gradually all changes would also be gradual. But the photographsshowed that, in places, "the forest is retreating in large patches," hesaid. "Those retreats have been about 200 meters over 70 years."

Visiting some of those sites of large retreat, Poulter noticeddead trees with fire scars amid the invading marsh grasses. That ledhim to consult U.S. Fish and Wildlife Service fire history records forthe area, which since 1970 have identified "ignition points" where eachwildfire originated.

When he digitally superimposed those ignition points on thedigitized landscape he found a correlation. "It appeared that the morefrequent the fires were the more likely the forest was to retreat inthis patchy way," he said.

At a site where a forest of loblolly and pond pines had beenburned in May 2004, he found only marsh grasses growing and no signs ofregenerating pine seedlings. He hypothesized that, in the absence oftrees to release water vapor into the air, the water table had movedupward.

Attempting to explain the lack of forest regeneration, Poulterinitially hypothesized that when marsh grass species migrate infollowing a fire, plant competition and the higher water table preventthe pines from returning.

But after further experiments showed that pine seedlings wereable to grow prolifically under the marsh plants at high water tables,Poulter developed a new hypothesis.

He suggests that after a fire both marsh grass and pineseedling can coexist and proliferate in burned areas. Then, if landmanagers prevent new wildfires for longer than 10 to 15 years, theforest may re-establish itself. "In some of those areas where fire hasbeen suppressed, you do see forests moving back out into the marsh," hesaid.

On the other hand, if a new fire occurs over a shorterinterval, "you would have to start the process over," he said. And thepine seedlings could lose out in the second round of competition.

"I think this is a great example of how there is some kind ofbuilt-in tolerance to environmental change that gets disrupted when adisturbance such as fire passes through the system," he said.

"The changes that you end up having are unpredictable, punctuated changes rather than gradual change."

According to Poulter, land managers are rethinking their oldextended fire suppression policies in his study area, a pristine regionof protected land especially rich in plant and animal species.

Plans call for restoring the environment to more natural andfrequent cycles of periodic burning and regeneration. "But undercertain situations there is the potential to increase the rates offorest retreat in this region," he said.

Poulter noted that both marsh grasses and pine forests actuallydepend on fire for periodic renewal. "These are such highlyfire-dependent systems that fire suppression as a strategy to reducemarsh invasion does not make much sense," he said.

"The reintroduction of controlled burning to minimize marshplant invasion of forests should be done under optimal weatherconditions and during the appropriate season of the year," he said.