November 30, 2010

Could Fire Help Chesapeake Bay’s Tidal Marshes?

Battered by rising sea levels, some Chesapeake tidal marshes could be rescued by fire. According to two University of Maryland scientists, controlled burns, if well managed, could help marshes rebuild themselves.  

Also known as prescribed burns, controlled burns are begun by igniting marsh vegetation and its undergrowth in strategic locations. Dating back to the Neolithic period, marsh burns were not only used by ancient civilizations like the Mayas and the Mesopotamians, today they are still used around the world in agriculture and environmental restoration work.

In the Chesapeake, controlled burns are meant to “deposit enough dead material to build up [the marsh] and keep it above sea level,” says Dr. Ray Weil, a specialist in soil science and one of the lead researchers on a marsh study funded by the Maryland Sea Grant College.

In Chesapeake tidal marshes, annual prescribed burns are set off during the winter season when most tidewater vegetation has died off. Burns are applied during high tide when sedge and grass roots are partially submerged. The dead upper stalks of these plants are burned away, depositing a layer of ash which the next generation of plants can use for nutrient uptake.  

In an effort to maximize the beneficial effects of controlled burns, the researchers set out to design a study that would separate out their various impacts.

Weil and Needelman sought to uncover whether ash deposition or the loss of upper vegetation, known as canopy removal, played the larger part in restoring and maintaining tidal marshes. According to the prevailing belief, vegetation would benefit from the combination of the two processes.

The researchers discovered, however, that canopy removal, not ash deposition, plays the largest part in restoring tidal marshes in the Chesapeake ecosystem.

 “Ash contains nutrients, right?” says Weil. Before the burn, “those nutrients are tied up in the plant material and not readily available for the next generation of plants. In the ash, nutrients are highly available like fertilizer, but that’s assuming that the nutrients in the ash are what the plants are lacking. Probably not, it turns out.”
 
The ash generated by the burns lacked enough nitrogen to replenish vegetation growth, says Weil who discovered that increased light exposure and higher temperatures resulting from canopy removal were more important in regenerating the marsh.

Although canopy removal caused a greater change in vegetation growth than ash deposition, the scientists also discovered that growth was relegated to the sedge species, Schoenoplectus americanus, only. As a result, continuing controlled burns could change the composition of Chesapeake tidal marshes over time, favoring the sedge species over various grass species. 

The scientists believe that their study and others like it, such as the Blackwater National Wildlife Refuge’s study of prescribed burns and their overall effectiveness, could hold the key to restoring Chesapeake marshes to a healthier state.  Though marshes will continue to be lost to sea level rise, healthy marshes can survive longer. “Marshes are able to accrete and increase their elevation along with sea level rise when they’re healthy,” says Needelman. “The healthier the marsh, the faster it’s accreting and the faster it can keep up with sea level rise.”