Barro Colorado Island, Panama
September 16, 2013

Legumes Help Tropical Forests Heal Themselves

redOrbit Staff & Wire Reports - Your Universe Online

Tropical forests that are timber harvested or cleared for agriculture can help reduce their own recovery time by capturing natural nitrogen fertilizer and carbon dioxide more quickly when making a comeback, according to research appearing in the journal Nature.

In addition, researchers working at the Smithsonian Tropical Research Institute (STRI) in Panama found that an unusual symbiotic relationship between a specific group of tree-species and a carbo-loading bacteria could play a role in how well tropical forests can absorb CO2 from the atmosphere.

Their research suggests that the role of these ecosystems in offsetting atmospheric accumulation of carbon from fossil fuels is dependent upon the diversity of the trees growing there, especially in second-growth forests. The study authors compared tree growth rate and nitrogen levels that were growing on pastures that had been abandoned two, 12, 30 and 80 years ago with those growing in mature forests.

Tree species that actively participated in nitrogen fixation – the process by which atmospheric nitrogen (N2) is converted to ammonia (NH3) – added carbon weight up to nine times faster than their non-fixing neighbors during the early stages of forest recovery. In fact, the researchers reported that nitrogen-fixers provided enough fertilizer to store 50,000 kilograms of carbon per hectare during the first 12 years of growth.

“This is the first solid case showing how nitrogen fixation by tropical trees directly affects the rate of carbon recovery after agricultural fields are abandoned,” STRI staff scientist Jefferson Scott Hall said in a statement. “Trees turn nitrogen fixation on and off according to the need for nitrogen in the system.”

Tropical forests, the researchers explained, thrive on natural nitrogen fertilizer that is pumped into the soil by trees in the legume family (which includes peas and beans). Hall and his colleagues studied second-growth forests in Panama, and found that the presence of legume trees was responsible for that 12-year period of rapid growth. In fact, forests that contained legumes had accumulated up to 40 percent of the carbon found in fully mature forests in just 12 years time, with over half of the nitrogen required for that feat coming courtesy of the legumes.

“Diversity really matters,” said first author Sarah Batterman, a postdoctoral researcher at Princeton University who worked on the project. “Each tree species fixes nitrogen and carbon differently so species important at 12 years drop out or become less common at 30 years. You can really see how different players contribute to the development of a mature tropical forest and the ecosystem services it provides.”

“This study is showing that there is an important place for nitrogen fixation in these disturbed areas,” she added. “Nitrogen fixers are a component of biodiversity and they're really important for the function of these forests, but we do not know enough about how this valuable group of trees influences forests. While some species may thrive on disturbance, others are in older forests where they may be sensitive to human activities.”