Nitrogen-Fixing Trees Help Tropical Forests Make a Comeback

Nitrogen fixing trees can help tropical forests make a comeback, a new study has found. Researchers say that some species of trees can restore lost nutrients in deforested areas, helping other trees to grow.

The study was conducted by researchers at Smithsonian Tropical Research Institute and their colleagues at Princeton University, Wageningen University, the University of Copenhagen and Yale University. Their field study in Panama showed that denuded forests were efficiently restored in a few years, mostly with the help of leguminous plants that fix atmospheric nitrogen and pump it into the soil.

The tracts of land studied in the research were parts of a tropical forest in Panama and had been used for agriculture from five to 300 years ago. Researchers found that nitrogen-fixing trees helped secure a large amount of carbon in just 12 years. In just about a decade, the new forests had accumulated 40 percent of carbon found in old, mature tropical forests. Legume trees contributed to nearly half of the carbon "sink".

"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. Trees turn nitrogen fixation on and off according to the need for nitrogen in the system," Jefferson Hall, STRI staff scientist and one of the study authors, said in a news release.

The study showed that legume trees accumulated carbon nine times faster than non nitrogen-fixers during the early stages of forests' comeback. These trees even provided enough nitrogen for other trees to grow.

The legume trees' secret lies in their relationship with a kind of bacteria known as rhizobia, which help the trees fix nitrogen. Tropical forest soil is often low in nitrogen, according to a news release from the Princeton University.

Leguminous trees are essential for a forests' recovery and so their loss could lead to long-term problems for many species of trees, researchers said.

"Diversity really matters. 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,"  said Sarah Batterman, first author of the study.

The study is published in the journal Nature.