Tropical forests 'fix' themselves after logging

Researchers have found that tropical forests jump start their own recovery after logging or agricultural clearing -- capturing nitrogen and carbon faster in response.

Scientists with the Smithsonian Tropical Research Institute (STRI) in Panama believe the discovery that trees can "turn up" their ability to "fix" nitrogen -- or capture it from the air and release it into the soil -- has far-reaching implications for forest restoration projects.

"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," said Jefferson Hall, STRI staff scientist. "Trees turn nitrogen fixation on and off according to the need for nitrogen in the system."

Scientists from Princeton University, Wageningen University, the University of Copenhagen and Yale University collaborated with STRI analyzing more than a square mile of the Panama Canal Watershed.

They compared tree growth rate and nitrogen levels on pastureland abandoned two, 12, 30 and 80 years ago with growth rates in mature forests, and found that nitrogen-fixing tree species put on carbon weight up to nine times faster than non-fixing species during early stages of forest recovery.

Nitrogen-fixers provided enough nitrogen fertilizer in the soil to facilitate storage of 50,000 kilograms of carbon per hectare during the first 12 years of growth. Legumes in particular -- including beans and peas -- are vital to early nitrogen-fixing during regrowth.

"Diversity really matters," said study author Sarah Batterman. "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."