Herbivores limit tropical forest growth by reducing nitrogen

In a new study led by the Cary Institute of Ecosystem Studies, researchers have found that nitrogen-fixing trees experience more herbivory than other trees in tropical forests. By selectively feeding on these particular trees, insects and other animals reduce nitrogen availability, which can limit forest growth. 

“Recent observations suggest that the large carbon sink in mature and recovering forests may be strongly limited by nitrogen,” wrote the study authors.  “Nitrogen-fixing trees (fixers) in symbiosis with bacteria provide the main natural source of new nitrogen to tropical forests.”

“However, abundances of fixers are tightly constrained, highlighting the fundamental unanswered question of what limits new nitrogen entering tropical ecosystems. Here we examine whether herbivory by animals is responsible for limiting symbiotic nitrogen fixation in tropical forests.”

With the help of soil microbes, nitrogen-fixing trees transform nitrogen gas into a type of nitrogen that plants can use for growth. In some tropical forests, nitrogen-fixing trees are the primary provider of new nitrogen, yet these trees are rare. 

“Tree growth in many tropical forests is limited by lack of nitrogen,” said study co-author Sarah Batterman. “Given the substantial benefit of nitrogen to these forests, it has long been a mystery why nitrogen-fixing trees represent just 5-15% of trees. We suspected that herbivores might be preferentially targeting fixers due to their nutritious, nitrogen-rich leaves.” 

In a field experiment in Panama, the team analyzed leaves from 23 fixer species and 20 non-fixer species. The researchers found that overall, fixers experienced 26 percent more herbivory than non-fixers. The leaves of fixers were attacked 21 percent more than those of non-fixers.

“Our findings suggest that nitrogen-fixers bear higher herbivory costs than non-fixers, and that herbivory may be substantial enough to limit the success of nitrogen-fixing trees and their ability to alleviate nitrogen deficits in tropical soils. This has management implications for the species mixes used in reforestation efforts,” said study lead author Will Barker from the University of Leeds.

“Mature and recovering tropical forests are a large and important carbon sink, yet this sink is weakening due to climate change and potential limitation by nitrogen,” said Batterman. “The widespread cost of herbivory for nitrogen-fixers should be incorporated in climate change models as a constraint on symbiotic nitrogen fixation and future tropical forest growth.”

The study is published in the journal Nature.

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By Chrissy Sexton, Earth.com Editor

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