By partnering with soil microbes, nitrogen-fixing trees turn atmospheric nitrogen gas into a form of nitrogen that is available to plants. When fixers shed their leaves, they enrich soils with nitrogen, benefitting nearby plants. In nitrogen-poor tropical forests, nitrogen-fixing trees are the main source of new nitrogen to soils. Yet they are also rare.

Sarah Batterman, a Tropical Forest Ecologist at Cary Institute of Ecosystem Studies and co-author on the paper, explains, "Tree growth in many tropical forests is limited by lack of nitrogen. 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."

With colleagues, Batterman set out to reveal if the diets of insects and other herbivores were a constraint. Their three part study looked at: (1) whether nitrogen-fixing trees experienced more herbivory than non-fixers, (2) the carbon cost of herbivory, and (3) if herbivory was due to herbivore preference for nitrogen-rich leaves.

Fieldwork was performed at Barro Colorado Island in Panama on a 50-hectare plot of mature tropical lowland forest. Seedling trees, a life stage vulnerable to herbivory, were assessed. Leaves were analyzed from 23 fixer species and 20 non-fixer species, representing 350 seedlings and 1,626 leaves. Herbivory on mature leaves was quantified by scanning leaves, and assessing damaged leaf area. For a subset of trees, active herbivory was tracked over a three month period.