Herbivores munch the most helpful trees in tropical forests

The ability of tropical forests to grow and store carbon is limited, in part, by herbivory. Insects and other animals prefer to feed on nitrogen-fixing trees, reducing the success of fixers and the nitrogen they provide. The research by Will Barker and colleagues shows that that herbivory may be sufficient to limit tropical symbiotic nitrogen fixation and could constrain its role in alleviating nitrogen limitation on the tropical carbon sink.

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.”

To compare herbivory across the wide range of fixer and non-fixer species present in biodiverse lowland tropical moist forest, Barker and colleagues sampled individuals from 23 fixer species and 20 non-fixer species in a 50 hectare plot on Barro Colorado Island, Panama.

Overall, fixers experienced 26% more herbivory than non-fixers. Their leaves were attacked 21% more than non-fixers, consistent with them being preferentially targeted by insects and other animals. Fixer seedlings had a higher proportion of leaf area lost than non-fixers, but this number was smaller than expected, indicating that fixers have evolved defense strategies to prevent herbivores from consuming large areas of their leaves. 

These findings from dozens of tropical mature forest tree species advance previous evidence of high herbivory on fixers from grassland ecosystems and several tropical tree species in the canopy of secondary forests. They are also consistent with palaeoecological evidence that fixer abundance promotes ecosystem-level herbivory. Combined, the findings suggest that across ecosystems and biomes, fixers bear higher herbivory costs than non-fixers. As these costs reduce growth and survival, they create a pressure on the abundance of fixer seedlings, potentially sufficient to constrain nitrogen fixation in tropical forests. 

Barker et al. 2022.

📰 More details are in the press release from Eurekalert.
🔬 Widespread herbivory cost in tropical nitrogen-fixing tree species is in Nature.
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