Nitrogen-fixing trees can’t help the fact that they’re delicious. These distinct tropical plants, members of the bean family, work alongside bacteria to produce nutrient-rich leaves that hungry herbivores crave just a little too often. And thanks to a paper published in the journal Nature late last year, the reasons behind this tale as old as evolutionary time have been unearthed.

Will Barker, the paper's lead author, helmed the research while completing his graduate studies at the University of Leeds, in the U.K. During that time, he was able to center his attention on the complex relationship between herbivores and so-called fixer trees. "It was my only niche area of expertise, and it was applicable for that one Ph.D. And then, it kind of carried on from there for another six or so years," he said.

Nitrogen-fixing trees are their own unique biological category. And what makes them noteworthy is the superpower they possess: the capacity to reconstruct an essential element in a way that both flora and fauna can use. Nitrogen, which constitutes over 75% of our atmosphere, is necessary for all life on Earth—without it, proteins and DNA would not exist.

However, most living things, including plants, cannot utilize its natural gaseous state. And so, fixer trees turn it into ammonia, a usable form, with the help of some microscopic creatures. "The tree builds a nodule on its roots, like a little ball, kind of like a pea, and invites bacteria to come and live inside," said Sarah Batterman, an associate scientist at the local Cary Institute of Ecosystem Studies and the study's corresponding author.

What occurs then is a mutually beneficial relationship—in exchange for carbon from the tree, the bacteria generate ammonia and fertilize the plant. From there, herbivores that feed on those fixer vegetation, primarily insects but also mammals, gain the nutrients they need, as do carnivores who prey on those same plant-eaters. As such, the process of nitrogen fixation is a win-win for organisms up and down the food chain. "It's a source for new nitrogen to enter a forest, rather than just constantly competing to cycle the same nitrogen that's going through that forest," said Barker.

To collect data for his investigation, Barker spent the wet seasons of 2017 and 2018 on Panama's Barro Colorado Island, a research site favored by scientists across the globe. Within a 50 hectare plot of lowland tropical forest, he identified all the plants that contained signs of herbivory activity; namely, by spotting holes in their leaves. By the time he completed his field work, Barker scanned and analyzed 1,626 mature leaves of 350 seedlings—with the help of species identification tags. "I could just go out to the forest…and take a random sample of the trees that matched the questions I wanted to ask…and I could go find them using a compass and a map," he said.

In total, Barker documented 43 tropical tree species, including 23 of the nitrogen-fixing variety. And after closer inspection, the study authors found that these fixer trees attract 26% more herbivores to their leaves, compared to the non-fixers. But contrary to what some may assume, no correlation was present between the amount of nutrients in the leaves and the number of plant-eaters who decided to pay a visit. "Whether you were a fixer or not definitely explained whether you got more herbivory, but whether or not you had high nitrogen in your leaves didn't," said Barker.

One possible theory to explain this phenomenon lies within the forces of coevolution. Over time, the trees learned to develop defense mechanisms, such as spines and thorns, to protect themselves from plant-based predators. In turn, the herbivores caught up to speed with this "evolutionary arms-race," as Batterman called it, and figured out how to sneak past these natural barricades. In other words, the plants and the animals genetically grew together, and herbivores evolved their skills to seek out nitrogen-fixing plants.

According to Barker, only up to 10% of tropical trees are nitrogen-fixers—but that doesn't come as a surprise. After all, the researchers identified that nitrogen-fixing tree species experience a 34% higher loss of opportunities to use carbon, the costs of which are both structural and physiological. During photosynthesis, plants make their own food by absorbing carbon through their leaves and transforming it into glucose sugar. As such, a reduced leaf area for fixers also decreases the amount of carbon that can be absorbed over time for this process.

In addition, when herbivores consume fixer leaves, the plants are required to dedicate more resources, more carbon that is, towards rebuilding tissue versus storing it away for nitrogen fixation. "It makes total sense that herbivores really like it, but that's why being able to provide a home for the bacteria is not as spread as it could have been if it was more beneficial to the plants themselves," said Radka Wildova, visiting assistant professor of environmental science and policy at Marist College.

As such, nitrogen-fixation in and of itself might not be touted as the most beneficial of genetic traits. Considering this, the more mature trees have fine-tuned the ability to turn the trait on and off when needed, depending on whether the cost of delivering carbon to their bacterial partners runs too high. "They can start that mutualism back up very quickly if they need it, such as if the tree next to them falls down and dies, and then, they suddenly have a lot of light and can grow quickly," said Batterman.

Herbivory damage to young fixer trees is an important aspect to consider when it comes to incorporating biodiversity into tropical reforestation initiatives. Indeed, sowing multiple species of fixer and non-fixer seedlings across a landscape is vital to capturing a forest's full ecological potential. "If we're planting nitrogen-fixing trees, they're probably going to be hit hard by herbivores, and they're not going to be able to fix as much nitrogen as we'd hoped. So maybe, we need to plant a little bit extra nitrogen-fixing trees in order to offset that reduction in nitrogen fixation," said Batterman.

According to Wildova, current research shows that terrestrial systems are currently experiencing a trend in nitrogen deficiency. And since non-fixer plants rely on bacteria in the surrounding soil for the usable nutrients they need, there is simply not enough nitrogen to go around. As such, adding more fixer plants to the world couldn't come at a better time. "We may potentially see more presence of these nitrogen-fixing species in the future. They have the nitrogen available to them more readily," she said.

If enough nitrogen-fixing tropical species are lucky enough to grow old, they can serve as a source of green carbon capture. In fact, a 2019 study published in Nature Communications, of which Batterman was also an author, found that mature forests with fixer trees absorb 10% more carbon than those without. "They have a huge potential to serve as a major carbon sink, taking up carbon dioxide from the atmosphere and storing it, which would help us offset our carbon emissions and reduce the effects of climate change," she said.

But for young fixer trees whose leaves are targeted as food, carbon capture abilities are limited. Instead, Barker hypothesizes that more benefits may be derived from the herbivores themselves, who can carry fixed nitrogen throughout an ecosystem. "A herbivore comes along and eats a leaf and then it flies away. And it will drop waste, and it will die. And that nitrogen that was going to be in one area has now been spread further throughout the forest," said Barker.

Amidst all the complexities and lingering questions, one result remains clear: nitrogen-fixing trees and herbivores are yet another reason to appreciate the interconnection of the natural world. "Usually, people and scientists don't necessarily think about both the animals and the plants," Batterman said, "and it's just so cool to think that animals can have that much impact on the plant world."

When Conserving Tropical Trees, Remember the Herbivores

MARIST CIRCLE