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The black locust: An arboreal hero?

Foliage of a black locust (Robinia pseudoacacia).
Norbert Nagel, Mörfelden-Walldorf, Germany
WikiMedia Commons
Foliage of a black locust (Robinia pseudoacacia).

“Black locust” sounds to me like either a great name for a TV drama or a terrible plague I’d flee to avoid.

But it’s actually a tree that, for years, foresters thought of as an arboreal hero.

It’s certainly an attractive tree, with its feathery foliage and fragrant flowers. It grows quickly and can tolerate poor soils, and even its scientific name is appealing: Robinia pseudoacacia. 

But what makes the black locust a biological hero is that like the other legumes it’s related to, it enriches the soil with nitrogen, an element needed by all living things to make proteins, amino acids and DNA.

Nitrogen is often in limited supply. Although nitrogen gas makes up 80% of our atmosphere, plants and animals can only get access to it when it’s been transformed to other chemical forms, like nitrate.

Some types of bacteria can convert gaseous nitrogen to nitrate through the process of “nitrogen fixation.” The bacteria provide nitrogen to the tree, and find a stable home in their roots, so black locust leaves have are high in nitrogen. When those leaves fall and decompose, they enrich the soil for themselves and the plants around them.

So, for years, foresters introduced black locusts in plantations to create “natural fertilizer.” They can add 200 pounds of nitrogen per acre each year, a huge boost to timber trees growing in poor soils.

But a recent study has hoisted a warning flag. In some forests, that “natural fertilizer” can dramatically favor invasive plants, like morning glories and honeysuckle vines, pushing out the native plants and reducing the animal populations that depend on them.

This pattern can persist for more than 15 years after black locust trees are removed, suggesting that their presence leaves a legacy that will continue to favor non-native species even after they’re gone.

So is the black locust a hero or something closer to its dreaded namesake? It all depends on where it’s planted and what’s around it. Trees are never simple.

Dr. Nalini Nadkarni is an emeritus professor of both The Evergreen State College and the University of Utah, one of the world’s leading ecologists and a popular science communicator. Dr. Nadkarni’s research and public engagement work is supported by the National Geographic Society and the National Science Foundation. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation. @nalininadkarni
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