The Kentucky Coffeetree: An Evolutionary Anachronism?

18 October 2018
The fleshy interior of a Kentucky Coffee Tree seed pod
The fleshy interior of a Kentucky Coffee Tree seed pod. Photo by Anna Schumacher ’21.

On Carleton’s campus and in a few spots in the Arboretum, a curious reminder of North America’s past awaits discovery. The Kentucky Coffeetree (Gymnocladus dioicus) is a member of Fabaceae, the legume family. The most noticeable feature of the tree in fall is its large seedpods, which resemble out-sized field pea pods and peanuts. But unlike those cultivated legumes, the interior of the pod is a gooey, rich fruit. The existence of fruit indicates that, over the course of the coffeetree’s evolution, large, animal-attracting fruits were a better strategy for reproduction than many small seeds.

A fruitless seed is an adaptive strategy among many grasses and forbs in Carleton’s prairies, where wind is a reliable dispersal agent. But not in the forested conditions where the coffeetree evolved. Scientists previously guessed that dispersal by water was been the reason behind its large bean pods. However, the pods are not very buoyant, and the fruity interior is not a requirement for water dispersal. Is it possible that the plant’s dispersal agent is now extinct? Some scientists think so.

The coffeetree pods are similar to fruits that are dispersed by rhinoceroses and elephants in Asia. Extinct mammoths and giant ground sloths that once roamed North America are prime candidates for dispersal agents. The bean’s chemical makeup suggests it could have potentially passed through their digestive tracts unscathed. As we would expect from an evolutionary anachronism, the coffeetree has been on the decline. Early naturalists rarely documented it away from floodplains or Native American settlements, suggesting that humans and water (to the extent that the seed pods can float) are the tree’s last remaining dispersal agents. Coffeetrees can also reproduce asexually, leading to small clonal colonies in some areas.

The tree’s native range, where it is now rare, covers much of Missouri and the Ohio Valley, extending northward into Minnesota along the Mississippi River. There is a male coffeetree (no seed pods!) in front of Boliou and a pod-producing female on Lilac Hill just to the north of the Rec parking lot. The raw beans contain the toxin cytosine, but are said to make a delicious coffee substitute when roasted.

–Callen Inman ’19, for the Cole Student Naturalists

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