Sunset for Kentucky Coffee Tree?

Its numbers are declining in nature. A look into its distant past could explain why.

 

A female Kentucky coffee tree, Gymnocladus dioicus, in November 2020.

Kentucky coffee tree (Gymnocladus dioicus) cuts a striking silhouette in the landscape. Its stout branches, plated bark, and chunky, persistent pods stand out, especially in winter. Nothing else looks like this.

And nothing else has quite its combination of puzzling traits. Its range today is strangely limited, its fruits should attract herbivores but don’t, and natural distribution of its seeds, heavy and nonbuoyant, is mostly by water. Ecologically, the tree doesn’t make sense. That wasn’t always so.

A Glimpse Into the Past

Kentucky coffee tree is thought to have arrived here sometime during the Miocene, an epoch of geologic time that extended from 23 million to 5 million years ago (Zaya & Howe, 2009, citing Tiffney & Manchester, 2001). Grasslands and savannas were widespread then, and so were many herbivores. In fact, North America then is thought to have resembled the African savanna today, at least in terms of the diversity of animals and the structure of their communities (MacFadden, 2000, citing Webb, 1997, 1983).  

Important to the story of Kentucky coffee tree, many of those herbivores were huge. They were the megafauna, and they included North American rhinos, camels and elephant-like animals called gomphotheres (MacFadden, 2000; Zaya & Howe, 2009, citing Webb, 1983, and Janis et al. 2004).  Long after the Miocene, land bridges brought additional large mammals to North America, including giant sloths and armadillos from South America and mammoths and bison from Eurasia (MacFadden, 2000).

The exact diets of the megafauna aren’t known, but it’s speculated that they included the large fruits of plants like Kentucky coffee tree. The big animals would have been tall enough to reach the tough pods and strong enough to open them, enticed, perhaps, by the sweet green pulp inside. The extremely hard seeds could have withstood their forceful bites and been passed through their digestive systems intact, arriving, finally, in a pile of dung, there to begin another generation (Zaya & Howe, 2009).

Hints of a Different Life

Kentucky coffee tree thus could have spread wherever its herbivores roamed. The tree itself hints that it was once more abundant and widely distributed. Although the natural range of the tree today is confined mostly to floodplain terraces (Smith, 2008, 2018), at one time it was likely more common in open, early successional (colonizing or re-colonizing) habitats. Evidence comes in part from its growth habits: It reproduces vegetatively, and vigorously, from root sprouts, its seedlings don’t tolerate shade, and the tree tolerates drought, a combination of traits that would suit it for life in disturbed uplands (Zaya & Howe, 2009, citing Huxley & Griffiths, 1992).

The flowers, too, hint that Kentucky coffee tree was once more abundant. Most trees produce either male (pollen producing) or female (fruit producing) flowers on separate plants. Separation ensures outcrossing and offers the potential benefits of genetic mixing – an advantage in changing environments – but it would have been a disadvantage if opposite individuals had been few and far between. That disadvantage could be overcome if the flowers were pollinated by specialists. Insects dedicated to Kentucky coffee tree would gather pollen only from those flowers, and they would travel some distance to do so. That isn’t the case with this tree, however. Its flowers are likely pollinated by generalists, insects that gather pollen from a variety of sources and are unlikely to go far (Zaya & Howe, 2009).

Range map from USDA PLANTS database, 

December 2020. 

Like the flowers, the seeds plant suspicions of a wider distribution. In its modern floodplain habitats, the seeds are spread primarily by water. That’s unexpected, judging from their heft. Water-borne seeds tend to be small, light and buoyant; those of Kentucky coffee tree are large and heavy and they sink quickly. By all appearances, they aren’t adapted for spread by water. They are adapted for spread by animals. Why would they be enclosed in a sweet, green goo – a lure and reward for herbivores – if they’re intended to be carried passively, and inanimately, by water?

Survival in a Changed World

Together these anomalies point to a vastly different life. Times have changed, drastically. The megafauna that could have helped the tree spread began declining 15,000 years ago, victims of climate and habitat change, disease, overhunting or some combination of causes (MacFadden, 2000; Barlow, 2001). By 10,000 years ago at the latest, most of them were gone, and as far as Kentucky coffee tree is concerned, nothing has replaced them. Its fruits are poisonous to cows, sheep and other modern herbivores (Rowe & Geyer, undated). Even the largest plant eaters don’t have the voluminous digestive systems of megafauna, so they lack the greater diversity and number of intestinal microbes that are thought to have metabolized the toxins (Zaya & Howe, 2009, citing several studies). The fruits now fall and rot, uneaten, or the seeds germinate under the parent tree. Either way, distribution is severely limited. Even if the seeds are transported in streams, they won’t germinate there (Zaya & Howe, 2009, citing van der Pijl, 1982, and Murray, 1986), and their waterlogged journey would continue to confine them to lowland habitats.

Changes in its environment have made Kentucky coffee tree a rare find in the wild, so it is designated a species of special concern in Minnesota (Smith, 2018). It’s in no danger of extinction, however. Humans have been spreading its seeds, intentionally and unintentionally, for centuries. Early Native Americans used various parts of the plant for medicine and food, including a coffee-like drink made from roasted seeds (VanNatta, 2009; NAEB, 2020). In fact, the presence of Kentucky coffee tree on floodplains today may reflect the movement of Native Americans along stream corridors in the past (VanNatta, 2009). European immigrants also used the seeds as a coffee substitute and, like Native Americans, utilized the seeds as game pieces (Zaya & Howe ,2009; VanNatta 2009).

Today the tree is most often planted as an ornamental or shade tree, as a “rewilded” tree returned to its probable upland haunts, or as a curious link to the past. No one is certain what, if anything, ate its fruits. If nothing did, the tree’s investment in pods, pulp and seeds had no payoff. That seems unlikely. Surely this unique tree fed something besides our imaginations.

References

Barlow, C. (2001). Ghost Stories from the Ice Age. Natural History, 110(7), 62.

MacFadden, B.J. (2000). Cenozoic mammalian herbivores from the Americas: Reconstructing ancient diets and terrestrial communities. Annual Review of Ecology and Systematics 31, 35-59.

Native American Ethnobotany (NAEB) Database. Accessed November 28, 2020. http://naeb.brit.org/uses/search/?string=Gymnocladus+dioicus

Row, J.M., and Geyer, W. (n.d.). Plant Guide: Kentucky Coffeetree. USDA NRCS Plants Database. https://plants.usda.gov/plantguide/pdf/cs_gydi.pdf.

Smith. W.R. (2008). Trees and Shrubs of Minnesota. University of Minnesota Press.

Smith. W. (2018). Rare Species Guide: Gymnocladus dioica. Minnesota Department of Natural Resources. https://bit.ly/3mE5Ea2

USDA, NRCS. 2020. The PLANTS Database (http://plants.usda.gov, 3 December 2020). National Plant Data Team, Greensboro, NC 27401-4901 USA.

VanNatta, A. (2009). Ecological importance of Native Americans Culture to the Kentucky Coffee Tree (Gymnocladus dioicus). University of Wisconsin Stevens Point. https://www.uwsp.edu/forestry/StuJournals/Documents/NA/avannatta.pdf

Zaya, D.N., and Howe, H.F. (2009). The anomalous Kentucky coffeetree: megafaunal fruit sinking to extinction? Oecologia, 161, 221-226. https://doi.org/10.1007/s00442-009-1372-3.