The anomalous Kentucky coffeetree: megafaunal fruit sinking to extinction?

Abstract

The Kentucky coffeetree (Gymnocladus dioicus, Fabaceae) is an ecological paradox. A rare tree in nature in eastern and central North America, G. dioicus produces legumes that are only known to be dispersed by water, but appear similar to fruits consumed and dispersed by elephants and rhinoceros. One would expect the pods to be consumed by livestock, but the pulp and seeds are toxic to cattle and sheep. We examine the puzzle of G. dioicus dispersal in light of its other reproductive and life history characteristics and find that it probably is a botanical anachronism, in terms of both a set of dispersal agents long extinct and habitats, including what we term megafaunal disclimaxes, which have disappeared. Large seeds, the megafaunal gestault of the fruit, a dioecious mating system, and shade-intolerance combined with vigorous cloning suggest a widely dispersed pioneer of Miocene through Pleistocene habitats profoundly altered by large-mammal herbivory. As to what ate it, we can only say there were once many candidates. We hypothesize that the plant is an ecological anachronism, sinking to extinction in the wild.