Since arriving in Virginia, Karen and I have frequently marveled at the Osage orange, a fruit-producing tree not encountered in my native Pennsylvania or in New England where we attended college and resided for decades.
The Osage orange was evidently ill-advisedly imported into Virginia as a decorative tree, and it responds to that hospitality by covering the ground every Fall with enormous bumpy fruits that nothing eats and which simply lie on the ground and rot.
I wondered out loud recently why a tree would bother to produce enormous fruits in great quantity that were inedible. Fruit production, after all, constitutes a system of bribery by members of the botanical kingdom. The tree or bush produces a tasty fruit or berry, and birds and animals consume them and consequently carry away and redistribute the plant’s seeds.
There are all those Osage orange trees busily producing gigantic, but inedible, citrus fruits that nobody wants. Why is this? I wondered. It just seemed very strange.
Happily, Karen found the answer just a few days later, in American Forests.
It turns out the Osage orange fruits, like certain others, used to have customers who liked eating them. Unfortunately, their natural Pleistocene megafauna audience went extinct.
[L]et’s return to the forlorn fruit of the Osage orange. Nothing today eats it. Once it drops from the tree, all of them on a given tree practically in unison, the only way it moves is to roll downhill or float in flood waters. Why would you evolve such an over-engineered, energetically expensive fruit if gravity and water are your only dispersers, and you like to grow on higher ground? You wouldn’t. Unless you expected it to be eaten by mammoths or ground-sloths.
According to my field guide, Osage-orange has a limited natural range in the Red River region of east-central Texas, southeastern Oklahoma, and adjacent Arkansas. Indians used to travel hundreds of miles for the wood, prized as the finest for making bows. Then European settlers planted it widely as living fences, taking advantage of the tree’s ability to spread via shoots from lateral roots. But Osage-orange persisted, and became widely naturalized long after the invention of barbed wire rendered them useless to farmers. The tree can now be found in 39 states and Ontario. If Osage-orange does so well elsewhere, why was it restricted to such a small area?
The answer likely lies in the disappearance of its primary disperser. Without mammoths, groundsloths, and other megafauna to transport its seeds uphill, the range of the species gradually shrank to the Red River region. In fact, fossils tell us that Osage-orange was much more widespread and diverse before the megafaunal extinctions. Back then, Osage-oranges could be found north up to Ontario, and there were seven, not just one, species in the Osage-orange genus, Maclura.
Another anachronistic tree is the Kentucky coffeetree, so named because early Kentucky settlers used its beans as a coffee substitute. Coffeetrees have tough, leathery pods with large, toxic seeds surrounded by a sweet pulp. Water cannot penetrate the thick seed coat to begin germination unless it is abraded or cut. Sounds like mammoth food to me. The natural range of coffeetrees is concentrated in the Midwest, but without its megafauna disperser, it is generally rare and mostly limited to floodplains.
Much the same can be said about the honeylocust, with its sweet seedpods up to 18 inches long. It is more common than coffeetrees, and is found in upland areas because cattle have filled in for the mastodons, camels, or some other dearly departed megamammal with a sweet tooth. The big-fruited pawpaws, persimmons, desert gourds, and wild squash may also have been dispersed more efficiently by recently extinct mammals.
Now when you see an Osage-orange, coffeetree, or honeylocust, you might sense the ghosts of megafauna munching on treats made just for them.