The Anachronistic Kentucky Coffee Tree

To see a Kentucky coffee tree (Gymnocladus dioicus) in the wild is a rare event. Each year your chances of doing so are diminishing. This interesting and beautiful legume is quite rare, growing in small scattered populations throughout eastern and Midwestern North America. Presettlement records hint that its rarity in nature is not necessarily a recent phenomenon either. It seems that, at least since humans have been paying attention, this tree has always been scarce. 

Despite its rarity in the wild, the Kentucky coffee tree has gained a lot of popularity as a landscape tree. It is an attractive species with contorted branching and large, airy leaves. It's about this time of year when folks start wondering if they have killed the new tree they planted last fall. I often hear complaints from folks new to this species that their trees must have lost their buds over the winter. The reason for this lies in its generic name. "Gymnocladus" is Greek for "naked branch." The leaf buds are not exposed like they are in other tree species. Instead, they are imbedded within the twigs, hidden under a hairy ring of bark. Kentucky coffee tree does not leaf out until late spring, well after most other trees have broken dormancy.

In the wild, Kentucky coffee tree can be found growing on floodplains and, very occasionally, scattered through upland habitats. As such, water has been invoked as the only known dispersal agent. This is a strange mechanism to call on as nothing about this tree (other than its current habitat) suggests adaptations for water dispersal. Its seed pods are quite heavy, chock full sweet pulp, and don't float very well. What's more, the pods often remain on the tree all winter and the large seeds within require ample scarification before they will germinate. They are toxic to boot. 

Even more perplexing is just how well this species does when planted outside of floodplains. It seems equally at home growing in a yard or along the sidewalk as it does on a floodplain. Taken together, all of these clues seem to suggest that the Kentucky coffee tree is missing something. Perhaps it is missing a preferred seed disperser? 

The megafaunal dispersal syndrome has become a sexy topic in ecology. Essentially it posits that North America was once home to a bewildering array of large mammals that flourished leading up to the end of the Pleistocene. With that many large animals haunting this once wild continent, many have suggested that North American vegetation evolved to cope with and even exploit their presence. Certainly we see this happen on a smaller scale with things like birds and small mammals. We see it on a much larger scale with animals like elephants and rhinos in Africa and Asia. Could it be that when the Pleistocene megafuna went extinct in North America, the plant species they dispersed suffered a huge ecological blow? 

The limited range of species like the Kentucky coffee tree would certainly seem to suggest so. Though it is a hard theory to test, the fruits of this tree seem adapted to something much more specific than running water. The large pod, the sweet pulp, and the hard seeds would suggest that the Kentucky coffee tree requires a larger mammalian herbivore to eat, scarify, and pass its seeds. No animal native to this continent today does the trick effectively. Most animals avoid the seeds entirely, which is likely due to their toxicity. Sure, the occasional seed germinates successfully, however, based on its limited natural range, the fecundity of the Kentucky coffee tree has been diminished. 

Photo Credit: Roger Latourwww.floraurbana.blogspot.ca

Further Reading:

http://1.usa.gov/1WUB4YX

http://bit.ly/239BSbW

Osage Orange

As a kid I used to get a kick out of a couple trees without ever giving any thought towards what it was. My friend's neighbor had a some Osage orange trees (Maclura pomifera) growing at the end of his driveway. Their houses were situated atop a large hill and the road was pretty much a straight drop down into a small river valley. After school on fall afternoons, we would hang out in my friends front yard and watch as the large "hedge apples" would fall from the tree, bounce off the hood of his neighbor's car (why he insisted on parking there is beyond me) and go rolling down the hill. I never would have guessed that almost two decades later the Osage orange would bring intrigue into my life yet again. This time, however, it would be because of the evolutionary conundrum it presents to those interested in a paleontological mystery...

The fruit of this tree are strange. They are about the size of a softball, they are green and wrinkly, and their insides are filled with small seeds encased in a rather fibrous pulp that oozes with slightly toxic white sap. No wild animal alive today regularly nibbles on these fruits besides the occasional squirrel and certainly none can swallow one whole. Why then would the tree go through so much energy to produce them when all they do anymore is fall off and rot on the ground? The answer lies in the recently extinct Pleistocene megafauna. 

The tree is named after the Osage tribe who used to travel great distances to the only known natural range of this tree in order to gather wood from it for making arrows. It only grew in a small range within the Red River region of Texas. When settlers made it to this continent, they too utilized this tree for things like hedgerows and natural fences. 

What is even stranger is that recent fossil evidence shows that Maclura once had a much greater distribution. Fossils have been found all the way up into Ontario, Canada. In fact, it is believed that there were once 7 different species of Maclura. It was quickly realized that this tree did quite well far outside of its current natural range. Why then was it so limited in distribution? Without the Pleistocene megafauna to distribute seeds, the tree had to rely on flood events to carry the large fruit any great distance. With a little luck, a few seeds would be able to germinate out of the rotting pulp. Botanists agree that the Red River region was a the last stronghold for this once wide ranging species until modern man came on the scene. 

Another clue comes from the toxicity of the fruits. Small animals cannot eat much of it without being poisoned. This makes sense if you are a Maclura relying on large animals as dispersers. You would want to arm your fruit just enough to discourage little, inefficient fruit thieves from making a wasteful meal out of your reproductive effort. However, by limiting the amount of toxins produced in the fruit, Maclura was still able to rely on large bodied animals that can eat a lot more fruit without getting poisoned. Today, with the introduction of domesticated megafauna such as horses and cows, we can once again observe how well these fruits perform in the presence of large mammals. 

Finally, for anyone familiar with Maclura, you will notice that the tree is armed with large spines. Why the heck does a large tree need to arm itself so extravagantly all the way to the top? Again, if you need things like mammoths or giant ground sloths to disperse your seeds, you may want to take some extra precautions to make sure they aren't snacking on you as well. It takes energy to produce spines so it is reasonable to assume that the tree would not go through so much trouble to protect even its crown if there once wasn't animals large enough to reach that high. The Pleistocene megafauna went extinct in what is evolutionarily speaking only the blink of an eye. Trees like the Osage orange have not had time to adapt accordingly. As such, without the helping hand of humans, this tree would still be hanging on to a mere fraction of its former range down in the Red River region of Texas.

Further Reading:
http://plants.usda.gov/core/profile?symbol=MAPO

http://www.americanforests.org/magazine/article/trees-that-miss-the-mammoths/

http://www.plosone.org/article/info:doi%2F10.1371%2Fjournal.pone.0001745