Philip Crim
12-7-2022
Species interactions can be a major factor driving natural selection, and these relationships often present themselves in the appearance and function of both plants and animals. Whether it be attractive fruits to draw animals to aid in dispersal, or thorns to repel herbivory, these features can have a big impact on the appearance of the plant and how it goes about its daily business. A common horticultural example of this is the American honeylocust (Gleditsia triacanthos L.) which is generally cultivated as thornless and often seedless street trees. However, some cultivated specimens do bear fruits, yielding seedlings that are more typical of their thorny, wild-type progenitors. These thorns are often compound (the thorns have thorns!) and are among the most noteworthy and vicious of any temperate tree species; indeed, they are believed to have warded off megafauna that are now extinct. While the trees seem literally built to repel large browsing creatures, the long, twisted, sweet pods seem designed to be a delicacy that even a human could enjoy.
The American honeylocust is not alone in terms of having a physical trait that seems to signal an ecological anachronism. Kentucky coffeetree (Gymnocladus dioicus (L.) K. Koch) also bears rather unusual fruits that seem optimized to be a food source for long-extinct creatures. Unlike the honeylocust, coffeetrees do not bear thorns; rather, they have a different response to herbivory and stem damage: they are prolific root-sprouters. As a result, coffeetrees can form large clonal colonies that can be easily spotted from a distance in any season by their unusually thick, gaunt stems. Coffeetrees are believed to be functionally dioecious, which means that there are male and female genders on separate trees; however there is some evidence that this is not always the case. When female trees are present, the pods make the trees very easy to spot from a distance in fall and winter.
While honeylocusts seem to be doing fairly well without their assumed megafauna associates, the same cannot be said of the Kentucky coffeetree. The proliferation of large ungulates that Europeans introduced during the colonization of North America seems to have been a boon for the honeylocust, but the coffeetree pods do not seem to be nearly so appealing to horses and livestock. Without passing through the digestive tract of a large mammal, the seed coat of coffeetree seeds remains virtually impermeable, leading to very low germination rates. Few animals seem to feed on the pods of coffeetree, although I have witnessed eastern grey squirrels (Sciurus carolinensis) chewing on them and ripping them apart. Whether this is for the sweetish gummy pulp inside the pods, or merely a recreational activity, I cannot say. It does appear that the squirrels sometimes leave behind seeds that have been abraded by chewing, so perhaps the squirrels are aiding the persistence of this species.
Without the widespread aid of animal dispersers, Kentucky coffeetrees are often found in floodplain habitats where moving water can transport and possibly abrade the extremely durable seed coats. In some cases, populations can be found growing in rocky limestone soils, although this appears to be less common in the present day. While it makes sense that the species can persist in areas where floodwaters can sometimes replicate the seed coat abrasion that had previously been performed by a large mammal, it is less clear how contemporary populations are persisting in upland areas.
As a child, I spent a lot of time reading my Golden Book on Trees of North America that was given to me by my Uncle Tom. One day, my father and I were cutting firewood near the Cook Lot at Limeledge and encountered a bizarre tree that neither of us could identify. It had shaggy, light gray outer bark over an inner bark with orange and pink tones. His initial suggestion was shagbark hickory, but the twigs were all wrong for hickory. We were also finding unusual pods on the forest floor. Eventually, we found a branch with pods still attached and I remembered the illustration from my book. I was sure it couldn’t be a coffeetree though, since the range map showed that it shouldn’t be anywhere in New York and we were in a suitably desolate area that the plants had probably not been introduced.
Well, I was wrong about that. A closer look at the range map showed me that there was a small disjunction in the native range… directly over Central New York! I was ecstatic and decided to tell my great-grandfather about my discovery since he had spent his life exploring the area and keeping an eye out for unusual plants. As a teenager, he had even tried to key out and classify the willows on his farm. He was unimpressed in a way that only a man in his late eighties can be; he had found these trees as a child and claimed that during the Great Depression, his family had gathered pods from the “coffeewoods” and roasted, ground, and made coffee from the seeds in the same spirit of folks during colonial times. There are varying claims when it comes to the issue of flavor, and roasting is necessary to detoxify alkaloids that would otherwise be potentially lethal if consumed by a human.
Since my childhood discovery, I have seen many dozens of coffeetrees in cultivation, if not more. Yet none of them bear much of a resemblance to the bark of our trees in Elbridge. I have also been able to track down some other patchy populations in CNY around Weedsport, Port Byron, Jordan, and Jack’s Reef. All of these trees are in fairly desolate locations where introductions are unlikely, and most or all of them (I haven’t seen some of them in several years) bear the same unusual shaggy bark as our putative native Elbridge stand. In 2019, a new Flora of Onondaga County was published that described coffeetree as having been likely extirpated. This is likely true from some of the old locations described from the 1800s and 1900s, including near Onondaga Lake and at least one site in Tully. Time and land use change can take a toll, and the western portion of the county seems to be strangely unexplored botanically.
Why our native stand has persisted on a rocky Limestone hillside when floodplain habitats seem to be much more favorable is a bit of a mystery. There are rumors from neighbors and other locals that there are patches of coffeetrees nearby on other properties. If this is true, it is likely that the species is in the towns of Skaneateles and Marcellus, as our native Elbridge stand is very close to the border of each. In addition, I have discovered a second, much smaller stand on Flat Rock road in the Town of Camillus, and a larger population has been reported elsewhere in Camillus. Examining these stands may help answer questions about how distinct the coffeetrees in CNY truly are; could they just be an unusual local variant, or are they worthy of some special taxonomic recognition? To help answer this question, seeds have been shared with the Brenton Arboretum in Dallas Center, Iowa, which hosts a coffeetree collection from seeds collected all over the native range of the species, and with the USDA. A study on genetic diversity across the range of the species could tell us a lot about our coffeetrees, and where the species as a whole might be trending in the absence of its associated megafauna. Spoiler alert: it’s not looking good.
Conservation of our wild, likely native Kentucky coffeetrees is an important example of protecting biodiversity in a changing world. Limeledge- both the road and the arboretum- are aptly named, as they occur along a Limestone outcrop that is ideal for quarrying. There are multiple quarries within just a few miles of Limeledge, two of which are quite large, and the companies that own them are looking to buy. Protecting this unique habitat and our coffeetrees is one of the top priorities of Limeledge, as it is a rare opportunity to have an on-site example emphasizing the importance of education, conservation, and biodiversity.
