All too often we think of a species' niche as a sort of address. Species will be present in suitable habitat and absent from unsuitable habitat. Certainly this oversimplification has been useful to us, however, it often ignores context. Species, especially long lived ones, can often be found in unsuitable habitat. Similarly, biotic interactions such as pollinators and seed dispersers are regularly overlooked when considering "suitable habitat." The absence of factors such as this can leave plants stranded in suboptimal conditions.
A recent paper published in PLOS One tackles this very idea by looking at a species of tree many of us will be familiar with - the honey locust (Gleditsia triacanthos). This central North American legume is widely planted as a street/landscape tree all over the United States. Ecologically speaking, honey locusts can be found growing wild in open xeric upland sites. In places like the southern Appalachian Mountains, however, they can also be found growing in mesic bottomlands. Regardless of where it is found, the honey locust seems to be severely dispersal limited (except in cases where cattle and other livestock have been introduced).
Before modern times, honey locust likely relied on Pleistocene megafauna to get around. The end of the Pleistocene marked the end of these large mammals. Left behind were many different plant species that had evolved alongside them. For a small handful of these plants, humans were a saving grace. Such is the case for the honey locust. Inside the honey locust pods there is a sugary pulp, which in southern Appalachia, the Cherokee were quite fond of. The Cherokee also used the tree for making weapons and gamesticks. As such, the honey locust holds great cultural significance, so much so that the Cherokee named at least one settlement "Kulsetsiyi" (more commonly known today as Cullasaja), which translates to "honey locust place."
Author, Dr. Robert Warren, noticed that in southern Appalachia, "Every time I saw a honey locust, I could throw a rock and hit an archaeological site.” What's more, the trees were not recruiting well unless cattle or some other form of human disturbance was present. This species seemed to be a prime candidate for testing persistent legacy effects in tree distributions.
Using seed germination experiments and lots of mapping, Dr. Warren was able to demonstrate that honey locust distributions in the southern Appalachian region are more closely tied to Cherokee settlements than its own niche requirements. The germination experiments strengthened this correlation by showing that mesic bottomlands had the lowest germination and survival rates.
Additionally, these sites are well known as former sites of Cherokee settlement and agriculture. Because this tree held such significance to their culture, it is quite likely that in lieu of Pleistocene megafauna, Native Americans, and eventually European livestock, allowed the honey locust to reclaim some of its former glory. Of course, today it is a staple of horticulture. Still, the point is that despite being found growing in a variety of habitat types, the honey locust is very often found in unsuitable habitat where it cannot reproduce without a helping hand. In the southern Appalachian region, honey locust distributions are more a reflection of Native American cultural practices.
Photo Credit: Cambridge Botanic Garden