Photograph © Andrew Dunn, 13 September 2005. Website: http://www.andrewdunnphoto.com/ licensed under CC BY-SA 2.0

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

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Photo by Joel Abroad licensed under CC BY-NC-SA 2.0

There are some trees out there that you probably shouldn't hug. Couroupita guianensis is one such example. You certainly wouldn't want to risk standing at the base of one for any length of time. What looks like a vine covering the trunk of each tree is actually the reproductive structures of this species. Beautiful flowers give way to hefty seed pods, earning this tree its common name, the cannonball tree.

A native to Central and South America as well as parts of the Caribbean, the distinctive flowers of this tree are born on long stalks that emerge right out of the trunk. This is known as "cauliflory." Trees like this can cause you to do a double take. Indeed, it is strange seeing flowers on a trunk instead of at the tips of branches. It is likely that this type of flowering has evolved as a form of resource partitioning. Instead of vying for pollinators or seed dispersers way up in the canopy, trees like C. guianensis may opt for them at lower levels in the forest where competition may be lower.

In the case of C. guianensis, the main pollinators are carpenter bees. The peculiar flowers don't produce any nectar, however, they make up for this by offering copious amounts of pollen. The strangest aspect of this is that two different type of pollen are produced. Each flower has two sets of anthers, one set forms a ring around the center of the flower and the other set is located at the tip of the petal that is bent inward forming a hood. What's more, the pollen grains produced by each set differs in appearance with the ring pollen being white and smaller and the hood pollen being yellow and larger. As it turns out, the hood pollen is mostly sterile whereas the ring pollen is fertile. When a bee lands on the hood of the flower looking for pollen, it is attracted to the larger grains. As it harvests pollen from the hood its body is pushed up against the ring pollen, which is carried to the next flower, where the process is repeated and the flower fertilized.

Photo by Mauricio Mercadante licensed under CC BY-NC-SA 2.0

After fertilization, large capsules are produced that sort of resemble coconuts or canon balls. Being a member of the Brazil nut family, these capsules can measure upwards of 8 inches in diameter and are chock full of pulp and seeds. Each capsule eventually falls from the tree, cracking open as it smashes into the ground. The capsules can be so large and heavy that anyone unfortunate enough to be standing under one when it fell is likely to be killed by the impact. The pulp inside is said to smell quite awful, which is a attractive to various seed dispersers around the forest. Peccaries as well as large rodents like the paca eat the seeds, which germinate quite well after passing through their gut.

Couroupita guianensis has been planted far outside of its natural range for a variety of reasons. It is likely that anyone visiting a botanical garden in the tropics will come across one of these odd trees. Any gardener worth their weight would do well to keep this tree away from footpaths. This is a species best admired from a distance. Aside from avoiding a head crushing blow from one of those seed capsules, this is a tree that must be seen in its entirety to truly appreciate.

Photo Credits: [1] [2]