The presence of sharp, woody projections directly on a tree’s trunk is a remarkable adaptation that captures attention and prompts questions about identification and function. These defensive structures, often mistakenly called thorns, represent a significant investment of a tree’s energy, suggesting a powerful evolutionary pressure to protect its main stem. This article clarifies the distinct botanical nature of these structures and identifies the most notable tree species that utilize them for defense.
Defining Tree Armaments
While common language groups all sharp plant protrusions under the term “thorns,” botanists use three distinct terms to differentiate these structures based on their tissue origin.
A true thorn is a modified branch or shoot, meaning it contains vascular tissue and is organically connected to the plant’s internal transport system. Because they are derived from stem tissue, true thorns are difficult to remove without tearing the wood beneath.
Spines are botanically distinct as they are modified leaves, stipules, or parts of leaves, but like thorns, they also contain vascular bundles for support. You might find spines replacing entire leaves, particularly in arid environments where they also serve to reduce water loss.
The final category is the prickle. Prickles are outgrowths of the plant’s epidermis and cortex, the superficial layers of the bark, and they completely lack internal vascular tissue. Because they are only surface-level attachments, prickles are typically easier to snap off or remove cleanly, as is the case with the familiar structures on a rose stem.
Common Trees with Trunk Thorns
One of the most widely recognized examples of a tree with formidable trunk armaments is the Honey Locust, Gleditsia triacanthos, native to central North America. This tree possesses true thorns, which emerge from the trunk and lower branches as reddish-brown, often branched clusters. These woody spikes can reach impressive lengths, sometimes growing up to eight inches long, forming a dense, impenetrable barrier along the lower parts of the tree. These clustered thorns are a defining feature of the wild Honey Locust species, but many commercially planted varieties are thornless cultivars, known as Gleditsia triacanthos var. inermis.
In tropical and subtropical regions, the Silk Floss Tree, Ceiba speciosa, displays an equally striking form of trunk defense. The trunk of this South American native is often swollen or bottle-shaped and covered in large, conical, spike-like structures. These projections are botanically classified as prickles, as they originate from the outer bark tissue and are not vascularized like true thorns. The prickles on the Silk Floss Tree are typically stout and triangular, protecting the trunk, which often remains green on younger specimens due to a high chlorophyll content that allows for photosynthesis.
Another species, the Devil’s Walking Stick, Aralia spinosa, is a smaller North American tree or large shrub known for its stout, sharp spines covering the trunk, branches, and even the leaf stalks. These pointed structures often form noticeable rings around the trunk, giving the plant one of its common names, Hercules’ Club.
The Biological Purpose of Trunk Thorns
The primary evolutionary purpose for a tree to dedicate resources to growing sharp armaments on its trunk is defense against herbivory, particularly from large browsing mammals. Trunk spikes act as a physical deterrent, preventing animals from stripping the nutrient-rich bark or eating the tender, low-hanging shoots. The positioning of these structures, often concentrated on the lower trunk and branches, targets terrestrial herbivores that feed at ground level.
The formidable nature of the thorns on species like the Honey Locust is often cited as an anachronistic defense mechanism, having evolved in response to browsing by now-extinct megafauna. Large Pleistocene mammals, such as giant ground sloths or mastodons, would have been capable of consuming substantial amounts of bark and foliage, and the tree’s massive thorns would have offered a necessary shield against them.
For trees in tropical environments, like the Silk Floss Tree, the prickles serve the dual purpose of deterring terrestrial grazers and preventing climbing animals, such as monkeys and rodents, from reaching the canopy’s valuable foliage and fruit. By protecting the main stem, the tree ensures its survival and reproductive success.