While not terribly common, many trees bear sharp thorns along their twigs, branches or trunks. Often, thorns are seen as problematic traits, and trees that bear them are often avoided during installations.
Some have even produced thornless cultivars to address the issue. However, thorns are not always bad, and they can even help you accomplish goals in some cases.
Many people use the terms thorn, spine and prickle interchangeably, but they do refer to different things. Thorns are modified shoots, and characteristic of hawthorns (Crataegus spp.), citrus trees (Citrus spp.) and honey locusts (Gleditsia triacanthos), among others. Spines are similar to thorns, in that they feature internal vascular tissue, but they arise from leaf tissues, not shoots. Spines adorn a number of shrubs, including barberry (Berberis vulgaris).
Prickles are the most divergent of the tree types, as they only attach to the surface of the stem or trunk, and have no vascular tissues connecting them to the main body of the plant. Perhaps not surprisingly, prickles (as are found on roses) are easier to remove from a stem or branch than spines or thorns are.
The primary reason trees developed thorns in the first place was to deter hungry herbivores. These mechanical deterrents are not the only strategy plants use for defense; others have evolved chemical defenses and produce urticating, noxious or toxic compounds. Nevertheless, sharp pokey things have stood the test of evolutionary time.
Thorns are not effective for deterring all primary consumers -- they fail to deter arthropods, for example – but they are effective enough that thorns are worth the resources they require, which could be used elsewhere. Most herbivores prefer the easiest meal they can find, so when faced with the prospect of being stabbed repeatedly by a tree, they look elsewhere for food.
Nevertheless, thick hides and long tongues help many animals avoid thorns, and feed on the delicate foliage. In some places with ample herbivore pressure, thorns become very common. Much of the African Savannah, for example, is blanketed in a carpet of acacia thorns.
Serving Human Needs
With a bit of forethought and careful planning, thorn-laden trees can provide important services for people. While you may think that deliberately planting these spiked species is an exercise in arboricultural sadism, it is possible to harness the repulsive power of these trees and use their powers for good.
For example, many police departments recommend using thorn-bearing trees (or other noxious plants) to dissuade criminals from targeting your home and land. While even the thorniest trees fail to provide an impenetrable shield, they often offer enough resistance that your average criminal will avoid your home and look for easier pickings elsewhere.
Thorny trees also make good wildlife management tools, but they do so in two entirely different ways. For example, a hedge of hawthorns or honey locusts may encourage local deer to keep their distance, or a tangle of blackberry bushes (Rubus spp.) may prevent cats from lurking beneath bird feeders. Birds may preferentially chose such areas for nesting, as they presumably offer some protection from predators. Some birds, notably shrikes, even impale their prey on the thorns and spines of trees. So, you while you can repel some forms of wildlife with thorns, you can also attract desirable critters with thorny trees.
As you can see, tree care often means dealing with thorns. But you don't have to deal with pin-cushion plants yourself. For residents of San Francisco, tree care is only a few clicks away! Contact Arborist Now and let us weather the thorns, spikes and prickles covering your trees and shrubs.