Gleditsia triacanthos L. (Fabaceae)


Honey locust

Hand-coloured plate of Gleditsia triacanthos from Michaux's 'The North American Sylva' (1818, vol. 2, t.79). Trunk spines of Gleditsia triacanthos.

The long, branched and clustered thorns covering the trunk and branches of the honey locust make it a formidable-looking tree. These thorns give the species its scientific name (triacanthos, literally 'three-spined'). Honey locust is a deciduous, leguminous tree native to forests in moist river valleys of central North America. It grows rapidly and thrives in poor conditions, and as landscapes have been transformed in parts of North America it has become a serious invasive species. Honey locust is a useful garden plant and was introduced to Britain at the turn of the eighteenth century.

Individual honey locust trees have a mixture of leaf types; some are pinnate, whilst others are bipinnate. The tiny, strongly scented flowers are mixed on individual trees; some are male, whilst others are female or hermaphrodite. The tree's common name does not refer to its potential value as a nectar source for bees but to the sweet, edible pulp inside the mature, many-seeded, indehiscent pods; the pulp has long been used by the Cherokee to make beverages.

The heavily-armed honey locust trunks, with their woody thorns up to 40 cm long, have led to speculation why there should be so much investment in protection, when the herbivores that might find the tree appetizing today are not deterred by them. One suggestion is that honey locust evolved in an environment where browsing herbivores larger than deer were common. Large mammals (megafauna) roamed the Americas for twenty million years but approximately twelve millennia ago, at the close of the Pleistocene, they went extinct. The well-armed trunks and branches may have evolved as protection against browsing megafauna and the thorns we see today are merely remnants of a once important evolutionary strategy. Thornless individuals are sometimes found in natural populations, some of which have been introduced into cultivation; saving safety-conscious gardeners from having to remove the trunk thorns.

Dispersal biology has implications for a species' natural distribution. Extinction of the American megafauna is thought to have robbed some large-seeded American trees, including honey locust, of their main dispersal agents. Today, honey locust seed dispersal appears to have been taken over by introduced mammals, such as cattle, horses and humans. Given the importance of honey locust in the diets and pharmacopeias of some Native American peoples it is natural to ask whether these people cultivated honey locust and hence how the current distribution of the species reflects past anthropogenic activities.

Further reading

Barlow C 2001. The ghosts of evolution: nonsensical fruit, missing partners, and other ecological anachronisms. Basic Books.

Catling PM 2001. Extinction and the importance of history and dependence in conservation. Biodiversity 2, 2-14.

Janzen DH, Martin PS 1982. Neotropical anachronisms: the fruits the gomphotheres ate. Science 215, 19-27.

Stephen Harris

BBC Radio Oxford clip about this week's plant