The term “gum tree” is a common name applied primarily to the genus Eucalyptus, a group of plants famous for their potent, unmistakable aroma. This designation is often ambiguous, as the name is also used for several unrelated trees across the globe. Characterized by their distinct, often pungent, volatile oils, these trees dominate much of the Australian landscape and hold significant ecological and economic importance.
The Eucalyptus Genus
The true gum trees belong to the genus Eucalyptus, part of the myrtle family (Myrtaceae), which includes over 700 recognized species. The great majority of these species are native to the Australian continent, where they are the dominant plant life in most forest and woodland ecosystems. Eucalypts are evergreen, retaining their leaves year-round, and have adapted to a vast range of environments, from arid deserts to high-rainfall coastal regions. This diversity ranges from multi-stemmed shrubs called mallees to towering forest giants like the Mountain Ash.
Key Identification Features
Identifying a Eucalyptus tree involves examining three variable characteristics: its bark, its leaves, and its fruit. The bark is a particularly useful feature, categorized broadly as smooth (or gum), stringybark, box bark, or ironbark. Smooth-barked species often shed their outer layer annually in long strips or large flakes, revealing a smooth, colorful layer underneath.
The adult leaves are typically lance-shaped and hang obliquely or vertically, contributing to the characteristic patchy shade beneath the canopy. These leaves are rich in oil glands, and crushing the foliage releases a strong, volatile scent. The reproductive structures are distinct, forming woody seed capsules commonly known as “gumnuts.” These fruits vary widely in size and shape, often appearing as small, hard cups, barrels, or bells.
The Origin of the Name
The name “gum tree” originates not from the leaves’ volatile oils but from a sticky, resinous substance the trees exude when the bark is damaged. This dark, reddish-brown material is technically called kino and is released as a protective response to injury. The copious flow of this material led early settlers to apply the common name “gum tree” to the entire genus.
This exudate is chemically classified as a resin, not a true gum, because it is largely insoluble in water. The substance is composed primarily of kinotannic acid, making it highly astringent. Historically, Australian kino was used in traditional Aboriginal medicine and for tanning leather.
Other Trees Known as Gum Trees
The common name “gum tree” is not exclusive to Eucalyptus, leading to frequent confusion, particularly in North America. In the United States, two prominent, unrelated tree species carry this designation. The Sweet Gum (Liquidambar styraciflua) is a deciduous tree distinguished by its unique star-shaped leaves and spiky, spherical fruits called “gumballs.”
It gets its name from a fragrant, sweetish sap or resin that flows when the bark is wounded, similar to Eucalyptus kino. The Black Gum, or Nyssa sylvatica (sometimes called Tupelo), is a highly valued ornamental tree known for its brilliant scarlet and orange fall foliage. The Black Gum’s common name likely stems from its dark, blocky bark and its moist habitat, rather than any significant resinous exudate.