Eucalyptus trees are a defining feature of many landscapes, yet their remarkable diversity, encompassing over 700 species, can make identification challenging. These iconic trees, often referred to as “gum trees,” present various forms from towering specimens to multi-stemmed mallees. Understanding the distinct characteristics of their leaves, bark, and fruit provides practical methods for accurate identification. This article will explore these features to help distinguish different eucalyptus species.
Eucalyptus Leaves as Identifiers
Eucalyptus leaves offer several distinct features for identification, though they can change significantly from juvenile to adult stages. Adult leaves are commonly lance-shaped or sickle-shaped, often appearing waxy or glossy green. They often hang downwards. Many species display an alternate leaf arrangement on mature trees.
A notable characteristic is the presence of oil glands within the leaves, which become visible as translucent dots when held against a light source. Crushing a leaf releases a distinctive aromatic scent, a result of these oils. While adult leaves are lanceolate, juvenile leaves are often rounder, broader, and sometimes glaucous (blue-grey). Some eucalyptus species even retain their juvenile leaf form into maturity, which can be a distinguishing trait.
Unveiling Clues from Eucalyptus Bark
Eucalyptus bark provides varied clues for identification. Bark types are broadly categorized by their texture and shedding patterns. Smooth-barked eucalyptus species shed their outer layer annually, revealing a fresh, often brightly colored, smooth surface underneath. This shedding can occur in large slabs, small flakes, or long ribbons that may remain partially attached, creating a “ribbon gum” appearance.
Rough-barked species retain their dead outer bark, which dries and accumulates. Within this category, “stringybarks” have long, fibrous bark that can be pulled off in long pieces, often spongy. “Ironbarks” are characterized by hard, deeply furrowed bark, often dark red to black due to impregnated kino (sap). “Box” bark is short-fibered and may be furrowed or broken into plates, sometimes with a tessellated appearance. Some species are “half-barks” or “blackbutts,” retaining rough bark only on the lower trunk while the upper sections are smooth.
Distinctive Eucalyptus Flowers and Fruit
Eucalyptus reproductive structures, particularly their flower buds and fruit, are distinctive identifiers. Eucalyptus flowers lack traditional petals; instead, their stamens are enclosed within a cap-like structure called an operculum. This operculum, derived from fused sepals and/or petals, detaches as the flower opens, revealing numerous fluffy stamens. The presence of this operculum is a defining feature of the genus Eucalyptus and its close relative, Corymbia.
After flowering, the ovary develops into a woody fruit, commonly known as a “gumnut” or capsule. These capsules vary in shape and size (cup-shaped, urn-shaped, or globular), with their dimensions, internal valves, and surface textures aiding species differentiation. Their woody nature protects seeds, and their morphology indicates a eucalyptus species.
Differentiating Eucalyptus from Other Trees
Distinguishing eucalyptus from similar Myrtaceae family trees like Angophora and Corymbia requires attention to differentiating features. Angophoras, sometimes called “apple trees,” share visual similarities but lack the operculum on their flower buds; their flowers have small, visible petals. Angophora adult leaves are arranged oppositely, while Eucalyptus and Corymbia species have alternate adult leaves.
Corymbias, often referred to as “bloodwoods,” were formerly classified under Eucalyptus but are now recognized as a distinct genus. While they possess an operculum like eucalyptus, their fruit often has an urn-shaped appearance, and their bark can be scaly or flaky, differing from eucalyptus bark types. Corymbia flower clusters (inflorescences) are arranged in “corymbs,” where flower stalks arise from different levels but terminate in a similar plane, contrasting with the simpler umbels found in Eucalyptus.