The gum tree, a symbol of the Australian landscape, represents one of the most species-rich genera of flowering plants in the world. These trees, shrubs, and mallee forms dominate the vast majority of Australian forests, demonstrating an incredible evolutionary success story. They are often simply referred to as eucalypts, a term encompassing a group of related genera. This complexity raises a question for botanists: exactly how many species belong to this iconic genus?
The Current Count and Taxonomic Debate
The number of recognized Eucalyptus species is not a fixed figure, but rather a dynamic range that reflects ongoing scientific investigation and debate. Currently, the genus Eucalyptus contains approximately 700 to over 750 described species. If one uses the broader, colloquial term “eucalypts,” which includes closely related genera, the total number of species rises to more than 850 or nearly 900. This variability stems from factors including the discovery of new species and the reclassification of existing ones.
Molecular phylogeny, the study of evolutionary relationships using DNA, continues to reshape the classification of this group. A significant challenge is the frequent natural hybridization that occurs between species in the wild, which produces intermediate forms difficult to categorize. Furthermore, some species appear to grade into one another across geographical areas, making a clear boundary for a distinct species challenging for taxonomists.
Defining the Genus Eucalyptus
The genus name Eucalyptus is derived from the Greek words eu (well) and kalyptos (covered), a reference to the genus’s defining characteristic. This feature is the operculum, a small cap or lid that covers the flower bud, protecting the developing stamens underneath. As the flower opens, the operculum is shed, revealing the numerous, often showy, stamens.
The modern taxonomic view has led to the reclassification of many species formerly placed in Eucalyptus into two other genera. The Bloodwoods, now classified under Corymbia, were split based on differences in their fruit, bark, and inflorescence structure. The Apple Gums, forming the genus Angophora, are easily distinguished because their flower buds lack the characteristic operculum entirely and instead have small, visible petals.
Key Adaptations and Diversity
The high number of species reflects the ecological and morphological specialization achieved by the genus across the Australian continent. Species have adapted to environments ranging from arid inland deserts to high-rainfall mountain slopes. This specialization is often most visible in the bark, which can be smooth and shed annually in strips (ribbon gums), or permanently rough and fibrous (stringybarks). Ironbarks develop a thick, dark, and hard bark infused with kino, while scribbly gums have distinct insect larvae tracks etched into their smooth surface.
Growth habit is another major differentiator. Species exist as towering forest giants, such as the Mountain Ash (Eucalyptus regnans), which can exceed 100 meters in height. Others are multi-stemmed shrubs known as mallees. Mallees possess a large, woody underground swelling called a lignotuber, which stores energy and allows the plant to rapidly resprout after a fire or drought.
Furthermore, many species exhibit leaf dimorphism, where the juvenile leaves are rounded and opposite. This contrasts sharply with the narrow, sickle-shaped, alternate leaves of the mature adult form.