Photo: Australian National Botanic Gardens


Eucalyptus trees are known to be some of the hardiest, most adaptive plant species on Earth, allowing them to thrive under a variety of terrain, soil, moisture, and ecological conditions.  Millions of years ago, rainforests covered a much greater portion of the Australian continent than at the present, and the ancestral precursors to eucalypts likely inhabited the outer fringes of these rainforests because of their adaptability to drier, less enriched soils.  When the drying effects of Map of Australia's regional fires by season.climate change eventually caused Australia’s moist, green canopies to retreat, the early Eucalyptus species were well-prepared to adapt and become the dominant forest vegetation.  Today, the rainforests of the island continent are like islands themselves, relatively small pockets of dense, green canopy surrounded by more open eucalypt woodlands.

The most vital survival tool of most modern Eucalyptus species is their cooperation with the cyclical fire patterns that punctuate life in the Australian ecosysytems.  As the climate became more arid over the millennia, the warm air coupled with abundant dry plant matter created ideal conditions for frequent thunderstorms and subsequent forest fires.  Species in the Eucalyptus genus possess a number of adaptations which allow them to survive these blazes, including thick, tough bark to protect the vulnerable heartwood layers beneath.  Concealed under this natural armor, many Eucalyptus species have a reserve of leaf buds; astoundingly, the extreme heat of the fire activates chemical reactions in these buds, causing a new set of leaves to grow after the fire.  In fact, the leaves themselves are characteristically rich in oil (read more about eucalyptus oil on the Interactions page),Eucalypts' ecological success is due in large part to their cooperative adaptations to fire. which causes them to burn intensely but very rapidly; not only does this protect the inner heartwood tissue from prolonged heat exposure, it also increases the fire’s ability to spread to other trees.  This is especially important for reproduction in some species of Eucalyptus, such as E. regnans, whose seeds will only germinate after a fire has swept the ground.  

Additional adaptations found in this genus include a rapid growth rate, as is seen in E. gunii, an ornamentally cultivated tree common in Great Britain that can grow up to 1.5 meters in a single year.  Eucalyptus trees are evergreen, and their leaves typically hang vertically to avoid the desiccating effects of the Australian sun.  The leaves are also covered with a waxy surface to help keep some types of A Christmas beetle approaches the waxy leaves of a flowering peppermint eucalypt.herbivorous beetles from getting a foothold, and the oils in the leaves are toxic to many predators (with a well-known exception being Phascolarctos cinereus, the koala).

This arsenal of survival tools led to the adaptive radiation of the ancestral eucalypts into the more than seven hundred Eucalyptus species known today.  In their native Australia, eucalypts occupy diverse habitats, such as the snow gum trees that grow in the alpine regions, or the brightly flowering species that spring up in forests near streams.  Their readiness to adapt destined the Eucalyptus trees for travel, and over two hundred species have been artificially introduced to other parts of the world, including California in the nineteenth century, as well as East Africa, Sri Lanka, and Portugal.                                                        

Photo: HM Rawson

Read on to find some surprising Facts about the eucalyptus trees!

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