The Quintessential Australian Tree
Many of us regard eucalypts as the quintessential Australian tree. They are adaptable, resilient and persistent. They have proved themselves over millions of years of evolution to be a dominant and successful tree species, but they also have their place. Eucalypts tend to dominate the coastal fringe of Australia, where most of us live. As the climate becomes more arid and the soils more ancient and nutrient poor, eucalypts are replaced by Acacia species until they too can no longer cope with the harsh conditions, and woody trees and shrubs no longer dominate.
With about 800 species, most coming from the genera Eucalyptus and Corymbia, eucalypts are a remarkably diverse and versatile group. There is a eucalypt for almost any urban or gardening situation – tall, short, single or multi-trunked, varied leaf shapes and shades and spectacular or almost unnoticeable flowers. Eucalypts are essential components of Australian ecosystems and support many other plant and animal species that live around them. They are species that will be vital to the sustainability of both natural and urban ecosystems as climate change takes effect across Australia. They are often the remnant links and corridors that connect a disrupted, disjointed and diminished complex of landscapes and ecosystems across the continent.
Understanding Eucalypt Shedding
Given their urban familiarity, it is amazing how so few people understand eucalypt biology and surprising the antagonism that many people show towards eucalypts. One of the most common complaints about urban eucalypts is that they shed leaves, flowers, buds, fruits and small (and occasionally larger) branches. They do, but no-one asks why. The trees often shed more leaves when soils are dry. It is a common and efficient adaptation to aridity, but it also adds to the organic and nutrient content of often poor soils. The value of eucalypt litter should not be under-estimated as it can be crucial to the survival of the soil micro-flora and fauna and great for general soil health. Eucalypts are wonderful nutrient recyclers and this is essential for the survival of other species in their ecosystems.
The tough leathery leaves of eucalypts are described as sclerophyllous (plants or vegetation with hard, leathery leaves) and are a well-known adaptation to dry conditions and nutrient poor soils. So many Australian species exhibit sclerophylly that the Australian flora as a whole is often described as sclerophyllous. The tough leaves are also well adapted to fire. If you can keep soil moisture levels higher, many eucalypts will retain leaves for longer and you may avoid major leaf dumps over short periods of time. The sclerophylly of eucalypt leaves partly explains why they break down so slowly and why they can be slippery on paths and roads. If you grow your eucalypts in larger mulched beds, the shedding becomes part of a natural cycle of events that requires a minimum of human intervention.
Lignotubers and Epicormic Buds
Among the many adaptations that make eucalypts so resilient and persistent are their lignotubers and epicormic buds. Epicormic buds are dormant buds that grow each year just below the bark so you cannot see them and most people don’t even know that they are there. They only develop into shoots after damage to the canopy and are a rapid response system to replace photosynthetic foliage after major stresses such as fire, heavy insect or other grazing, storm damage or major branch shedding. Lignotubers at the base of the trunk are a store of dormant buds that again develop into shoots after an even greater level of stress. They have evolved to replace the trunk and branching canopy of a tree and many people would know of them from the fame in eras past of mallee roots as a source of fire wood. Nearly all eucalypts possess epicormic buds, but not all species or even all populations of a species possess lignotubers. Their presence in a species often depends on the levels of environmental stress a population faces or the different response strategies after fire.
Dormant buds are marvellous assets for those species that possess them. They allow rapid recovery from significant stress through the re-establishment of a functional tree canopy. One of the most spectacular examples is the very rapid production of epicormic shoots after a bushfire. About 6 weeks after a fire these shoots appear in great numbers, giving trees a toothbrush like appearance and providing an early indication of ecosystem recovery. This recovery benefits many other species after significant stress events by quickly providing food resources and habitat. Similar things happen when a tree or branch is heavily grazed or when there has been poor pruning or lopping of eucalypts.
Often Overlooked Benefits of Eucalypts
The possession of epicormic and lignotuberous buds is an adaptation that allows individual tree longevity, rapid stress recovery and resilience and persistence. Dormant buds are also wonderfully useful assets for arborists and horticulturists. With so many buds available it is possible to formatively and structurally prune eucalypts to minimise risk and prolong their useful lives, especially in gardens and urban landscapes. When mistakes are made or trees are damaged, eucalypts can be remarkably forgiving because they have the options of rapidly producing new shoots.
The bark of eucalypts is as diverse as the trees themselves – smooth-barked, gum-barked ironbark, stringybark and candlebark. The characteristics of the bark are often adapted to the environment within which a species grows and are often used to identify species. Some barks protect against grass and bushfires while others can protect against flooding, heat and cold. The bark provides habitat and refuge for a host of fungi, bacteria, insects and spiders, all of which are part of the rich web of life that eucalypts support.
The importance of eucalypt roots systems can easily be over-looked, but they are wonderfully adapted to Australian soils. Depending on the soil type, they can go deep or spread far and wide. Some species produce exudates that free up otherwise unavailable nutrients making them available not only to the trees themselves but other species that are dependent upon them. Without eucalypts many other species would be unable to survive.
The presence of lignotubers at the base of the trunk of so many eucalypts means that the buds within and the roots that supply them with water and nutrients are protected by a superb insulating layer of soil, which explains their near miraculous and speedy recovery from bushfires. Root systems consolidate soils on steep slopes preventing land slips and along waterways they can slow flood waters and reduce erosion of river banks. The roots and the other species of the rhizosphere can also sequester significant amounts of carbon.
Essential Trees for the Future
Not all eucalypts are tall, single-trunked and impressive; some are smaller multi-trunked mallee species. However, all sequester carbon above and below ground and they provide canopy shade and cooling through transpiration if soils are moist enough. Both can be important in reducing ambient temperatures and reducing the urban heat island effect (UBIE). Sometimes people argue that eucalypts only produce a mottled shade, but they forget that eucalypts can often continue transpiring in hot dry environments and so continue cooling when other species have closed stomata and ceased transpiring. Providing irrigation to eucalypts early in the morning of a very hot can not only help retain leaves but can maximise shade and cooling, both of which can be important for human health during heat waves.
While many people complain about the untidiness and shedding of eucalypts in their gardens, street and parks, and compare them unfavourably with some of the great exotic tree species from other parts of the world, I am amazed how well they do without the centuries, and sometimes millennia, of breeding and selection that has gone into the success of these exotic species. Australians have a history of exploiting our natural resources, including tree species, rather than investing in and developing them. Imagine how many great eucalypts we might have for urban use if we properly bred and selected them for specific sites and purposes.
Love them or loathe them, eucalypts will be essential parts of natural and urban ecosystems in many parts of Australia in future. They are capable of coping with climate change and will be a key to creating liveable and sustainable towns and cities across much of the continent. They are great assets in both natural and created ecosystems. In an urban context we should develop an understanding their superb biology and take the necessary steps to manage them properly so that they realise the full potential of their life spans and the many benefits that they provide.
Written by G M Moore. School of Agriculture, Food and Ecosystem Sciences, University of Melbourne, Burnley Campus, 500 Yarra Boulevard, Richmond, 3121, Australia.