I work in a university biological sciences department that has a large number of international staff and students. Shortly after the arrival of new international members to the department, once they have ventured out into the Australian bush a few times, I often hear the following summation about our landscape from them: ‘The Australian bush is so grey.’ They are, of course, correct. The foliage of many of the dominant trees across the Australian landscape is quite grey compared to the verdant greens of the broad-leaf, deciduous forests, or the deep greens of the boreal pine forests of the new arrivals’ homelands. But rather than making our native vegetation drab and un-interesting, this difference is one of the most obvious gateways into the many fascinating adaptations our flora has developed to cope with the particular challenges life in Australia presents.
Why do the leaves of eucalypts typically have a grey-green colour? The reason is not driven by the plants themselves, but rather the environment they live in. The primary role of a leaf is to capture enough sunlight to allow the plant to photosynthesise. Here in Australia, our bright, sunny skies provide ample sunlight that is higher in ultraviolet radiation than in most parts of the world. This means that most trees are easily able to meet their requirements for intercepting enough energy from the sun, and in fact, they are in danger of being damaged by too much heat and solar radiation. Under these conditions, it becomes advantageous to deflect some of the sunlight that hits the leaf surface. The grey colour of the leaves, formed by a waxy layer called the cuticle, does just that. By preventing the leaf from overheating, the cuticle reduces the amount of water that the leaf loses to the environment. This is important across much of Australia because as well as our bright, blue skies, we are also renowned for our dry climate.
Have you ever sat under a eucalypt in the middle of a baking-hot day, looked at the smattering of dappled shade around you, and wished that the tree above you provided more of it? This too is an adaptation to conserve water and avoid excessive heat loading of the leaves. The leaves of most eucalypts hang quite vertically. This means that under the intense sun in the middle of the day, when the sun sits high in the sky, very little of the leaf surface is pointing directly at the sun. However, in the early morning and late afternoon when the sunlight is less intense, the leaf surface is largely perpendicular to the incoming sun rays, enabling efficient light capture for photosynthesis. As an added bonus, the pendulous leaves allow easy passage of rain through the canopy so that rainwater ends up falling close to the roots where it can be taken up by the tree.
There are many other ways that our plants have come to conserve water. Australia’s floral emblem, the Golden Wattle, has a water-saving trait common to many acacias. The foliage of many species of Acacia is not comprised of leaves at all. These species may have juvenile leaves, but most of the photosynthetic tissue on a mature plant is in the form of phyllodes. Phyllodes are modified leaf stems and these lose less water than true leaves because they have fewer pores, called stomata, through which gases, including water vapour, can pass.
Many other Australian native plants have reduced the size of their leaves, which also serves to reduce the number of stomata that water can be lost through. In the case of many hakeas, small leaf surface area is possible because the leaves have evolved to become cylindrical spikes that also deter herbivores from browsing on them. Allocasuarinas have developed a tactic similar to acacias and do most of their photosynthesis using modified branchlets. Their leaves have been reduced to such an extent that these tiny, teeth-like scale leaves can only be seen by very close inspection – often a magnifying glass is required to see them properly. The photosynthetic branchlets have longitudinal ridges and grooves, and the stomata are situated deep within the grooves. This shelters the stomata from wind, which would otherwise cause water vapour to evaporate away from the stomatal opening more rapidly. Therefore, by reducing their leaves to tiny scale leaves and conducting photosynthesis in specialised branchlets, allocasuarinas are able to minimise the amount of water they lose.
Australia’s soils are nutrient-poor when compared to other parts of the world. This is because our continent has been geologically stable for a very long time, and has experienced prolonged leaching of soil nutrients without them being replaced through tectonic activity. To persist in low-nutrient environments, many Australian plants have evolved to possess thick, leathery leaves that are high in the compounds cellulose and lignin. Leaves with these traits are referred to as sclerophyllous leaves. Sclerophylly enables leaves to have a longer lifespan because they are less palatable to herbivores, and are also more resistant to damage by drought and general wear and tear. Long-lived leaves are able to use nutrients more efficiently because for each unit of nutrients they invest in leaf growth and maintenance, the leaf is able to provide more energy through photosynthesis because its photosynthetically active lifespan is so much longer.
It is easy to see why my new colleagues are quick to label the Australian bush as ‘grey’. However, with the advantages that grey leaf colouration confers under Australian environmental conditions, it is little wonder that so much of our flora has adopted this colour scheme. Australia is a land of extremes, and in order to survive under these harsh conditions, our plants have evolved a multitude of specialist traits. By appreciating the selective pressures that have gone into shaping the vegetation of our surrounding landscape, you will find that our bush is every bit as interesting as the vegetation in other parts of the world. It may just take more than a cursory look to see it for what it truly is.
Banner image courtesy of Rowan Mott.