EUCALYPTS

 

Australian eucalypts have traditionally been named according to their bark. Smooth-barked species are generally called Gum or Mallee. Rough-barked species are known as Box or Stringybark.

There are four smooth-barked eucalypts, three Gum and one Mallee, that typify different parts of the borderlands of Western Victoria. They have become the predominant canopy-trees in areas with particular geographic characteristics.

From north to south, these are:

(i) Dumosa Mallee, Eucalyptus dumosa, sometimes called White Mallee, is the most widespread of the Mallees. It grows in a great arc across southeastern Australia, from south of Dubbo and Louth in NSW, through northwestern Victoria and the Murray Mallee of SA, to Eyre Peninsula and the Flinders Ranges. Early explorers were not impressed.

John Oxley's Journals of Two Expeditions into the Interior of New South Wales (1820), mentions "those dreadful scrubs of eucalyptus dumosa". Allan Cunningham, his accompanying botanist, is probably responsible for this classification. The Latin word dumosas means 'shrubby', describing the growth habit of all mallees. As the frustrated Oxley put it, "thick brushes of eucalyptus that spread in every direction around us, eucalyptus dumosa, the dwarf gum, as they never exceed 20 feet in height...spreading out in a bushy circle from their roots in such a manner that it is impossible to see farther than from one bush to another".

(ii) Yellow Gum, Eucalyptus leucoxylon, is found throughout West Wimmera (and across the border into the Upper South East of South Australia, where it is called Blue Gum!). It has smooth yellowish bark with some rougher cover at its base. Its leaves have a glaucous hue of blue-grey or blue-green.

There are three main sub-species; leucoxylon leucoxylon, leucoxylon pruinosa, and leucoxylon stephaniae. These range according to relative dryness of habitat, from 'well-watered' around Dergholm - Langkoop to the semi-arid sandy soils of Little and Big Deserts. As a tall tree it can resemble a Red Gum, with its woolly butt and grey and cream patches on trunk and branches. However, Yellow Gum occupies lighter soils that often lack essential minerals.

(iii) Red Gum, Eucalyptus camaldulensis, sometimes known as River Red Gum, is "a familiar and iconic tree", " a tree that is endemic to [and emblematic of] Australia". It is the most widespread of all the eucalypts, found over most of the continent alongside water courses and permanent water. The common name refers to the colour of its wood.

There are seven sub-species but only one of them, camaldulensis camaldulensis, is found in South West Victoria and South East South Australia. This variety is mostly associated with riparian environments, rivers, creeks, lakes and swamps. In the higher rainfall zones they also spread out over plains and plateaus with heavier soils and shallow aquifers.

(iv) Swamp Gum, Eucalyptus ovata, favours poorly drained country, tolerating flooding in winter and short summer droughts. The botanical name ovata is Latin, referring to its oval, 'egg-shaped' leaves. Long strips of bark are shed in summer and autumn from the upper trunk and branches, leaving a smooth skin.

There are two subspecies in Victoria, ovata ovata being the most usual, and ovata grandiflora occupying the farthest southwest corner of the state.

The distribution of these eucalypts in western Victoria is a story about water. The availability of water varies according to its source, essentially rainwater, surface water, or water-table. Red Gum is an apparent master of this diversity. 

While it is best known for riverine and lacustrine environments (including intermittent creeks and seasonal swamps), it is not restricted to these scenarios. As mentioned, large open forests of Red Gum exist in the Dundas region of South West Victoria, and a broad belt of the same species follows the border south from Kybybolite - Naracoorte in South Australia.

Research from the Chowilla floodplains on the Murray River has revealed a plant-hydrology that is deliberately mixed. A reasonable expectation might be that the extensive Red Gum forests along the Murray are watered by the river itself, supplemented by seasonal flooding. This, at least, is what can be observed above ground.

Results from three sites were taken, called the stream site, the channel site, and the inland site. "The trees at the Stream site had continuous access to stream water and it generally made up to 50% of the water taken up by them. The trees on the ephemeral stream had access to surface water 50% of the time [but] stream water accounted for no more than 30% of the water used by the trees when the stream carried water. At the Inland site, trees did not appear to take up any stream water when flooded for two months".

These findings indicated a deliberate reliance by Red Gums on subterranean water. There are clear evolutionary advantages to this strategy. "Firstly, groundwater is a highly reliable source of water...Adaptation for its use would aid the trees' survival during droughts and give them a competitive advantage over shallow rooted species in dry seasons...Secondly, the level of stress imposed on the trees by relying on soil water and ground water may, in the longer term, be less than that imposed by committing resources to utilise the available surface water...even when groundwaters are highly saline".

To the south, higher rainfall and shallower water tables benefit the Red Gum, and Swamp Gum. In the north, though, rainfall decreases, water-table depth increases, and both by a considerable degree.

  

In this hydrologically poorer situation, species like Dumosa Mallee, and Yellow Gum, face a survival challenge. Part of of their response to semi-aridity was to develop lignotubers, a structure at or near ground level, between their trunks and their roots (for example, 'mallee stumps'). Lignotubers function as "resource sinks", storing water and carbohydrates, as well as "maintaining a pool of bud-forming sites" to rapidly recover shoot growth after fire or drought. 

At a microscopic level, that is at the level of each individual cell that makes up a living plant, scientists have sought to examine the 'soup' that is within the cellular walls. They want to know the response that each 'building-block' of the plant is making to cope with water stress.

We can start with a statement: "One of the principal mechanisms by which plants cope with water deficit is osmotic adjustment, to maintain positive cell turgor via the active accumulation of solutes." This sentence becomes comprehensible when we define some of its terms. Osmosis, as in "osmatic adjustment", concerns the diffusion of liquids through something porous, like the cell's walls. Turgor is the state of being full, which a cell does by balancing osmotic pressure and the permeability of its walls. Solutes are substances that are dissolved in the cell's internal fluid, such as carbohydrates.

In addition to carbohydrates involved in primary metabolism, the sugars like sucrose, glucose and fructose, there are "several compatible solutes" like quercitol and mannitol that are collectively called cyclitols. These and other dissolved chemical compounds are what constitute the 'soup', and their relative concentrations are a guide to what is happening within the plant.

An extensive experiment was conducted to determine the constituents of cell fluid from the leaves and roots of 13 different eucalypts. These were varieties from a range of natural environments. Mesic species that received annual rainfall above 500 mm included the Red Gum and the Swamp Gum. Xeric species that received rainfall of less than 500 mm were represented by the mallees E. oleosa (Oil Mallee) and E. viridus (Green Mallee) that grow in northern Victoria.

The eucalypts were grown from seed and their saplings then subjected to serious drought conditions. Cells in mesic species reacted by reducing water content and accumulating concentrations of sucrose. Sugar is a quickly converted source of high energy. This is a good tactic for higher rainfall species used to regular summer lack and winter plenty, because they were then positioned to make a prompt recovery. Cells in xeric species also responded to induced dryness by  reducing water content, but they accumulated concentrations of cyclitols instead, particularly quercitol.

Increased concentrations of "highly stable, metabolically inactive, cyclitols" have the effect, in simplified terms, of 'slowing' osmosis. The mallees are conditioned to longer summer lack and irregular seasonal recovery. Drought years, not months, are part of their normal climatic cycle. So it makes sense for them to prepare for prolonged periods of water deficit. They do this by diverting carbon away from growth processes, but not enough to threaten cell turgor.

In conclusion, the existence of quercitol (in small amounts) in all of the examined trees indicates that this compound is widely present in the eucalyptus family, but it is expressed differently in species from habitats prone to drought conditions. 

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REFERENCES:

PJ Thornton & GR Walker, 1994, 'Variations in stream water uptake by Eucalyptus camaldulensis with differing access to stream water',Oecologia, 100, 293-301

A Merchant, M Tausz, SK Arndt, & MA Adams, 2006, 'Cyclitols and carbohydrates in leaves and roots of 13 Eucalyptus species suggest contrasting physiological responses to water deficit', Plant, Cell and Environment, 29, 2017-2029