CASUARINAS

There are three main types of Casuarinaceae growing in western Victoria. Ranging from the southwest coast (Southern Ocean) northwards to the Milewa Mallee, each species tends to occupy its own geographic zone.

Nearest to the sea is the Drooping Sheoak, Allocasuarina verticillata, (formerly Casuarina stricta), also called the Coast Sheoak.

In the middle zone of the Wimmera is the Buloke, Allocasuarina luehmannii, sometimes spelled Bull-Oak.

Farthest inland is the Belah, Casuarina pauper, (so-named because it is a smaller version of Casuarina cristata that occurs in New South Wales and Queensland).

Drooping Sheoak, Buloke, and Belah also abut and cross over the border into South Australia.

Casuarinas resemble pine trees with their needle-like foliage and woody seed-cases. The foliage is actually made up of segmented branchlets, with tiny blades (residual leaf tips) radiating from each joint of the thin stem. Their distinctive appearance is described in the word 'casuarina', which comes from a Malay word 'kasuari' meaning Cassowary. This term likens the tree's mass of drooping leaf-forms to the feathers of the flightless bird.

Drooping Sheoak prefers well-drained soils in open sun. It is widespread on coastal cliffs and secondary dunes subject to salt spray but not usually on saline soils. It is commonly found an the lava plains and volcanic hills of the Western District, and on drier, rocky situations in the Central Highlands. Bark on the trunk is fissured and grey and mature trees grow to 10 metres high. The crown of the tree is grey-green and rounded in shape with drooping foliage. In the right light the foliage reflects in iridescent blues and greens.

 

Buloke tolerates poorly drained, heavy soils that alternate between the drought conditions of hot dry summers and the waterlogging of cool wet winters. A local farmers' term identifies this habitat as 'buloke and crab-hole country', an ironic reference to its periodic saturation. Mature trees have a straight trunk with dark furrowed bark and grow up to15 metres tall. The  segmented branchlets are typically upright, unlike many other casuarinas, and are 'dull' brown in colour. In mixed woodland settings, bulokes seem to 'absorb' light, creating dark shadows in between eucalypts.

 

Belah usually grows on red-brown soils with light-textured topsoil. Rainfall range in the Sunset country is only 200-300 mm. Mature trees have a straight trunk, vary from 5 to 15 metres tall, and form a dense foliage at the crown. Out in the open they can be an impressive sight, compared to the low mallee and shrubs that surround them.

Competing For Place.

The fluctuation of species distribution and relative dominance in Australian climatic conditions is illustrated by the pollen record for the past 100,000 years or so from Lake Wangoom, near Warrnambool. 

While Australian plants like the Casuarinas were already well-suited to drier conditions, the pattern (or proportions) of forests, grasslands and deserts was constantly changing. This was in large part because of the waxing and waning of the polar ice-caps during the Late Pleistocene. Climate in Australia cycled between icehouse phases (cold, dry conditions) and greenhouse phases (warmer, wetter conditions).

Lake Wangoom lies near the southern margin of western Victoria's basalt grasslands and is 10 kms north of the sea. It is a volcanic maar, formed by explosions when hot magma rose up and collided with subterranean water tables. The shallow crater is 1.2 kms in diameter and is surrounded by a tuff-ring of ejected limestone. In the 1980s two drill cores were extracted from accumulated sediment in the centre of the basin. Analysis of slice-samples in the 1990s revealed a botanic history of the area.

The data is presented here in the form of percentages of the total dryland pollen count contributed by each of the plant families. From left to right:

     Rainforest trees and tree-ferns, mainly Nothofagus, Cyathea, Dicksonia.

     Eucalyptus, the sum of gums, box, and mallee.

     Casuarinaceae, the sum of Casuarina and Allocasuarina.

     Asteraceae, herbaceous daisy-type wildflowers and low bushes.

     Poaceae, native grasses.

This arrangement is really a sequence of vegetation regimes or habitats; from closed (canopied) temperate rainforest to less dense mixed woodlands to open grassland.

The standout zone is LW4, which shows the highest incidence of rainforest (otherwise minimal). More importantly, there is clear eucalyptus dominance and low paoceae values, indicating a time of major forest expansion under significant levels of rainfall. (This possibly reflects the peak of the most recent Interglacial about 50-60,000 years ago, roughly at the midpoint between the last two Glacials). Similar spikes in precipitation occur in LW6 and the beginning of LW8.

In contrast, zones LW1, LW3, LW5, and LW7 show Casuarinaceae percentages equaling or exceeding Eucalyptus results, coupled with consistently strong responses for wildflowers and grasses. In these landscapes, trees are less concentrated in an environment experiencing a relative lack of moisture.

Zone LW1 is particularly interesting. It covers the Holocene Epoch, the past 10-12,000 years before present (YBP). The pollen record since the last Glacial projects high grass values while the proportions of Eucalypt and Casuarina tend to alternate ― when one is comparatively high, the other is comparatively low, and vice-versa.

The Lake Wangoom investigation supports the idea that Casuarinas replace Eucalypts in low rainfall conditions. In these circumstances, adaptation to drought resistance is critical for species' success. The distinctive foliage of the Sheoak, Buloke, and Belah is central to their competitive advantage over the better-known Gums, Boxes and Stringybarks.

Adaptation To Aridity

Current understanding is that the plant family of Casuarinaceae contains four genera, not just the single genus Casuarina.

Gymnostoma:  containing 18 living species; one in NE Queensland (Gymnostoma australianum), the rest in Melasia, Melanesia, Polynesia.

Ceuthostoma:  with 2 living species in Melasia (Borneo to New Guinea)

Casuarina:  containing 17 surviving species, most in the region from Bay of Bengal to Polynesia, but 6 in Australia.

Allocasuarina:  with 58 species, all in Australia.

These genera grow in the tropics, but Casuarina and Allocasuarina extend into the temperate areas of southwestern and southwestern Australia. The story of Casuarinaceae's evolutionary development is consequently divided into two parts.

In brief, there has been a scleromorphic response to poor soils, predating a xeromorphic response to low moisture, resulting in the segmented foliage of Australia's Casuarina and Allocasuarina. This  is explored in the diagram below, by drawings of Casuarina leaf-stems in cross section. 

 

The structure and mechanics of these "distinctive slender wiry branchlets" are evidence that "low soil nutrients, especially phosphorus, and high water availability in a relatively light limited environment, were the original drivers for evolution in the Casuarinaceae. Reducing water availability in a progressively higher light environment were the major drivers of the post-Eocene evolution in this unique plant family."

Gymnostroma today occupies a ecological niche. It is "best described as a habitat specialist, occurring in marginal low nutrient sites within various types of rainforest". Seedlings establish in relatively high light areas not favoured by other tropical plants dependent on richer soils. The elongated stems of this ancestral genus of the family maximised the available area for photosynthesis, positioning leaf stomata over the entire surface to allow rapid exchange of oxygen and carbon dioxide. (Stomata are the small openings that connect and regulate the exchange of elements between the 'outside' atmosphere and the 'inside' cells of the plant.) This was a more energy-efficient form of leaf design for a plant occupying the 'edges' of a hot, humid environment.

The next selection pressure was the drying of the climate from the Oligocene onwards. The Casuarinaceae response to aridity was to move its exposed stomata into furrows stretching along each stem. In practice, this "migration" has the effect of minimising water loss through the plant's stomata. 

The first part of this "encryption" process was achieved by Ceuthostoma, illustrated by model (f) in the diagram above. Increasing the number of fused leaves and deepening the grooves between them turbo-charged the adaptation process. The 'amplification' of drought resistance is visible in Casuarina and Allocasuarina, represented by (g) and (h) above.

Evolution of furrows with a tighter fold protected stomata while maintaining nutrient efficiency, and provided later derivatives of Casuarinaceae with the ability to occupy a 'new' ecological niche ― dry, open grass and woodland on poor Australian soils.