Wannon

Location

This region includes that area south of the Little Desert to the Victorian coast, and west of the Grampians, extending into and including the South-East province of Laut et al. (1977a, 1977b) in South Australia. The boundary between this region and the Wimmera has been arbitrarily defined as the edge of the nearest north–south running aeolian sand-ridge to the west of Natimuk (crossing the Wimmera Highway approximately 44 km WSW of Horsham). This ridge is part of a series of parallel sand-ridges that alternate with mixtures of grey clays, loam soils and swamps. The south-eastern boundary between this region and the Victorian Volcanic Plain Region is largely defined as the most westerly limit of the Upper Cainozoic (Quaternary) basalts of the Newer Volcanics. However, communities typical of the Wannon Region occur in several places where sands have been deposited on these basalts (e.g. near the Nigretta and Wannon Falls). Extensive clearing in this area has often made it impossible to use vegetation changes to identify the junction between this region and the Volcanic Plain. Allestree (Portland Bay) has been arbitrarily chosen as the south-eastern coastal limit of this region.

Major grids C—E. Approximate area 13 632 km².

Major landforms

The major landform characteristic of this region is that of the Miocene and Pliocene marine sediments of the Otway Basin. This coastal plain exhibits a regular series of calcareous sand-ridges that cover a large area to the west of the lower Glenelg River. Along most of the coastline (extending into South Australia) there is a narrow zone of active dunes rising steeply from the sandy beach. Behind this zone are extensive lagoons with occasional outlets to the sea. During the Pleistocene and Recent periods the sand was extensively redistributed by wind, forming sheets and low dunes. Large areas (e.g. near Casterton and Coleraine) were deeply weathered to form laterites that are characterized by high amounts of aluminium and iron oxides as gravel and ‘ironstone’ in the upper soil layer (Kenley in Anon. 1972a; Gibbons & Rowan, this volume). Lavas extruded during the Newer Volcanic activity filled and blocked parts of the pre-Upper Pliocene drainage system (e.g. Wannon River) (Kenley, in Douglas, M. H. & O’Brien 1974). The two main riverine systems, the Glenelg and Wannon Rivers, arise outside the region. Only small freshwater lakes occur in the region. Large quantities of groundwater occur, and these aquifers are used for urban and industrial use. Portland and Heywood (and Naracoorte, in South Australia) obtain water from the underlying sands of the Dartmoor formation, whereas Mt Gambier and Penola (South Australia) obtain their water supplies from the Gambier limestone formation (Kenley in Anon. 1972a) that extends into the region. Salt lakes and swamps are mostly confined to the near-coastal zone.

The topography ranges from loosely compacted formations, particularly in near-coastal and coastal sands (e.g. Kentbruck Heath and Discovery Bay), to grey or brown deep loams and clays (e.g. in the Casterton–Coleraine area). Much of the remaining area is characterized by grey and brown sands and loamy soils with a distant clay subsoil. Duplex soils are common in the region. Ironstone gravel is frequently present at the base of the top soil. Yellow-brown calcareous sodic duplex soils are common to the west of the Black Ranges, and currently support sheep and cattle grazing. Reddish-brown to red gradational soils (terra rossa soils) occur exclusively on limestone as parent material. These show only limited horizon development (see also Gibbons and Rowan, this volume). All formations are below about 270 metres altitude.

Climate

The region is characterized by a cool moist temperate climate, with the rainfall decreasing progressively as the distance from the coast increases. The mean annual rainfall is between about 400 mm in the north and 850 mm in the south (Anon. 1972a, 1979). The summer months are dry (driest months are January and February) and warm, with most of the rain falling during the winter months (wettest months are June–August), especially evident in the coastal zone. The mean daily maximum temperatures range from 19°C to 21°C (on the coast in summer) to about 30°C (inland), with mean daily minima ranging from 7°C to 9°C (on the coast in winter) to 4°C to 5°C (inland) (Anon. 1972a, 1979). Westerly to south-westerly winds are generally cooler (more frequent in winter and often rain-bearing) than north-easterly to north-westerly winds that are warm and often dry (more frequent in summer). Refer also ‘Climate of Victoria’, this volume.

Vegetation

Coastal communities

The coastal communities form a narrow continuous zone that is composed of beach and dune vegetation, coastal cliffs and saltmarshes. Relatively few species occur on the deep infertile sands of the beaches and dunes (e.g. at Discovery Bay). These sands are often unstable and are directly exposed to the salt spray. Atriplex cinerea, Cakile edentula, C. maritima* and Spinifex sericeus commonly colonize bare sand. Other common species that occur on the seaward face of dunes include Ammophila arenaria (extensively introduced to control sand erosion), Actites megalocarpa, Carpobrolus rossii, Lepidosperma gladiatum, Leucophyta brownii, Senecio lautus and Stipa slipoides.

The crest of the dunes and the landward slopes usually support a dense shrubbery of Acacia sophorae, Correa alba, Leptospermum laevigatum, Leucopogon parviflorus, Melaleuca lanceolata and Rhagodia candolleana. This community is often locally dominated by Allocasuarina verticillata. Other common species include Acaena novaezelandiae and Exocarpos syrticola.

The coastal cliff communities occur in a very exposed environment, in shallow skeletal soils, with the plants frequently growing in rock crevices. Many of the species that occur on the beach and seaward slopes of the dunes also occur on the cliffs. Alyxia buxifolia, Muehlenbeckia adpressa and Tetragonia implexicoma are also common components of this habitat. The very localized species, Eucalyptus diversifolia and Lasiopetalum schulzenii, are virtually confined in Victoria to Cape Nelson (Plate 6G).

The coastal saltmarshes occur on flat, wet, highly saline areas. They are usually dominated by the samphires, Sclerostegia arbuscula or Halosarcia halocnemoides and Sarcocornia quinqueflora. Frankenia pauciflora and species of Atriplex, Hemichroa and Suaeda are common. Disphyma crassifolium subsp. clavellatum frequently occurs on the margin of the saltmarsh.

The vegetation of the freshwater swamps is made up of a complex mixture of sedges and rushes.

Heaths and tree–heaths

Wet heathlands occur on very infertile soils (gleyed podzols) that are frequently waterlogged (e.g. towards the Lower Glenelg River and in the Kentbruck Heath, approximately halfway between Heywood and the South Australian–Victorian border). These heath communities, up to 2 metres high, are dominated by species of Acacia, Allocasuarina, Epacris, Gymnoschoenus, Hakea, Lepidosperma, Leptocarpus, Leptospermum, Leucopogon, Melaleuca, Patersonia, Persoonia, Utricularia and Xanthorrhoea (Plate 6H). Tree–heaths differ only in the presence of scattered low trees (Willis in Douglas & O’Brien 1974), usually of Eucalyptus baxteri (Lower Glenelg and Mt Richmond areas) or an association of E. leucoxylon and E. melliodora. E. willisii is common as a mallee-type shrub on the Kentbruck Heath. The canopy of the trees ranges from 6 to 10 metres in height.

Fire is a common factor within the communities, with many plants exhibiting fire-adapted reproductive strategies (see Gill, this volume).

Woodlands

These formations are characterized by trees, 6– 30 metres high, that have broad, open crowns and short boles. The trees are often widely separated so that the formation is usually open to very open. Woodlands occur throughout much of the region, particularly in the northern parts (e.g. near Edenhope and Hamilton). This formation occurs on moderately fertile duplex soils. Large areas of woodland have been cleared for agriculture.

Eucalyptus camaldulensis (River Red-gum)

Eucalyptus camaldulensis occurs commonly in the northern half of the region. Apart from its association with watercourses (e.g. Wannon River, Cavendish), this subcommunity characteristically occupies the wetter duplex soils that are often flooded or waterlogged during winter. It usually does not occur on sandy, shallow or stony soils. Eucalyptus camaldulensis frequently forms pure stands or occurs in association with E. ovata in wetter sites. It occurs with E. leucoxylon or E. viminalis in the better-drained sites. The understorey is dominated by low grasses (often Danthonia and Stipa species), sedges and herbs. Occasional shrubs are also present. Eucalyptus baxteri and E. viminalis tend to replace E. camaldulensis on sandy or more lateritic sites.

Eucalyptus leucoxylon (Yellow Gum)

Eucalyptus leucoxylon is most common north of the Casterton–Penola road (from near Dergholm), and extends into the Wimmera Region. It also occurs on duplex soils. The associated species include Acacia melanoxylon, A. acinacea and Hakea rostrata. Eucalyptus fasciculosa, E. ovata and E. viminalis occur with it in areas of more restricted drainage (Plate 61).

Eucalyptus pauciflora (Snow-gum)– E. ovata (Swamp Gum) and E. viminalis (Manna Gum)

This alliance was widespread in the southern part of the Dundas Tableland, south of the Eucalyptus camaldulensis boundary (Gibbons & Downes 1964). The dominant Eucalyptus is determined by microtopographical differences. The grassy ground-layer is dominated by both Themeda triandra and Poa species. It survives only as roadside remnants in the region.

Damp sclerophyll forests

Tall open-forest occurs in well-drained, moderately fertile soils, particularly common in the Hotspur area.

Eucalyptus obliqua (Messmate)

Eucalyptus obliqua usually occurs on moderately fertile, well-drained soils. It commonly forms almost pure stands (e.g. in parts of the Annya State Forest where it dominates an understorey of Acacia melanoxylon, A. verticillata, Acrotriche serrulata, Banksia marginata, Bursaria spinosa, Epacris impressa, Leptospermum continentale, Leucopogon ericoides and Pteridium esculentum). In the wetter sites Eucalyptus ovata and E. viminalis occur as co-dominants with E. obliqua (e.g. near Portland). Other common co-dominants include E. willisii and E. baxteri.

Eucalyptus baxteri (Brown Stringybark)

This species is widely distributed throughout the region. Pure stands occur over wide areas. In sandy soils it occurs with Eucalyptus viminalis and occasional E. aromaphloia (sensu lato) (south of Mt Dundas). In the northern part of the region, it is replaced by the very similar E. arenacea which occurs with E. leucoxylon, Banksia ornata and Xanthorrhoea australis (e.g. on sand ridges between Edenhope and Natimuk).

Eucalyptus viminalis (Manna Gum)

Eucalyptus viminalis rarely forms pure stands (as at Lyons and in the Lower Glenelg). It usually occurs as a co-dominant with other eucalypts (e.g. with E. ovata and occasional E. obliqua in the Hotspur and Heywood areas). In the south-eastern part of this region it occurs on clayey soils as a co-dominant with E. willisii and E. ovata. The understorey mostly comprises Acacia melanoxylon, Pteridium esculentum, grasses (frequently Themeda and Stipa spp.) and occasional sclerophyllous shrubs.

Eucalyptus ovata (Swamp Gum)

Eucalyptus ovata occurs widely throughout the region and is one of the most common eucalypts in the wetter southern parts. It usually is a minor component in stands of E. baxteri, E. obliqua or E. viminalis (e.g. in the Mt Richmond National Park).

Land use

This Region has been extensively cleared for pastoral agriculture, particularly for wool, livestock and dairy production. Pinus radiata plantations are a notable feature of the sandy soils of the southern and western parts of this region. Eucalyptus obliqua, and to a lesser extent E. baxteri, E. ovata and E. willisii, have been selectively logged in southern parts of the region (e.g. Annya State Forest, Hotspur area and Lower Glenelg).

National Parks

• Lower Glenelg—27 300 ha;
• Mt Richmond—1733 ha.

State Parks

• Cape Nelson—210 ha;
• Dergholm (proposed)—10 400 ha;
• Discovery Bay Coastal Park—8590 ha.

Source: Conn, B.J. (1993) Natural regions and vegetation of Victoria, in: Foreman, D.B. and N.G. Walsh (eds), Flora of Victoria Volume 1, pp. 79–158, Inkata Press.