Banksia Woodland is found on flat to gently undulating, sandy soils in near-coastal regions of southern and eastern Victoria. The altitude range is generally between 10 and 100 m above sea level and the rainfall varies from 700 to 1000 mm a year. About 65% of the area once supporting Banksia Woodland in Victoria falls on public land while about 40% is represented in conservation parks and reserves. About 25-30% of all Banksia Woodland has been permanently cleared for agriculture or urban development.
There are two basic forms of Banksia Woodland one which is strictly coastal (Coast Banksia Woodland - CBW) and one which is near-coastal (NCBW) but occurs up to 30 km inland. In East Gippsland the two forms often grade into each other.
CBW is found on calcareous sands and Banksia integrifolia (Coast Banksia), to 10 m tall, is the principal canopy species. It often grows in association with Acacia longifolia var. sophorae (Coast Wattle), Leptospermum laevigatum (Coast Tea-tree) and Allocasuarina verticillata (Drooping Sheoak), over an understory of small shrubs, herbs (many of which are succulent), sedges and grasses. In many areas it will border and share many species with Coastal Scrub on the seaward side and will abut Heathland further inland.
NCBW develops on siliceous sands, east of Wilson's Promontory, and
Banksia serrata (Saw Banksia),
to 10 m tall,
is the main canopy species which may grow in association with
Eucalyptus globoidea (White Stringybark),
Eucalyptus sieberi (Silvertop Ash),
Eucalyptus consideniana (Yertchuk)
or Eucalyptus botryoides (Southern
Mahogany). The understory is made up of small to
medium-sized, small-leafed shrubs, sedges, perennial herbs and grasses.
NCBW often borders and shares many species with
Heathland and may merge into
CBW towards the coast.
The two forms of Banksia Woodland are, in many ways, versions of Coastal Scrub and Heathland with a banksia canopy. CBW plants all need to deal with the high pH of infertile calcareous soils and with occasional wind-borne salt spray and high winds from coastal storms. NCBW plants must cope with the low pH of extremely infertile, siliceous soils and frequent fires. Both forms of the ecosystem need to deal with sandy soils that have a poor water-holding capacity. Consequently plants within Banksia Woodland will exhibit adaptive mechanisms such as succulence (Rhagodia candolleana, Tetragonia implexicoma) for salt-tolerance, underground storage and regrowth organs and woody seed capsules (Banksia, Allocasuarina Leptospermum, Melaleuca), for regrowth after fire, and specialised roots systems with plant-fungal symbiotic relationship (most shrubs) or plant-bacterial symbiotic relationships (peas, sheoaks) or dense masses of fine roots with long root-hairs (sedges, sheoaks, banksia, hakea) for dealing with low nutrient availability.
There are eight species of Banksia that are native to Victoria, four of which are common in Banksia Woodland. The two dominant species, Banksia integrifolia and Banksia serrata are the largest banksias in the state and although their presence may not make a great difference to the ecosystem in terms of species numbers they do make a substantial ecological one.
The concept of foundation species is a rather loose one developed in the 1970s, which refers to usually dominant plant species that have a significant effect on the function and survival of an ecosystem. It can be argued that both large banksias in Banksia Woodland fit that concept and that the loss or reduction in number of these plants would have a flow-on effect to the other inhabitants of the ecosystem. The most obvious key feature of both banksias is the number and size of their clusters of nectar-producing flowers. These provide a major source of food for honeyeaters, spinebills, wattlebirds, lorikeets and pygmy-possums, as well as a large number of nectivorous invertebrates upon which insectivorous vertebrates will feed. The flowering time is also important as it is principally in the summer and autumn, rather than spring, thus significantly expanding the period of availability of nectar.
After flowering the banksias produce large woody cones containing two-valved capsules in which the seeds develop and remain. A large plant will support hundreds of cones that release their seeds only after fire or the death of a branch. In the meantime they hold a substantial source of food and cockatoos (principally Yellow-tailed Blacks) may break open the cones to feed on the seeds or beetle larvae which often invade the woody fruit.
The banksias, when mature, are densely foliaged and may develop hollows in their trunks, both structural features which make them attractive to birds, bats and possums for nesting or roosting.
The large size of the banksias is a function of their ability to draw nutrients from the infertile soils by means of a shallow root system with specialised, highly absorptive structures called proteoid roots. A mature tree will provide a dense canopy as well as a thick leaf litter in which relatively few understory species will grow. Consequently a large proportion of the biomass of the ecosystem will reside in the trees. The relatively frequent fires in Banksia Woodland will return many of the nutrients to soil, enabling access by other species, and at the same time release banksia seeds from their cones. Fire will not usually kill the banksias which will often regenerate directly from lignotubers (underground woody storage organs) or epicormic shoots (growth buds beneath the bark). Very hot or very frequent fires, however, may kill the trees which will mean that regeneration will need to start from seeds. At this stage of development the ecosystem is in most danger as a fire which takes place before the banksias are reproductively mature - that is, before they produce very many seeds, before a lignotuber can develop and before the protective bark is very thick - may cause local extinctions.
The fauna of Banksia Woodland has a superficial similarity to that of the Heathland and Coastal Scrub ecosystems, in that the range of species overlaps considerably. There are, however, important quantitative differences. Mammals make up a much larger proportion of vertebrate records in Banksia Woodland than for most other ecosystems. Ground feeding mammals (Bush Rat, Swamp Rat, Antechinus, Wombat, Black Wallaby, Potoroo) have been recorded more commonly in Banksia Woodland than elsewhere as have some arboreal species (Common Ringtail Possum, Eastern Pygmy-possum). Similarly large snakes (Red-bellied Black Snake, Tiger Snake, Lowland Copperhead) and goannas (Lace Goanna) are, based on recorded sightings, most abundant in Banksia Woodland.
The presence of large banksias in Banksia Woodland appears to affect the ecosystem by effeciently gathering nutrients from the infertile soils and converting these into structures that provide food, shelter and breeding sites for a range of animals. The fact that large predators (by Australian standards) such as snakes and goannas are more abundant here than elsewhere suggests that productivity within Banksia Woodland is considerably enhanced by the principal trees and that the term foundation species can be reasonably applied to them.
© Paul Gullan, Viridans Biological Databases