Australian Acacia a general overview

Acacias belong to the large world-wide leguminosae, or legume family. This family includes many well known and widely cultivated plants, such as peas, clover, beans etc. The leguminosae is divided into three sub-families, ceasalpiniaceae, fabaceae and the mimosaceae, which is the section that contains acacia, as well as mimosa and others.
Acacias have a world-wide distribution in tropical and sub-tropical areas of central and south America, Africa, parts of Asia and islands of the Pacific and Indian oceans as well as Australia, which has the largest number of endemic species. There are about 1200 species known world-wide, with at least 7-800 species in Australia, others still not properly described or named.
The word ‘acacia’ is derived from the Greek word ‘akis’ meaning sharp point. It was first used for Acacia arabica, a spiny tree used as a source of gum, ‘gum arabic’. Acacias have been used by people since neolithic times, Australian Aboriginal people had probably been using them for most of their at least 40 000 years in Australia.

In Australia acacias are the largest genus of plants, with acacias found in almost all habitats, except rainforests and some grassland or arid zones. Some habitat types, eg ‘mulga’, are almost exclusively acacia species, and acacias are generally a fairly integral part of the Australian flora, it is the largest endemic plant genus in Australia.
Within the genus Acacia there is however considerable variation over such a large number of species, with some quite exceptional species. Some species produce large numbers of flowers and can look very dramatic when in flower. Most are small trees to shrubs, with rare specimens, including cultivars, growing to large trees or prostrate ground covers. Generally they are quite fast growing, with many of them not living a long time (10-20 yrs), a few species may live longer than fifty years. The foliage can vary from fern like bipinnate leaves to phyllodes of varying size and shape in place of true leaves, the majority of Australian species are phyllodinous. Phyllodes are in fact the flattened leaf stems that act and look like ordinary leaves, using the sun to produce energy for the plant. This is believed to be an adaptation to the generally dry environment in Australia over a long period of time, though there is some dispute over wether the phyllodinous species are a more modern or primitive type to the bipinnate species. There can be huge variation in the phyllodes, in size and shape, as well as colour, texture, stickiness or hairiness.
All Acacias have pinnate then bipinnate leaves as the first true leaves, most of the phyllodinous species start to develop phyllodes soon after, sometimes bipinnate foliage may persist on some parts of the plant especially when it has been damaged. Some Australian Acacias have bipinnate foliage for their whole life.
As well as foliage, flowers are good means of identifying different species, varying from globular heads on solitary peduncles or along racemes, or in spikes. Almost all of them have cream to yellow-orange flowers, varying from pale to very bright and showy, one Qld species has pink-purple flowers. The colour of the flowers in each species seems fairly consistent and can aid in identifying different species. The flower heads are actually lots of little flowers bundled together. Interestingly Acacia flowers don’t produce any nectar, though the gland(s) found along the edge of phyllodes or leaf stems may sometimes exude ‘nectar’ in some species at some times of the year or under certain conditions, and they are often found near the base of the phyllode next to where the flowers appear. Ants and sometimes other animals use this as a food source. Bees still visit the flowers to gather pollen and Acacias seem mostly insect pollinated.

Like all members of the leguminosae Acacias produce pods or legumes which contain seeds, these too can be helpful in identification, varying from straight to highly coiled or twisted, smooth or covered in fine hairs. How the seed sits in the pod and the colour and size of the aril or funicle that holds the seed to the pod can also help in identification. This aril or seed stalk contains oils and fats and seems much relished by ants who will cart away the seed that has fallen to the ground, carrying the aril plus seed underground to store as a food source. In species with brightly coloured arils birds are thought to be attracted to them, thus dispersing the seed, the seed may remain attached to the pod via this aril even after the pod has split open also.
There can be a lot of variation amongst a single species due to many factors, eg climate, soil, genetic variation etc which can sometimes make identification a tricky process. Rainfall seems to be one of the most influential factors in the distribution of Acacias and other plants. Natural hybrids in both phyllodinous and bipinnate foliated species are also not unknown, eg a hybrid A. obtusifolia X A. sophorae from Qld, but don’t seem really common, and seem to be found mostly where there is an overlap in the range of two related species in localised areas. Hybrids do seem more common in cultivated forms of some bipinnate species, eg A. baileyana and A. decurrens.
Since European invasion the distribution of many plants has been dramatically affected, especially in the more populated east. Large areas of forest and scrub have been cleared for farming or residential purposes. We may have even lost some species of both plants and animals before we knew they were there ( in fact the first hybrid mentioned in the previous paragraph is now known only as a herbarium specimen because the area it was found in has been changed,it’s habitat no longer exists!).
Acacias have been affected by this in different ways. Some have proliferated where they were previously uncommon as they are one of the first species to reappear after clearing of land, an example is A. melanoxylon which grows over a lot of land that was previously rainforest in E and NE NSW as a regeneration species and is not found in mature rainforest due to it being unable to stand the low light intensity, or the many cultivated species (eg A. baileyana, acacia pycnantha...etc) have become naturalised in areas they previously were not found. Disturbed land such as along roadsides also tend to have many Acacias as common species, as well as many logged forests, where Acacias generally one of the first species to reappear. Others have obviously lost a lot of their habitat and their distribution may have shrunk. In some cases vast areas, 1 000’s of hectares, of plants were cleared, eg A. harpophylla, ‘brigalow’ in Qld. This greatly affected the numbers but not necessarily affect the distribution as some plants persisted along roadsides and other unaffected land. From the later part of last century to earlier this one 1 000’s of tonnes of Acacia bark was exported from Australia for use in the tanning industry.

Seen here is tons of acacia pycnantha bark being collected for export or use in the tanning industry from the Mt Lofty ranges SA, in some parts this along with general deforestation led to once tree covered hills becoming brown bare grasslands, as many still are today in some parts of SA.

Like many members of the leguminosae the roots of some Acacias have nitrogen fixing bacteria, which means they fix nitrogen from the air into the soil, thereby improving the nutrient level. This and their hardiness and generally rapid growth make some species especially good for bush regeneration, and to stop soil erosion.
Another consequence of European invasion has been the loss of knowledge concerning the native flora. Aboriginal people had a thorough and complex knowledge of the bush around them and much of what they knew is now lost, especially in the eastern part of Australia. Some people from the north and western areas still have some knowledge of local plants as they were able to maintain a more traditional lifestyle before their land was taken. From ‘Wild medicine in Australia’ there is a quote,"a report in the mid-nineteenth century on Victorian Aborigenes states that all their common ailments were effectively treated with lotions and decoctions of wattle bark and gum, which suggests that the medicinal use of acacia was probably much more widespread than the specific record shows".
Over the last few years there has been a lot more interest in the native flora and the possibilities of useful compounds from them, and an appreciation of the knowledge still held by the few older Aboriginal people of their local plants. So there have been a few publications and studies of plants used for food and medicine by Aborigenes, in an area where there has been little research. Research has found that some Acacia seeds are very nutritious, with good levels of protein, fats, carbohydrates and minerals, some species are being grown in arid areas of Africa and Asia for food and fuel.
A feature of many Acacias is also their hard wood, some species having extremely hard and heavy wood, some of the heartwoods also have distinct colouring or odours. The Aboriginal people used wood from different species for just about anything that they made out of wood, such as spears, spear heads, digging sticks, shields, woomeras, boomerangs etc. Europeans too recognised the value of the wood from some species and used the wood from a few species for cabinet and ornamental work, including coach building and for beer barrels. Others were employed in such things as gunstocks and even machine bearings, as well as fence posts.

Since earlier this century acacia has been recognised as a national floral emblem, specifically acacia pycnantha, ‘golden wattle’, though it was not officially accepted as the national emblem until 1988. Sep 1st was formerly celebrated as wattle day, on August 30 1909, government botanist and director of the botanic gardens, Sydney, J H Maiden along with Mrs H Clunies-Ross and Mrs A Kettlewell founded the Aust Wattle Day League. Wattle day is not a particularly well known or celebrated day as it was formerly. The founders of the wattle day league are however remembered in the names of three species of acacia, a. maidenii, a. kettlewelliae, a. clunies-rossiae.