Riparian Forest (River Forest)
Riparian Forest (RipFor) is found in narrow strips along the sheltered banks of rivers over a wide altitude range, mostly between 100 and 1300 m above sea level, with rainfall between 800 and 1500 mm a year. About half of the area once supporting RipFor in Victoria falls on public land while a little under one sixth is represented in conservation parks and reserves. About 45% of all RipFor has been permanently cleared for agriculture, urban development or drainage control. In most areas RipFor is bordered by Damp Sclerophyll Forest (DampSF) or Dry Sclerophyll Forest (DrySF) ecosystems.
RipFor is characterised by tall, straight trees, most commonly
Eucalyptus viminalis (Manna Gum) but often replaced by or
associated with Eucalyptus radiata (Narrow-leaf Peppermint),
Eucalyptus cypellocarpa (Mountain Grey-gum) or Eucalyptus elata
(River Peppermint - eastern Victoria). The understory consists of
climbers, broad-leafed and narrow-leafed shrubs, ferns (including tree
ferns), scrambling grasses and soft-leafed herbs.
RipFor is at once the most diverse and most disturbed forest ecosystem in Victoria.
Its diversity arises from the its linear nature, often less than 10 m from each river bank, which means that it will contain some species common to the surrounding forests as well as those that are best suited to riverine conditions. It is an example of what has often been referred to as an ecotone effect, where the juxtaposition of two ecosystems creates a narrow overlap zone (an ecotone) which is more diverse than either ecosystem because it contains animals and plants from both. In this case, the two ecosystems are the river and the forest so the RipFor ecosystem is, in effect, an ecotone itself.
The forest canopy of tall eucalypts, large wattles and broad-leafed shrubs supports 80% of Victoria's possums, gliders and bats and most of the common forest birds and arboreal skinks. The ground layer of ferns, large herbs and grasses, is suitable for browsers, so wombats and wallabies are common. The low herb layer is interspersed with a deep leaf litter which supports a rich invertebrate fauna sustaining rats, antechinus, frogs and ground skinks. The river banks and the rivers themselves provide hunting and breeding grounds for platypus, water rats, kingfishers, martins and swallows. The shallows along the river edges support sedges, rushes and wetland herbs in which frogs (more than 70% of Victorian species), water skinks and water dragons forage, as do the snakes that feed on them.
The disturbance in RipFor comes from three broad sources. The first is the complete removal of significant lengths of river-bank vegetation to improve access for stock, irrigation, industry or housing. The second is from runoff from surrounding farms, housing and industry which carries organic and inorganic chemicals, seeds from weed species, refuse and soil. The third is from direct interference from gold mining (in the 1850s), planting introduced species such as willows, deliberate dispersal seeds of non-native plants such as blackberry, deliberate release of non-native fish such as trout and carp, irrigation and water-supply construction, bridge construction and sub-standard forestry practices (those that do not meet the Victorian 'Code of Forest Practice').
With such a range of damaging influences it might be thought that few rivers support anything like their original vegetation and unfortunately this is the case in many places. The Yarra River, for example, extends from the northern end of Port Phillip Bay to the north-western slopes of the Baw Baw Plateau, 110 km to the east, and for more than 75% of its length the vegetation has been either completely removed or has been so dramatically changed that it can no longer be regarded as a native ecosystem. In addition, nearly all of its major tributaries have been dammed or altered for agriculture and urban development. Indeed the waterways upstream from the Maroondah, O'Shannassy and Upper Yarra Reservoirs are the only undisturbed sections of the entire catchment. Similar descriptions can be made for most other large river systems in Victoria which support RipFor (e.g. Gellibrand, LaTrobe, Thompson, Mitchell, Tambo).
The ecological and agricultural weed, Blackberry, and the English Blackbird are more abundant in RipFor than in any other ecosystem. Foxes and feral cats take advantage of the excellent cover, the ready supply of water and the abundance of small vertebrates to hunt in RipFor more frequently than elsewhere. And there is a greater range of weedy garden escapes in RipFor than any landscape outside of abandoned suburban building blocks.
Despite these drawbacks RipFor is still the best place to go to see Bell Miners, Manna Gums, Tiger Snakes and frogs. Most existing stands still have more native than non-native species, and more native species than most other ecosystems. And even comparatively simple management programs, such as hand removal of non-native shrubs and trees, can create long-term ecological benefits. These management programs may be a key part of the future of Riparian Forest.
In many parts of rural, and to a lesser extent urban, Victoria the only remnants of native vegetation are narrow strips along roadsides and fencelines, and often the only natural part of the vegetation is the trees. Much has been made of the importance of these strips as wildlife corridors, along which animals may move and plants may disperse, but as they are normally surrounded by a somewhat hostile, treeless landscape and they have little in common with the broad spans of vegetation from which they arose, there are ecologists who question whether they really function in the way that we would like. RipFor, on the other hand, is undoubtedly a wildlife corridor along which animals and plants are known to disperse, migrate and interact. Even when the forests that border RipFor have been depleted or removed, there is evidence that the integrity of the riverine ecosystem is not always greatly compromised. For example, RipFor in the Eastern Highlands is often rich in native species, although weedy, even when it runs through parklands, townships and pastures.
The importance of RipFor as both a biodiversity reservoir and a genuine wildlife corridor, along with its resilience to disturbance in adjacent land, should give land managers some confidence that a measured approach to replanting and rehabilitation of RipFor will be a worthwhile investment. Such programs are often relatively straightforward but they are also time-consuming, labour-intensive and easy to get wrong. Many poorly supervised weeding programs have removed native species that looked like weeds - for example, Rubus parvifolius (Small-leaf Bramble) is native even though the similar Blackberries aren't, Microlaena stipoides (Weeping Grass) looks like the introduced Couch, Senecio jacobaea (Ragwort) is an invasive weed but there are several native Senecios that belong, and there are many inexperienced workers who seem to believe that native forests would be better off without the abrasive Tetrarhena juncea (Wire-grass) and the spiny-fruited Acaena novae-zelandiae (Bidgee-widegee), despite them both being natives.
Replanting can be just as problematic without the careful oversight of an experienced ecologist, and should always be done with local species from local stock. Care should be taken not to emphasise the more attractive species or those which are thought to provide food for birds or butterflies (this is never as clear as we think), and the position in the landscape is also critical. A fern or sedge or lomatia which normally grows directly on a riverbank should not be planted uphill nor should species which cannot withstand occasional flooding be placed too close to the water. If these pitfalls are planned for, however, rehabilitation programs in RipFor have much to recommend them as valuable ecological measures.
© Paul Gullan, Viridans Biological Databases