Wet Sclerophyll Forest

Wet Sclerophyll Forest (WetSF) is found on deep loamy soils of sheltered hillsides mostly between 600 and 1300 m above sea level and with rainfall above 1100 mm a year.  About 80% the area once supporting WetSF in Victoria falls on public land while a little over one third is represented in conservation parks and reserves.

This is the tallest of all Victorian forest ecosystems with trees that often reach 75 m or more in height over an understory of climbers, broad-leafed shrubs, tree ferns, ground ferns, small herbs and coarse grass.  The principal tree species of WetSF is Eucalyptus regnans (Mountain Ash) which often grows in single-species stands but is sometimes associated with or replaced by one or two others, such as Eucalyptus obliqua (Messmate) and Eucalyptus cypellocarpa (Mountain Grey-gum) in central and western Victoria, or by Eucalyptus fastigata (Cut-tail) and Eucalyptus denticulata (Errinundra Shining-gum) in eastern Victoria, or Eucalyptus delegatensis (Alpine Ash) and Eucalyptus nitens (Shining Gum) at higher altitudes.

Wet Sclerophyll Forest
Wet Sclerophyll Forest 

The mammalian fauna consists primarily of medium-sized browsers (wombat, wallaby), small, ground insectivores and omnivores (antechinus, native rats), arboreal species, some of which are tall forest specialists (Mountain Brushtail Possum, Ringtail Possum, Leadbeater's Possum, Greater Glider, Yellow-bellied Glider, Sugar Glider) and bats.  Most of the birds are common forest species across Victoria (fantails, thornbills, treecreepers, whistlers, honeyeaters, rosellas, currawongs, robins, pardalotes, cuckoos, Kookaburra) but some are more abundant in WetSF than most other ecosystems (lyrebird, whipbird, Bassian Thrush. Pilotbird, King-parrot, scrub-wren, Sooty Owl).  The reptiles are mostly small arboreal skinks with occasional Tiger Snakes and White-lipped Snakes.  All common frogs are small, principally terrestrial species.

There are three key features of WetSF that distinguish it from other ecosystems - deep leaf litter, fleshy-fruited plants and very large trees. 

The litter on the forest floor is made up of portions of fern fronds, large leathery leaves from shrubs and eucalypts, twigs, branches and strips of bark.  None of these break down quickly, despite the usually moist conditions at ground level, so the litter, and the soil beneath it, becomes very deep and loose.  It is the kind of environment that is rich in invertebrates and fungi and as a consequence the commonest ground mammals are insectivores that specialise in fossicking around and under partly decomposed plant material (antechinus).  It is also the sort of terrain that favours the ground-dwelling Lyrebird, which forages by constantly raking the litter, with its large feet, in search of invertebrates.

WetSF has the highest proportion of fleshy-fruited plant species of all Victorian ecosystems except for Warm Temperate Rainforest.  Clearly the high rainfall and relatively humid conditions in the forest are accommodating factors for such a water-dependant reproductive strategy.  Most of the fruits are small - raspberry-sized or smaller - and they are fed upon, and their hard seeds spread, by omnivorous and frugivorous birds such as Silvereyes, omnivorous bush rats and, sometimes, antechinus.

The most striking feature of WetSF is of course the height and immense girth of its trees.  Not only do they grow large but they do it very quickly.   Twenty years after the 1939 bushfires, which killed a large proportion the Mountain Ash in the Eastern Highlands, most of the trees which regenerated from 2 mm long seeds were over 30 m tall and are today over 60 m.  Even forest giants of 90 m or more are seldom older than 250 years while equivalent-sized Redwoods from California are often 1000 years old before they reach this height (although usually of greater girth).  This size has economic and ecological consequences for the forest.  Economic, because it is the most productive of all Victorian forests for timber (growth rates of the trees are almost double that for other forests) and ecological, because it takes a long while for the forest to recover from logging and to re-establish its full compliment of plant and animal species.  It is not surprising, therefore, to find that a conflict between the quest for timber and the conservation of native animals and plants has developed into one with considerable heat.

The oldest trees in WetSF begin to develop hollows in their trunks and larger branches after they are about 150 years old.  Gliders, possums, owls, bats, parrots, kookaburras, kingfishers, treecreepers, pardalotes and some reptiles require tree hollows for nesting or roosting or both.  In young forests the hollows are in short supply and thus the diversity of forest animals is lower.  From a timber perspective logging rotations for WetSF are generally set at 100 years with some forests being cut at 60 years.  The timber industry doesn't want trees with hollows and has developed the term 'overmature' to denote vegetation which has reached the tree hollow stage, which also implies that the forest is past its best.  Conservation groups have, in reply, embraced the warmer expression of 'old growth forest' for the same stage of development and now that term is part of most modern arguments about forest management. 

In recent times new economic arguments and climate change have entered the debate.  There have been many challenges to the economic viability of timber harvesting over the past 30 years.  The earliest were simply claims that the state government, through its wide range of forest management and rehabilitation programs, fire research and control, ecological research and silvicultural research, was spending more money on WetSF for the timber industry than it was getting from timber royalties.  More recently the competing economics of logging versus eco-tourism have added to the debate and the latter seems to have won the day in marginal timber areas such as the Otway Ranges in south-western Victoria.  Hydrologists have long argued that mature WetSF in the catchments of the Yarra River are the reason Melbourne has a clean and constant supply of water.  One of the arguments is that a young forest uses much more water than a mature one, particularly in its first few decade when the growth rate is high.  This has created a problem for those who favour plantation timber farms over harvesting native forests, as plantations also have a heavy water requirement.

Research by scientists from the Department of Sustainability and Environment, on WetSF forest recovery after logging, has found that the understory regrowth will often contain plant species more indicative of Damp Sclerophyll Forest - a less productive forest of lower rainfall areas - in place of the originals.  The most significant species that failed to regenerate fully was the tree fern Dicksonia antarctica, which is killed by logging but not by natural events such as wildfire.  The regrowth vegetation was also more uniform, and hence less diverse, than it would have been before logging.  It was initially believed that the timber harvesting itself may have been the only cause of this alteration to the forest composition but an equally logical argument is that some of it is an indication of climate change.  If either of these explanations is true then many of the current stands of WetSF will be replaced by something else after the next logging rotation or even the next catastrophic wildfire.  

© Paul Gullan, Viridans Biological Databases