The tallest trees in Western Australia are also amongst the world’s tallest hardwoods. The majestic Karri (Eucalyptus diversicolor), with its smooth pale bark and straight stems growing up to 90m (300 feet) is one of the southwest’s iconic species. It grows in pure stands in the high rainfall areas on deep loamy soils that were prized for agriculture from the state’s earliest days, as well as in drier and shallower soils as mixed forests with jarrah and marri. Karri has long been valued for its wood, and mill towns dotted the forests for the first hundred and fifty or so years of Western Australia’s European settlement.
These days, thanks to years of community pressure and government responses, a large part of the karri forest is under reservation or other protection and now nature-based tourism, fine woodcrafts and other uses like production of karri honey are foremost in people’s use of the karri forests, although some logging still occurs. Much of the reserved areas of karri forest are now part of the Walpole Wilderness Area, and a number of national parks including D’Entrecasteaux, Shannon, Walpole-Nornalup and Mt Lindesay.
The heart of the southern forests is to the south of Nannup and Manjimup through to Denmark, mostly where rainfall exceeds 1100mm (about 43 inches) per year. However, a few karri forest outliers, including on the Leeuwin-Naturaliste Ridge, south of Margaret River, the Porongurup Range north east of Albany, and the Many peaks area, give hints of karri’s historical extent across a much wider area when climates were more favourable. In fact there are some steep, moist gullies in the northern jarrah forest where understorey more typical of the karri forest can still be found.
While the karri may have evolved under rainforest conditions, it has learned to cope with drier summers so can be truly considered a Mediterranean system. Its dense understorey helps keep the moisture for as long as possible over summer, and helps to moderate the summer temperatures too: while it’s the cooler part of Western Australia, it can still reach temperatures of 40C (over 100°F) in a hot spell. Small trees and shrubs such as the Peppermint (Agonis flexuosa), named for the strong peppermint aroma from its crushed leaves, Karri Sheoak (Allocasuarina decussata), the creamy-flowered Karri Wattle (Acacia pentadenia) and the oak-leaved shrub known as Karri Oak (Chorilaena quercifolia) provide the backdrop for the smaller wildflowers – the dramatic coral vine, the white and cream of old man’s beard, the bright blue native wisteria and the myriad orchids to name but a few. All up, there are around 2000 plant taxa in the mosaic of habitats that make up the karri forests, with a concentration of diversity and endemism within the Walpole Wilderness Area.
The dense understorey also provides shelter for animals, although species have been lost to habitat destruction and predation by the introduced European Red Fox. Some species like Quokkas (Setonix brachyurus), perhaps best known in Western Australia as a tourist attraction on Rottnest Island off the coast from Perth, survive in the moist dense thickets in the valleys of the jarrah-karri forests or around wetlands. While the more mature old growth forests are favoured by some species, it’s the more open forests, woodlands and heaths that are preferred by species such as the Western Brush-tailed Possum and Chuditch.
Birds are plentiful, although the tall forest doesn’t have quite the same species diversity as some of the more open habitats. Nevertheless, there are around 150 species to be found within the forest mosaic, with some like the white-breasted robin and red-winged fairy-wren preferring the karri forest itself.
As might be expected, it’s the moisture-loving animals that have their greatest expression in the southern forests. The water in the swamps and rivers here is the freshest of any in the southwest and this has allowed the development and survival of fish, frogs and aquatic invertebrates that are found nowhere else. The Forest Toadlet (Metacrinia nichollsi) for example lives among the leaf litter in the karri forest and the young toadlets hatch directly from eggs bypassing the tadpole stage. It’s been suggested that the bright purple and orange patches on its belly may mimic some of the fungi that are found in its habitat. Four related frog species from the Geocrinia genus are endemic to areas with rainfall higher than 1100mm, each having a very restricted range. The Roseate Frog (G. rosea) is the most widespread but nevertheless confined to pockets around Augusta, the karri forests around Pemberton, and a small area near Walpole. It has recently been bred and released from the Perth Zoo as part of a research project to help in the conservation of two of its more restricted relations, the Orange-bellied Frog (G. vittelina) and the White-bellied Frog (G. alba) which are nationally considered vulnerable and endangered respectively, and the very restricted Walpole Frog (G. lutea), found only in a small area near Nornalup and Walpole. The three have slightly different habitats associated with gullies or peat swamps, but all are dependent on year-round moisture.