The New Nature Read online

  Brushtail possums aren’t usually thought of as tree-killers, but evidence against them is mounting. Back in the 1870s Aborigines west of Melbourne were accusing them of killing trees. The possums were no longer hunted and their numbers had risen. As the Reverend Peter MacPherson put it, in earlier days ‘the tooth of the blackfellow operated upon the possum, and the tooth of the possum operated upon the leaves of the eucalypt’. Now it was just the tooth of the possum that operated. Possums also benefited when dingoes were culled. James Dawson, writing in 1881, said the Aborigines around Framlingham then lost a treat: manna (buumbul), a delicious sugar secreted by bugs in trees. ‘They say that, in consequence of the great increase of opossums, caused by the destruction of the wild dog, they never get any buumbul now, as the opossums eat it all.’ People and possums coveted the same sweets.

  In Tasmania’s Midlands today brushtails are killing many remnant paddock trees. Suspicion is also growing in Victoria that brushtails are currently killing trees in small forest remnants. ‘It’s now evident from our work that possums are a nightmare,’ Geoff Carr said. He told me of one site in the You Yangs where ‘possums have just killed the entire canopy of hundreds and hundreds of trees’. They removed the crowns of red boxes (Eucalyptus polyanthemos), yet seldom fed below 2 metres’ height, presumably from fear of foxes and dogs. Geoff has only recently decided that possums are a problem. ‘My eyes are open to it now,’ he said. ‘Wherever I go now we are finding gross possum damage. It appears to me that this is really a very bad issue. I suspect it’s probably been a pest in modified landscapes for a very long time. It is compounded by miners and nutrient enrichment.’ Miners help kill the trees, and added nutrients from cow dung or farm fertiliser produce more palatable leaves.

  Possums were freed long ago in the Keppell Islands of central Queensland, apparently by fur-traders, and there are now networks of possum trails and dying trees over the islands. With no dingoes to fear, the possums often wander about by day. Brushtails are Australia’s main problem possum, but in Victoria even ringtails stand accused of stripping trees.

  Insects are the culprits in a serious dieback problem on the tablelands of New South Wales. Lone paddock trees and small woodlots are often stricken by native beetles (Chrysomelids and scarabs). Intact forests are rarely harmed, the beetles preferring paddock eucalypts that are fed fertiliser by sheep and cattle camping below them. Beetles find the elevated nitrogen and phosphorus levels in the leaves irresistible. Grubs of Christmas beetles will feed in pastures right by the trees they attack as adults. Native fungi (Botryosphaeria ribis) then attack the stricken trees and often kill them. Tree seedlings don’t fare well in grazed paddocks, and dying trees are not replaced. It’s a sorry example of landscape decay. In another dire example, I’ve seen hundreds of dying tuart trees (Eucalyptus gomphocephala) along road verges south of Perth stricken by tuart borer (Phoracantha impavida), a limb-killing beetle. Experts suspect that groundwater depletion, in league with drought and frost, have given beetles the upper hand.

  But dieback can also strike well inside forests. In Victoria and Western Australia a fungus bearing pretty orange mushrooms deals death in the wake of loggers. Armillaria luteobubalina is a parasite that girdles tree roots, causing root rot and leaf drop. First noted in the 1950s, it’s become a lot worse since then. It creeps out from cut stumps to claim saplings and trees bruised by timber-cutters. Wattles, pea bushes, daisies and lilies also die, in patches up to 20 hectares in size. In central Victoria 2500 hectares are ‘moderately to severely affected’. South of Perth the fungus affects karri (Eucalyptus diversicolor) regeneration. Karri forests are brutally managed by taking out good trees and clear-felling the rest, creating a clean slate for regeneration. Armillaria luteobubalina festers in the stumps, then claims the next generation. Scientists hope that biological control by other fungi will check its advance.

  Another western eucalypt, jarrah (E. marginata), is coming under increasing attack from a miniature moth, the jarrah leafminer (Perthida glyphopa). In an elegant study Ian Abbott and colleagues peered at 329 specimens of dried jarrah foliage held by various herbaria in Australia and Europe dating back to 1791. A fifty-year interval separated the first European settlement of a district and the first evidence of leafminer attack. Abbott thinks that hot summer fires lit by Aborigines scorched eucalypt crowns and killed the caterpillars, whereas the smaller spring fires lit by settlers allowed their spread. This notion of hunter-gatherers influencing moth numbers high up in trees makes the idea of Aborigines controlling koalas much easier to accept. In both situations humans were helping trees by killing their enemies.

  Native mistletoes (Amyema miquelii, A. pendulum) also kill trees. They do best where scattered eucalypts grow on farms and along roads, away from bushfires and brushtails. Foxes, by preying on mistletoe-munching possums, help them succeed. Water running off roads and fertilisers on farms guarantee them lush hosts. Within forests, low branches carrying mistletoes are shed when nearby trees cast too much shade. But trees in the open keep all their limbs, and the mistletoes benefit as well from the extra sunshine. Inside forests these parasites rarely kill trees, but on farms they kill hundreds. In 1904 they were declared noxious weeds in Victoria.

  Other plant-killers include fungi that strike down rainforest beeches in Tasmania and banksias in Western Australia, and psyllids on pisonia trees on coral cays. The rising death tolls in recent years suggest some human involvement, although the mechanisms remain unclear. I should also mention some of the pests that strike plantations where eucalypts are grown in even stands, such as witchetty grubs (Endoxyla cinerea) in Queensland, sawflies (Phylacteophaga eucalypti) in southern Australia, and bullseye borer (Tryphocaria acanthocera) in the west. Plantations also invite attack by wallabies, parrots and native rats.

  In many parts of Australia today trees are dying and we need to know why. The immediate causes may be koalas, possums, insects, mistletoes or diseases, but the story is always more complicated than that. A forest is a complex web of checks and balances. You can’t remove some of its parts – be they dingoes, trees or fires – or reduce it to fragments, and expect the same laws to prevail. A small reserve is not a forest in miniature, it answers to different rules. That’s a key point to keep in mind as we grapple with all the animals, plants and diseases killing our trees.

  ‘Could it be that the trees . . . were awake, that the forest was rising, marching over the hills to war?’

  J.R.R. Tolkien, The Lord of the Rings

  Alfred Deakin, Australia’s second prime minister, ruled at a time when parliament met in Melbourne. To escape the hurly-burly of office, Deakin regularly withdrew to a bungalow nestled in 5 acres of coastal heathland at Port Lonsdale, near Geelong. Here his wife painted orchids while Deakin read. His retreat, an elegant raw timber home in Californian arts-and-crafts style, has remained with the family. In 1999 I was privileged to be taken there by bushland consultant Mark Trengove. Through a window I could see Deakin’s sitting room still intact, his books and magazines in place. Mark then drew my eye to a stone sundial outside. ‘Deakin built that in 1912,’ he said. ‘Notice anything strange about it?’ The sundial now hides in heavy shade cast by coast tea-trees (Leptospermum laevigatum). Most of Deakin’s 5 acres have been invaded by these small trees, converting sunny heathland into dark forest.

  Mark then showed me rotting trunks of ancient grasstrees extinguished by years of shade. He’s identified the seventeen different ground orchids Pattie Deakin painted in 1911, and only four grow here today. Mark and the current owners have hacked down groves of tea-trees and set them ablaze, working to bring the heathland back. The job looks daunting.

  These tea-trees probably grew from seeds dropped by nearby street trees. But tea-tree invasion is a growing problem even inside national parks. At Wilsons Promontory tea-trees began creeping into valued tracts of heathland during the 1950s, evidently in response to changed fire regimes. Diverse shrublands have given way to monocultures of small trees. A
tree once confined to coastal dunes is now spreading inland along many stretches of Victorian coastline.

  Coast tea-tree is just one of many native plants now invading new areas en masse. Whenever we change the way a forest is burnt or grazed, or feed in nutrients, the vegetation may change. One plant may invade in the thousands, or one community may overwhelm another. Four examples came up in earlier chapters: invasions of buttongrass in Tasmania, wet sclerophyll in north Queensland, grassland on Cape York Peninsula, and farmland by Melaleuca pustulata in Tasmania. It’s now time to consider this phenomenon more fully. It differs from the garden plant problem mentioned before, in that most invaders are local plants. Coast tea-tree grows naturally a short stroll from Deakin’s house.

  Earlier I mentioned Sheila Walkerton and the noisy miners in her garden in Northwood, Sydney. Sheila has seen many invasions in her valley, and none concerns her more than the spread of a local tree, sweet pittosporum (Pittosporum undulatum). ‘This valley has an interesting history,’ she said as we followed a winding trail through the park near her home. ‘It’s been worked since about the 1790s. It was a route through for the loggers. The bullocks used to come down through the valley. There are still places where you can see the wagon ruts.’ She pointed out ruts cut into a sandstone shelf near the stream. ‘I remember this area when I grew up as very dry, open eucalypt forest. I remember the smell of the eucalypts and boiling the billy and all those wonderful things.’

  We looked around. On both sides of the valley sickly eucalypts towered over a dense layer of sweet pittosporum. Many gum trees were dead. ‘See that eucalypt,’ she said, pointing. ‘There are too many pittosporums around the roots.’ I could see that the invaders were starving the trees of water and minerals. Smaller plants were being killed by the shade thrown. I peered under the pittosporums but there wasn’t much to see. ‘There used to be little Acianthus orchids on this rock,’ Sheila said, pointing to a block of sandstone in the gloom. ‘I haven’t seen them there for years.’ Hundreds of orchids, lilies, grasses and shrubs have vanished from this valley. The plant that grows best beneath sweet pittosporum is an exotic, privet (Ligustrum lucidum).

  Similar changes are underway all over Sydney. Houses perched on ridges to snare views are feeding nutrients into infertile forests below. Rich loads of phosphorus and nitrogen flow in from septic tanks, fertilised gardens, food scraps and dog droppings. Water runs in from leaking pipes, overflowing swimming pools and garden sprinklers, and as rain channelled off roofs and roads. And bushfires no longer rule. If you take eucalypt forest, add fertiliser and water and take out fire, you have the recipe for rainforest. The pittosporum invasion is really takeover by rainforest. Botanists call it ‘mesic shift’ – a switch to a wetter community. Because Sydney’s original rainforest supported few tree species, one is dominating the shift, although others, notably cheese tree (Glochidion ferdinandii) and bleeding heart (Homalanthus nutans), are spreading too. Rainforest is escaping from fertile, fire-protected valleys and sprouting everywhere. This is another kind of change induced by people: vegetation shift.

  Sheila works for a bushcare company engaged by Lane Cove City Council to quell the invasion. ‘Our brief is to take out every pittosporum within the vicinity of a eucalypt or angophora.’ Sheila’s views on this tree have reversed over the years. ‘I was very unwilling at first to think of it as a weed,’ she confessed. ‘Then I did a TAFE course on bush regeneration in 1987. By that time there were serious questions being raised. It was a hard one, but the evidence was irrefutable. I could see why they had to be removed.’

  Peter Beard, a ranger at Lane Cove National Park, describes pittosporum invasion as ‘one of the most sensitive issues around’. He became involved back in 1985. ‘There was a huge divide then and there’s still a divide now.’ I was shown dense stands invading eucalypt forest around an old caravan park, where changed fire regimes are more of an issue than nutrient enrichment. Peter said attitudes towards pittosporum had hardened with time: ‘Rangers would say, “Let’s just see what happens. Maybe with climate change it’s meant to be more pittos and cheese trees. The bush will evolve, why should we intervene?” My attitude is, we have, through error, already intervened, so we need to fix things up. We’re not trying to recreate 1788. But we are trying to let evolution take place. We are part of the process. Too many people view things as, There’s the ecosystem and we’re outside it.’

  This is all too much for some people to swallow – the idea that we affect forests so dramatically; that we should get out and ‘fix nature up’.

  Wherever possible, hot fires are lit to kill this ‘weed’. I visited Buckley’s Reserve on the Mornington Peninsula near Melbourne some weeks after a wildfire had raced through. Two-thirds of the reserve had been invaded by pittosporum, but now the woodland flora was coming back. Rising from the ash were sundews, milkmaids, early nancy, chocolate lilies and honeypots. ‘This is probably as good as it gets,’ explained Rohan Cuming, my guide for the day. ‘These are pelargoniums. I’ve spent ten years working through this reserve and found only one of them. Now there are thousands. And they’re going to seed their heads off.’ Rohan told me this place ‘wants to be grassy. It wants to have lots of things in the understorey.’ The plants we saw were lucky – pittosporum invasion was staunched early, before their store of soil seed was spent. Older sites must be replanted after fire to prevent weeds invading instead.

  Pittosporum is native to Sydney but not to the Mornington Peninsula. This tree, arguably our worst native weed, happens to be weedy both within and beyond its natural range, which runs from southern Queensland to (maybe) Western Port Bay in Victoria. No-one is sure about its natural range any more. I mentioned it before as a misbehaving garden plant. In Victoria it’s been listed officially as a Potentially Threatening Process, but also as part of a rare plant community (limestone rainforest). Rare orchids like to grow on its limbs. In places it is now hybridising with related trees, smudging their identity. It thrives on contradictions. Whether we call it a weed or an asset depends on where it is growing and how it is behaving.

  Mesic shift is serious in north Queensland, where wet sclerophyll forests abut the western edges of the Wet Tropics rainforests. The sclerophyll trees (eucalypts) need fire for renewal (their seeds sprout in ash), but fire in these areas has been actively discouraged. There are also cows consuming fuel and roads operating as firebreaks. Half the area of sclerophyll forest has vanished over fifty years, swallowed up by a rich assemblage of rainforest trees. The losers in this shift from open forest to rainforest include endangered tropical bettongs, rare gliders, and certain birds, not to mention the towering eucalypts that make up this unique forest type. The Wet Tropics Management Authority is concerned; so too are traditional owners. Yidinjdji elder George Davis says Palmerston National Park has deteriorated since the days when his grandfather fired the eucalypt forests to keep them ‘clean and healthy’. Weeds and saplings are now choking the land, he says, rendering it impassable to people and animals. The forest is now ‘very sick’. Unlike most Australians, George views people as part of wild systems.

  Further north, on Cape York Peninsula, the survival of critical habitat depends strongly on Aboriginal-style management. Golden-shouldered parrots survive only in two regions, where graziers regularly light hot fires to stop paperbarks (Melaleuca viridiflora) invading. When Lakefield became a national park, fires were suppressed at first. Paperbarks then marched in and the rare parrots vanished for good. In New South Wales and southern Queensland endangered eastern bristlebirds are also losing out to changed fire regimes. Fires in their heathland/tussock habitat are now too frequent or too few.

  Plant invasion is also a worry for graziers. In western Queensland and New South Wales the invaders on eroded red soil are ‘woody weeds’: mainly native hops (Dodonaea viscosa), cassias (Senna), and emu bushes and poverty bushes (Eremophila). Most of these plants are unpalatable and sometimes toxic to stock, spotted emu bush (Eremophila maculata) yielding cyanide. Fir
es suppress them by killing their seedlings, but stock now graze so low that grass fires no longer take, allowing these shrubs to claim the land. In one recent study Bradd Witt analysed layers of manure in an old shearing shed and found that sheep over the decades had been forced to eat more shrubbery and less grass.

  Woody invaders are causing heartache all over the world. In South Africa ‘bush encroachment’ of pastures has rendered more than a million hectares unusable by stock. In North America conifers are taking over from sagebrush, and mesquite from grassland. Farmers can pre-empt invasion by burning some of their grass instead of offering it all to stock, but few do so. When shrubs take over, succession is very difficult to reverse, because fires no longer take.

  On the tablelands of New South Wales the woody ‘weeds’ are daisies. Chinese shrub (Cassinia arcuata) has swallowed up more than 600000 hectares of sheep pasture, and wild curry (C. laevis, also called ‘cough bush’ because of its funny smell) is invading as well. On some farms they’ve been countered by moving around their enemies, native scale insects (Austrotachardia and Paratachardina), which multiply and often kill them. This didn’t work out on one farm, where a native sticky daisy (Olearia elliptica) grabbed the vacated land instead.

  Farmers may also lose out when one grass replaces another. No sheep graze eastern Queensland today, but last century there were vast flocks on the subcoastal grazing lands, growing plump on rolling pastures of kangaroo grass (Themeda triandra). Farmers torched the land each year to promote a flush of sweet new grass. Pioneer Harold Finch Hatton wrote of the Mackay district in 1885: ‘When the whole country is burning in patches for miles round, it is a very pretty sight to see the fire at night, creeping up the sides of the mountain’. But those ‘pretty’ fires conspired with drought and stock to exhaust the sweet kangaroo grass. It had not been burned by Aborigines that often. Over most of Queensland it departed from pastures, making way for coarser, more fire-adapted black spear grass (Heteropogon contortus). Spear grass is less sustaining than kangaroo grass and dries into useless straw. It also sprouts sheaths of spine-tipped seeds on wiry awns that bore into wool, ruining fleeces, piercing skin, triggering ‘scab’, and sometimes delivering death. Finch Hatton saw sheep ‘with their fleeces stuffed so full of grass-seed that they are absolutely incapable of moving, and can only stand still, with their legs wide apart, looking more like a hedgehog on stilts than a sheep’. The sheep industry collapsed.