The end of the line for our biggest trees?

By Professor William Laurance 14 February 2012
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Big trees evolved for longevity and stability, commodities in short supply these days, says Professor Bill Laurance.

BIG TREES ARE among the oldest and largest of all living organisms and store much of a forest’s carbon, locking it up safely rather than releasing it as heat-trapping greenhouses gases. In general they comprise less than 2 per cent of a forest’s trees, but due to their wide energy-absorbing canopies, they can make up to 25 per cent of a forest’s total biomass.

As a result, they are the breadbaskets of the forest, producing abundant crops of fruits, flowers, leaves and other food that many animals rely on for survival. And, importantly, big trees produce the lion’s share of seeds that constitute the next generation of trees, meaning degredation of these grandfathers of the forests can have wide-ranging effects.

Big trees, like Australia’s mountain ash, manna gum, kauri pine and giant strangler fig, were of course felled in great numbers in earlier land clearing and timber-cutting, but that is just part of the story. Even more insidious are a range of other threats – habitat fragmentation, droughts, windstorms, aggressive weeds, altered fire regimes, salinisation, a decline of their animal seed dispersers, and exotic pathogens and pests.

Giant trees: grandfathers of the forest

It has been revealed in the Northern Territory, for instance, that gamba grass – an agressive weed species from Africa – is overrunning many savanna woodlands. Trees in these woodlands are adapted for fire, but gamba grass grows up to four metres tall and burns so fiercely that it instantly destroys big, slow-growing giants. It has also been revealed that big trees are not resistant to low-intensity ground fires as was once thought and may actually die off two to three years after an event.

Another exotic weed, lantana, plagues seasonal rainforests such as those at Forty-Mile Scrub National Park in north Queensland. There, the lantana forms dense thickets that almost completely halt the growth and survival of tree seedlings.

Even more worrying is the death of adult giant trees. Computer models reveal that, for naturally long-lived species that produce big trees, even a fairly minor increase in adult mortality can seriously erode their population. 

Habitat fragmentation kills many adult trees. Isolated patches of forest encircled by pastures or croplands are much more vulnerable to winds, which tend to accelerate over the surrounding denuded lands. As they grow taller, the big trees get thick and less flexible, and hence strong winds can more easily uproot or snap them. 

Other threats to Australia’s big trees abound: die-back caused by the Phytophthora fungus is killing big trees in many parts of the country; the Sirex wood wasp, native to Eurasia and North America, has destroyed millions of trees in South Australia and Tasmania; and salinisation of soils is killing many others. 

Largest trees are susceptible to drought 

Droughts are also a serious worry, especially because many scientists believe droughts and heat waves could increase with further global warming. A big surprise is just how vulnerable big trees are to recent droughts, given that they certainly would have experienced droughts in the past. 

In north-east Australia, multi-year droughts have repeatedly triggered widespread deaths of Eucalyptus, Corymbia and Acacia trees. Big trees in Africa, Asia and the Amazon have also been highly drought prone. In one Amazonian experiment, in which half of the rainfall was captured by ducts and removed from the forest, the mortality rate of smaller trees doubled, but that of the biggest trees shot up by 450 per cent. 

Endangered relics of Gondwanan rainforest in Queensland and nothern New South Wales, which rely on cloud moisture, could also suffer as clouds are pushed higher by climate change.

The decline of big trees foretells a different world – one where forests shrink and store less carbon, where fewer tree-dependent animals survive, where giant cathedral-like canopies and the diverse ecosystems that rely on them to live become a thing of the past.

Dr William Laurance is a distinguished research professor and Australian Laureate at James Cook University in Cairns, Queensland. He also holds the Prince Bernhard Chair in International Nature Conservation at the University of Utrecht, Netherlands.