Australasian Science: Australia's authority on science since 1938

Fall of the Leviathans

By David Salt

Three of the world’s top forestry ecologists have warned that the planet’s stock of large, old trees is experiencing an accelerating decline.

They are among the biggest organisms on Earth, they form keystone structures in every landscape in which they are found, everyone loves them… and they’re disappearing before our eyes. Just as large-bodied animals such as elephants, tigers and many whales have declined drastically around the world, a growing body of evidence suggests that large old trees could be equally imperiled.

Three of the world’s most distinguished forest ecologists have made an impassioned plea in the international journal Science to do something about the accelerating loss of the planet’s stock of large, old trees.

“It’s difficult to underscore how critically important large, old trees are to the ecosystems in which they are found,” says the lead author of the report, Prof David Lindenmayer of the Australian National University’s Environmental Decisions Group. “They provide nesting or sheltering cavities for up to 30% of all vertebrate species in some ecosystems; store large quantities of carbon; play significant roles in local hydrological regimes; and provide abundant food for numerous animals in the form of fruits, flowers, foliage and nectar.

“In agricultural landscapes, large old trees can be focal points for restoration efforts, they facilitate ecosystem connectivity by attracting mobile seed dispersers and pollinators, and they act as stepping stones for many animals.

“Younger and smaller trees simply cannot provide most of the distinctive ecological roles played by large old trees, so with their passing something irreplaceable is lost.”

Together with colleagues Jerry Franklin from the University of Washington and William Laurance from James Cook University, Lindenmayer describes the unfolding situation. Populations of large old trees are plummeting in agricultural environments like intensively grazed landscapes in California, Costa Rica and Spain, where such trees are predicted to disappear within 90–180 years. In south-eastern Australia, millions of hectares of grazing lands are projected to support less than 1.3% of the historical densities of large old trees within 50–100 years.

But it’s not just production landscapes that are witnessing the decline. Large old trees are disappearing in forests at all latitudes. Larger trees throughout southern Sweden have declined from historical densities of around 19 per hectare to one per hectare. In California’s Yosemite National Park, the density of the largest trees declined by 24% between the 1930s and 1990s. Large old mountain ash trees in south-east Australia – the Earth’s tallest flowering plants – are predicted to decline from 5.1 trees per hectare in 1997 to just 0.6 trees per hectare by 2070. Fragmented Brazilian rainforests are likely to lose half of their original large trees in the first three decades after isolation.

“Study after study is showing the same thing,” says Lindenmayer. “Ecosystems around the world are rapidly losing large old trees.

“They are exceptionally vulnerable to intentional removal, elevated mortality, reduced recruitment, or combinations of these drivers. They are removed during logging, land clearing, agricultural intensification, fire management and for human safety. Droughts, repeated wildfires, competition with invasive plants, edge effects, air pollution, disease and insect attack can all increase tree mortality.”

According to the scientists, sustaining our stocks of large old trees will be a challenge. Appropriate population age-structures need to be maintained to ensure the perpetual supply of large old trees. This requires policies and management practices that intentionally grow such trees and reduce their mortality rates.

In addition, large old trees are more likely to persist in particular parts of landscapes such as disturbance refugia. Research is needed to determine the locations and causes of such refugia and to devise strategies to protect them. For example, timber and farming activities in managed landscapes might be concentrated where large old trees are least likely to persist or develop.

Targeted research is needed to better understand their key threats and devise strategies to counter them. Without such initiatives, these iconic organisms, and the many species that depend on them, could be lost forever.

David Salt is a researcher with the Environmental Decisions Group at the Australian National University.