"It is a very, very disturbing trend. We are talking about the loss of the biggest living organisms on the planet, of the largest flowering plants on the planet, of organisms that play a key role in regulating and enriching our world," says Professor Bill Laurance of James Cook University.
"Large old trees play critical ecological roles. They provide nesting or sheltering cavities for up to 30% of all birds and animals in some ecosystems. They store huge amounts of carbon. They recycle soil nutrients, create rich patches for other life to thrive in, and influence the flow of water within landscapes and the local climate.
"Big trees supply abundant food for numerous animals in the form of fruits, flowers, foliage and nectar. Their hollows offer nests and shelter for birds and animals like Australia's endangered Leadbeater's Possum (Gymnobelideus leadbeateri) and their loss could mean extinction for such creatures.
"In agricultural landscapes, large old trees can be focal points for vegetation restoration; they help connect the landscape by acting as stepping stones for many animals that disperse seeds and pollen," he says.
The alarming decline in old trees in so many types of forest appears to be driven by a combination of forces, including land clearing, agricultural practices, man-made changes in fire regimes, logging and timber gathering, insect attack and rapid climatic changes, says Prof. Jerry Franklin.
"For example, populations of large old pines in the dry forests of western North America declined dramatically over the last century because of selective logging, uncharacteristically severe wildfires, and other causes," he adds.
The researchers liken the global loss of big trees to the tragedy that has already befallen the world's largest mammals, such as elephant
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