Heavily cut area, post-fire in the wet forests of the Australian Capital Territory, showing losses of large old trees, stream erosion and weed invasion. (Photo : David Lindenmayer/ Australian National University)

The world's oldest trees are dying at alarming rates, leading to a significant global decline of the largest living organisms on Earth.

A team of international researchers have revealed that there is a major decline in trees that are 100-300 years-old in many cities, forests, savannahs and woodlands.

"Large old trees are critical in many natural and human-dominated environments. Studies of ecosystems around the world suggest populations of these trees are declining rapidly," lead author professor David Lindenmayer, of the ARC Centre of Excellence for Environmental Decisions (CEED), and his colleagues, said in their report.

The research team found about the loss of old trees while examining records dating back to the 1860s from Swedish forestry. Moreover, a 30-year study on mountain ash forests in Australia revealed that the old trees are dying in forest fires, 10 times the normal death rate in non-fire years - caused by drought, increase in temperatures and logging.

When researchers examined old trees across the globe including trees in California's Yosemite National Park, rainforests in Brazil, forests of Europe and the Savannahs of Africa, they noticed a similar increase in the death rates of old trees.

"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," said co-author Bill Laurance of James Cook University, Australia.

Trees play a significant role in the Earth's ecosystem. Besides providing shelter for 30 percent of birds and animals, they also store huge amounts of carbon and recycle soil-rich nutrients, said the researchers.

The decline in tree population has been attributed to various factors including land clearing, logging, agricultural practices, man-made changes in fire regimes and climate change, said Jerry Franklin, third author of the study from Washington University.

Franklin and his colleagues warn that the global trees might be imperiled, just like how animals such as elephants, rhinos and tigers have declined in many parts of the world due to human activities like poaching and illegal trading business.

They urged for a better assessment of the extent of tree loss and to identify areas where the trees would have better survival chances.

The findings of the study, "Rapid Worldwide Declines of Large Old Trees", are published in the journal Science.