Old postcards show North American redwoods large enough for cars to drive through, thousand-year-old kauri trees in New Zealand with trunks the size of tanks, and European oaks older than the Roman empire with branches covering half a football field.
Some of these monumental trees are still alive, but scientists say that the world’s oldest and largest trees are dying out fast as climate change attracts new pests and diseases to forests, and settlements and new roads fragment ecosystems.
The latest of the botanical giants to succumb are some of the world’s oldest baobab trees that dominate the southern African savannah and can live to well over 2,500 years.
New research published in Nature Plants records that nine of the world’s oldest 13 baobabs and five of the six biggest ones, have partially or completely died in the last 12 years. The Romanian and South African researchers involved in the study speculate that repeated droughts, linked to climate change, may be responsible, but they want more research.
“I don’t know the specific cause for the decline in the South African baobabs but we are certainly losing many of our giant old trees,” said Bill Laurance, Australian ecologist and world expert on the state of large trees. “There has been a massive and definite decline in large old trees globally over the last century. We see it happening everywhere. Loggers have targeted big trees and vast forests have been razed worldwide for farmlands and urban sprawl”.
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Laurance says large old trees comprise less than 2 percent of the trees in any forest but they can contain 25 percent of the total biomass (the mass of plants and animals in the area) and are vital for the health of whole forests because they seed large areas.
But paradoxically, he said, they are the most vulnerable to climate change. “One would imagine such behemoths had survived many climatic vicissitudes over their vast lifetimes. But in a climatically changing world, their great stature is a curse. They struggle to get water up to their foliage without suffering dangerous embolisms in their vascular systems. Droughts can be fatal.”
The state of big old trees reflects the health of the planet, he said. “A world where forests have been fragmented and where climate is increasingly unpredictable is more suited to weedy, short-lived trees that can grow anywhere.
“Big old trees are organisms which have adapted to live in stable conditions. But climate change is inherently unstable. These trees live close to danger. In a world with more drought and with higher temperatures, it does not take much to push them over the edge,” he said.
Forests and climate are intrinsically linked, with tree loss both a cause and an effect of the changing climate, said Duncan Macqueen, principal researcher for natural resources with the International Institute for Environment and Development (IIED) in London and a former U.K. government forestry research official.
Macqueen, who works in Africa and southeast Asia, sees climate change affecting forests everywhere. “Less predictable rainfall, as well as more droughts and inundations, are putting trees under greater pressure and are changing the populations of pests and pollinators. The climate is now effectively throwing chaos into what are finely tuned ecological systems,” he explained.
Warmer winters have encouraged huge outbreaks of pests and diseases, he said. “Northern temperate forests with large populations of the same species are in trouble. Pests are less of a problem in tropical forests because there is more diversity of species and the chances of a single pest wiping out a whole forest there is unlikely. Diversity protects forests from disease outbreaks.”
A warming climate has introduced the southern pine beetle — which destroys pine trees — into new parts of the U.S.
Corey Lesk, a researcher at Columbia University’s Center for Climate Systems Research, says he is astonished how the geographic range of these beetles in the U.S. has expanded. In only around 20 years, he says, they have moved hundreds of miles north into New Jersey, New York and Connecticut forests, and are now infesting and killing trees.
“At present [the beetle] is over 400 km beyond its historical range,” Lesk said. “At this rate, by 2080, the climate is likely to be suitable for [southern pine beetle] expansion into 700,000 sq km of previously unaffected forests throughout the northeastern United States and into southeastern Canada, disrupting industries and ecosystems.”
North American forests have been devastated by other pests, such as mountain pine and emerald ash borer beetles. Fabrice Parisi, a program manager with the charity Tree Canada in Ottawa, said: “Warmer winters and droughts have resulted in outbreaks of emerald ash borer and mountain pine beetles. The mountain pine beetles now have two breeding seasons and can kill a tree in three years.” Parisi said the beetles are spreading past the Rocky Mountains and have moved into Alberta, Canada.
“We have this perfect storm of beetles because it’s warmer,” said Diana Six, professor of forest entomology at the University of Montana. “More and more, the cause is climate change. There have been tens of millions of trees killed by pests in western North America. [Beetles] are going to have one of the biggest impacts on our forests and our ecosystems planetwide.”
Carlos Nobre, Brazil’s leading climatologist, said the Amazon is approaching an ecological tipping point because of climate change, beyond which the water cycle may never recover and the forest would be eventually lost. Tropical forest would turn into savannah, which holds much less carbon. This would lead to an increase in carbon emissions, adding to climate change.
The combined effects of deforestation, climate change and widespread use of fire, he says, “indicate a tipping point for the Amazon system to flip to non-forest ecosystems at 20-25 percent deforestation. The basin is about 18 percent deforested now.”
The severity of both droughts and flooding over the last decade-plus, Nobre said, “suggest that the whole system is oscillating.” He also pointed to the dry season southern and eastern Amazon increasing over the last two decades as a sign of a changing climate.
In the short term, very little can be done to stop climate change devastating both large old trees and forests. But scientists are racing to find trees that are genetically resistant to individual pests. These, in time, may be bred in large quantities and replanted.
In the long term, trees will advance to wherever the climate allows. The tree line is already shifting higher up mountains and moving north into the previously treeless Arctic tundra.
With the will, there is plenty that can be done, said Tree Canada’s Parisi. “Governments and people have certainly got the message that climate change can destroy economies. A huge conservation effort is needed.”
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