Climate change gives ancient trees growth spurt

Rising temperatures are causing some of the oldest trees on Earth to grow faster, new research suggests. But the change might not benefit the climate – it may simply cause the trees to die more quickly.

Previous research (pdf) suggested that Great Basin bristlecone pines located in the mountains of western US are growing more rapidly. But the reason for the growth spurt – and whether or not it is unusual – was unclear.

A new analysis of tree rings suggests that the growth spurt is indeed unprecedented: bristlecone pines have grown faster in the past 50 years than they have in 3.7 millennia.

"This well-designed study suggests that climate change is the factor causing the growth spurt," says Greg Wiles, who reconstructs past climates using tree rings at Wooster College in Ohio.

Growth in old age

Matthew Salzer of the Laboratory of Tree-Ring Research at the University of Arizona in Tucson and colleagues took cores from ancient trees at three sites in the White Mountains of eastern California and the Ruby mountains of Nevada.

They then measured the width of 420,000 rings from pines within 150 metres of their uppermost range, and noted their age. Lastly, the team compared trends in tree ring width with modelled and reconstructed trends in local temperature.

Tree rings in the last half a century have been 0.58 millimetres wide on average, compared to 0.38 mm wide in the 4750 years preceding the period, the study found.

What's more, the growth spurt appears to be accelerating: between 2001 and 2005, tree ring widths were almost twice as great as they were before 1951, says Salzer, with an average width of 0.68 millimetres.

The changes in tree-ring width are closely correlated with temperature, the study concludes: recent warming in western US is the best explanation for last century's unprecedented growth spurt.

Human impact

"This is a cautionary tale," says Michael Mann, who uses tree rings to gain insights into past climates at Pennsylvania State University's Earth System Science Center, most famously to create the "hockey stick" graph showing an increase in temperature. "Only the human impact of increasing greenhouse gas concentrations can explain that warming."

Christof Bigler of the department of environmental sciences at the Swiss Federal Institute of Technology in Zurich agrees that the growth spurt may be a warning sign. He says that if the ancient trees grow more rapidly they may simply die younger.

But not everyone thinks that the growth spurt is a bad thing. "In the sense of reducing atmospheric CO₂ concentrations, it is actually a good thing that these old trees are kicking into life again," says Jinbao Li of the Tree Ring Lab at Columbia University in Palisades, New York.

It's also possible that the growth spurt is partly due to a rise in CO₂ levels, which may "fertilise" forests, says Iain Robertson of the School of the Environment and Society at Swansea University, UK. "Temperature only explains about a quarter of the variance in ring widths, so CO₂ fertilisation could still play a minor role."

Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0903029106

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