a, C stocks. b, N stocks. The stocks are divided into above-ground tree biomass (grey fill), above-ground shrub biomass (black fill), roots (hash fill) and soils down to the underlying parent material (white fill). Above-ground stocks are shown above the x axis; below-ground stocks are shown below the x axis. The total ecosystem stock values (g m−2) are provided within each soil bar. All differences between the two ecosystems (except root C and N stocks) were statistically significant (P<0:001). Hartley, et al., 2012

By Ben Cubby, Environment Editor
18 June 2012 In a surprise finding, researchers have shown that as trees start to grow closer to the North Pole, replacing once-barren tundra, they release more greenhouse gases than they absorb. The study has global implications for measuring the speed of global warming because it had previously been thought that when forests colonise the frozen Arctic, they might act to slow climate change by soaking up extra carbon dioxide from the air. Instead, as temperatures rise and plants take root further north, the barren soils are “primed” by new growth and start to release long-held stocks of carbon. The amount of carbon activated by a change from tundra to forest outweighed that soaked up by the new trees, leading to a net increase in the amount that ends up warming the atmosphere. “We suggest that, as more productive forest communities colonise tundra, the decomposition of the large [carbon] stocks in tundra soils could be stimulated,” the researchers wrote in a paper to be released today in the journal Nature Climate Change. “Thus, counterintuitively, increased plant growth in the European Arctic could result in [carbon] being released to the atmosphere, accelerating climate change.” A team of eight researchers from Scotland and England braved frigid conditions in northern Sweden to painstakingly gather and measure soil samples from patches of forest and tundra. They carefully peeled the soil into centimetre-thick layers and measured the carbon and organic content of each, using the traces of radioactivity from atomic weapons testing in the 1950s as a marker to distinguish older carbon deposits from those which had been more recently absorbed. […] The study’s findings identify just one of the complications of recent warming that could lead to much faster temperature rises this century. They include the potential release of large amounts of heat-trapping methane from thawing Arctic soils and the large-scale release of emissions from peat in the tropics. A recent Arctic report card produced by the US National Oceanic and Atmospheric Administration concluded that the region “has undergone a fundamental shift in conditions, and that the Arctic of recent years – warmer, greener, less icy – is likely to be the new normal for the far north”. Between 1982 and 2010, the amount of vegetation in the Arctic had increased by 8 per cent, representing thousands of square kilometres of new forest and shrubland, the report said.

Carbon shown to rise as trees replace tundra