Science - October 13, 2011

Shrubs slow permafrost thaw

Tundra shrubs can obstruct permafrost thaw caused by climate change. Daan Blok draws this surprising conclusion in his thesis Shrubs in the cold, for which he received his PhD early in October.

Energy flow measurements in the Siberian tundra.
Blok researched how climate change affects the vegetation and the permafrost in the Siberian tundra. A warmer climate boosts plant growth. This extra vegetation causes more solar energy to be absorbed at the earth's surface, and some of this energy ends up in the soil. In theory, the soil should then thaw to a greater depth in summer. This would be bad for the climate as permafrost thaw releases a lot of carbon dioxide and methane which can speed up the greenhouse effect.
Effect of shade
However, this theory has to be adjusted in the light of Blok's research on the effect of the dwarf birch, the most common shrub in the Siberian tundra, on the depth of the thaw. He cleared all the dwarf birches from small test plots, monitored the thaw depth throughout two summers and measured the various energy flows between the atmosphere and the soil. The result is surprising and counter-intuitive: warming slows the thaw. This is because of the dwarf birch, thinks Blok. The shade cast by the shrub keeps the soil cool, and that cooling effect under the shrub counteracts the warming effect, because of the increase in vegetation. ‘So there is a buffer, a kind of self-regulating mechanism in nature itself', says Blok.
But this does not mean that the permafrost is not vulnerable to climate change. ‘There is bound to be a limit somewhere', says Blok, taking the wind out of the sails of the climate sceptics. ‘At some point the vegetation will stop increasing and the shade will no longer be able to compensate for the warming. And anyway, there aren't dwarf birches everywhere.' About 40 percent of tundra vegetation around the Siberian research station consists of dwarf birch. Enough, says Blok, for climate modellers to take it into account in future. ‘The system is a lot more complex than we thought.'