By John Upton
Trees and shrubs suck water up from the earth and into their leaves through super-thin straws in their trunks called xylem. When sunlight hits a leaf, it causes some of that water to evaporate, which in turn draws more water up from the roots through the xylem. This solar powered plumbing system keeps plants hydrated, allowing them to combine carbon dioxide from the atmosphere with water from the soil to create sugars essential for their growth. This process of photosynthesis also releases waste oxygen, which is breathed by animals and fungi.
But the system starts to fall apart when temperatures drop below freezing and the water turns to ice. Some plants produce antifreeze, and some have evolved to seize on short growing seasons when melted water is briefly available, but most simply cannot survive in the Arctic or Antarctic. Either the frozen water is too rigid to move through the xylem, or water freezes and thaws inside the plant, creating deadly bubbles that wreck its insides.
That’s the main reason that tundras are so barren — water in the soil is often locked up as ice that is useless to most plants, especially to tall trees.
As tundras heat up and ice wanes, forests are blooming in new places. Logically, one would expect these new and expanded forests to slow climate change by sucking up the atmosphere’s burgeoning carbon dioxide supplies and turning them back into oxygen.
But a new study published in the journal Climate Nature Change suggests that such logic might not prevail. That’s because the soils of tundras are home to vast reservoirs of carbon that could be stirred up and spewed back into the atmosphere by the return of the forests.
The researchers compared carbon levels in the heathlands of tundras in northern Sweden with those in nearby birch forests, which are replacing the heath as Earth heats up. They discovered that the amount of carbon stored in the low-lying heath vegetation and in the soil beneath it exceeded the amount stored in the forests’ plants and soil.
“Counterintuitively,” the researchers wrote in the paper, “increased plant growth in the European Arctic could result in C (carbon) being released to the atmosphere, accelerating climate change.”
To explain this counterintuitive result, the researchers found that frenetic plant activity in birch forests during the summer growing season triggered the decomposition of old organic matter in the soil, causing the ecosystem to leak more carbon than it captured.
Lead researcher Iain Hartley, a geography lecturer at the University of Exeter, warned that more studies are needed in a wide range of Arctic environments to determine whether expanding forests will indeed exacerbate global warming. (Unlike many other arctic tundra environments, those studied by the researchers lacked a permafrost.) But the findings point to that as a major risk.
“At the moment, the results are relevant to a particular change in vegetation,” Hartley told me in an email. “There is a lot of further study required to try and work out how carbon storage will change as forest colonises tundra in different areas of the Arctic.”