Melting permafrost in Arctic forests may cause trees to tilt to the side in ways that slow their growth, reducing the amount of carbon these “drunken forests” store.
The northern hemisphere’s boreal forest is a vast ecosystem that contains up to 40 per cent of all carbon stored on land. Rapid warming of the Arctic due to climate change is already affecting how these forests grow and thus how much carbon they store. It is also melting the permafrost beneath them, which has its own consequences for tree growth.
Thawing ground may release nutrients and water that boosts growth, and additional carbon taken up by the faster-growing forest could potentially offset the climate effects of greenhouse gases released by the thawed soil. On the other hand, the destabilised ground might also suppress growth by causing more trees to drunkenly tilt every which way.
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To get a clearer view of these patterns and what is driving them, at Wilfred Laurier University in Canada and her colleagues analysed 60 years of tree ring growth from more than a hundred sites in the Canadian Arctic. They also measured changes in permafrost at several of the sites and estimated the number of times trees have tilted at all the sites.
Previous work had shown that forests are responding differently to climate change depending on their latitudes. Satellite suggest that in warmer, lower-latitude areas, climate change appears to be impeding growth, as higher temperatures exacerbate drought. But at colder, higher latitudes, a “greening” trend indicates warming temperatures and rising CO2 levels are boosting plant growth.
Alfaro Sánchez and her colleagues found a similar trend in their tree ring analysis. Since the 1980s, growth has slowed in trees at both lower and, more recently, middle latitudes. Trees at higher latitudes have seen growth increases, although they may be reaching their limit as well.
“Now we’re seeing that these trees are hitting a tipping point,” says Alfaro Sánchez. “Basically, it’s becoming too warm even for them, and this is starting to reduce carbon storage. On top of all this, thawing permafrost is causing trees to lean, which negatively affects their growth and carbon storage, regardless of latitude.”
The researchers found that more tilting events were associated with lower growth, especially among trees at higher latitudes, where there is more permafrost. They linked these drunken forests to the thawing of permafrost underneath the “active” layer of soil where the trees grow. Tilted trees don’t grow as fast because they are forced to use more of their nutrients just to stay upright, rather than expand their trunk, says Alfaro Sánchez.
“This was kind of hidden in plain sight,” says at the NASA Jet Propulsion Laboratory in California. “It makes sense that shifting stability of the individual plants would change their ability to uptake carbon and thrive.”
While it remains unclear how widespread this effect might be, she points out that melting permafrost has implications far beyond destabilising forests. For instance, it also boosts methane emissions and may release pathogens long frozen in the soil. “All of these are really significant emergent issues, because we don’t currently know what happens when the Arctic thaws,” says Miner.
PNAS
