Climate change making Dahurian larch to grow faster in China's permafrost

Climate change in permafrost forest of north-eastern China is making Dahurian larch trees (hardy trees) to grow faster, found a study. The study was published in the Journal of Geophysical Research: Biogeosciences.

The study of growth rings from Dahurian larch in China finds that the hardy trees grew more from 2005 to 2014 than in the preceding 40 years. Also, the oldest trees have had the biggest growth spurts: trees older than 400 years grew more rapidly in those 10 years than in the past 300 years. The study's authors suspect warmer soil temperatures are fuelling the growth spurts by lowering the depth of the permafrost layer, allowing the trees' roots to expand and suck up more nutrients.

The increased growth is good for the trees in the short-term but may be disastrous for the forests in the long-term, according to the authors. As the climate continues to warm, the permafrost underneath the trees may eventually degrade and no longer be able to support the slow-growing trees. No other tree species can survive the permafrost plains this far north, so if the larch forests of northern Asia disappear, the entire ecosystem would change. "The disappearance of larch would be a disaster to the forest ecosystem in this region," said Xianliang Zhang, the lead author of the study.

Dahurian larch is Earth's northernmost tree species and its most cold-hardy: These larches are the only trees that can tolerate the frigid permafrost plains of Russia, Mongolia and northern China. Chinese locals refer to Dahurian larch as "thin-old-trees," because they grow slowly in the thin active layer of soil above the permafrost and can live for more than 400 years. Permafrost regions around the world have been thawing in recent decades due to rising temperatures, sometimes degrading into swamps and wetlands. In the study, Zhang and his colleagues analysed growth rings from more than 400 Dahurian larches in old-growth forests of north-eastern China to see how the trees are faring in a warming climate.

Tree rings allow scientists to measure how much trees grow from year to year. Much like people, trees do most of their growing while young. Dahurian larch generally grows rapidly until they become around 150 years old, at which point their growth slows. When the trees hit 300 years old, their growth basically stalls. The researchers used the width of each tree's growth rings to calculate how much area each tree gained in cross-section each year over the course of its lifetime.

The results show Dahurian larch trees grew more from 2005 to 2014 than from 1964 to 2004. Interestingly, the effect was most pronounced in the oldest trees: Trees older than 300 years grew 80 per cent more from 2005 to 2014 than in the preceding 40 years. Trees between 250 and 300 years old grew 35 per cent more during that time period, while trees younger than 250 years grew between 11 and 13 per cent more. The old trees' growth is unusual -- it's akin to a 100-year-old person suddenly getting taller, according to Zhang. The authors suspect older trees are growing more than younger trees because they have more developed root systems that can harvest resources from the soil more efficiently.

The researchers compared the trees' growth rates to climate factors like soil temperatures and precipitation data over the past 50 years to see what was causing the unusual growth. They found increased soil temperatures, especially in winter, are likely powering the growth spurts. They suspect the warmer temperatures lower the depth of the permafrost layer, providing the trees' roots more room to expand and access to more nutrients.

While this initial soil warming has benefitted Dahurian larch, further permafrost thaw could likely decrease tree growth and even cause the forest to decay, according to the authors. Dahurian larch can't survive in wet conditions, so permafrost changing to wetlands or peatlands would be detrimental to the forest as a whole, they said. "If the larch forest retreats in this region in the future, it is also not a good sign for the whole boreal forest," Zhang said. 

(With inputs from agencies.)