Previous analyses showed that tree growth in continental semi-arid forests is generally limited by the amount of soil water available during the growing season. However, researchers were unsure as to how tree growth was changing over time in response to climate change in these forests.

A recent study published in ERL by Xiuchen Wu and colleagues from Peking University, China has now addressed this issue by analysing a tree-ring network in north-western China together with associated climate and remote-sensing data.

The team found that the semi-arid forests in this area have been suffering since the early 1970s from limited growth mainly because of early (March–April) spring warming, which has led to reduced amounts of water in soil. The new study also showed that it is the long-lasting stress caused by these droughts that has greatly contributed to reducing regional tree growth in recent decades, and that it has led to a much slower growth rate in young trees.

“The dramatic early spring warming, without concurrent increases in precipitation, has reduced soil water content in continental semi-arid regions in north-western China,” said Wu, “possibly because of more water evaporating from the soil or the degradation of snow and seasonal frozen soil.”

These results have been obtained as part of the FASTER project (Forest And Steppe Ecotone Research) in Inner Asia conducted by an international team led by Hongyan Liu from Peking University. The researchers previously showed that a large number of semi-arid forests in Inner Asia have been experiencing growth decline and that many trees have even died in the eastern part of this region. These results were published in a recent issue of Global Change Biology.

Semi-arid forests are expected to become more vulnerable to changing climate in the future – especially if temperatures rise, if there are more frequent heat waves or if there are other climate-associated changes, such as increased forest fires.

“Our results could have important implications for forest management in the on-going and massive afforestation project in China – the Sanbei Forest Shelter Project,” said Wu. “But different tree species might respond to seasonal warming in quite different ways as indicated in another of our recent papers published in PLoS ONE. Therefore, it is still fairly difficult for us to provide reliable predictions for the future fate of these forests.”

Regional forest monitoring is urgently needed to improve our understanding of the mechanisms behind the decline in tree growth, and indeed forest die-off, which appears to be emerging as a global phenomenon.

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