Rule or exception?
Forests and soils are no longer the great hope they were once celebrated as in the fight against climate change. Too many studies cast doubt on expected carbon storage capacities. But an ancient forest in China doesn't play by the rules.
Venerable might be the term that comes to mind for the Dinghushan Biosphere Reserve in southern China's Guangdong Province. Original rainforests that are at least 400 years old cover the mountains and hills and provide the suitable backdrop for one of the most important Buddhist centers in the region. A million visitors flock to this first nature reserve in China every year – both a challenge and an opportunity.
But Dingshushan has another treasure to offer: Thorough research has been carried out in the forest area for decades, with a focus on the effects of global warming, accompanied by studies on the structure and biodiversity of forest ecosystems. Scientists can therefore draw on a rich pool of data that demonstrates how the forest, soil, air and living environment have changed over a much longer period of time than is usual in many places.
When evaluating such comprehensive time series, something surprising comes to light. Guoyi Zhou of the Chinese Academy of Sciences and his colleagues have now experienced this too: They found that the concentrations of organic carbon in the top twenty centimeters of the soil had increased from 1.4 to 2.35 percent since 1979. According to measurements on 230 soil samples, the amount of stored organic carbon had increased by 60 grams per square meter each year. At the same time, bulk density, a measure of the proportion of pores, had clearly decreased.
But that doesn't fit into the image of old forests at all. Because, according to the opinion so far, nothing exciting should happen here in terms of carbon storage: the amount absorbed would correspond to plus minus zero what is breathed out again, i.e. released in the form of carbon dioxide. Therefore, these forests were never discussed as sinks for carbon dioxide.
But how did this surprising result come about? "The carbon cycle processes in the soil of these forests are changing"
(Zhou et al.) Because no one has really checked what everyone was assuming beforehand, the researchers explain. But they notice something else: "The processes of the carbon cycle in the soil of these forests are changing, they react to a changing environment." So what may have been true before global warming – that old-growth forests do not store additional carbon – has been overridden by developments over the last few decades.
Is there a glimmer of hope on the horizon when it comes to carbon storage and forests? Maybe - it remains to be seen what other studies in similar systems will deliver. The annual gain of 0.035 percent is also well below the 2.6 to 2.9 percent that researchers achieved in other forest areas using nitrogen fertilization.
In any case, it's only one side of the coin. On the other hand, there is a grave warning: we have no idea what surprises climate change has in store for us. Only one thing is clear – even the traditional must be constantly re-examined.
