Are there ever more forests on earth or not? This question is hotly debated in science. A US study now strengthens the position that sees the tree cover of the planet on a good path. Researchers around the geographer Xiao-Peng Song of the University of Maryland report in the journal "Nature", that the earth won at tree surfaces, also bald areas would decrease.

The team evaluated satellite imagery from 1982 to 2016 and analyzed how the landscape changed. The researchers divided this into three categories: bare soils, short plant growth and tree cover, which included vegetation over five meters high.

The scientists found that the tree cover increased by over seven percent in the observation period. Specifically, these are 2.24 million square kilometers. The proportion of bare areas fell accordingly by 1.16 million square kilometers.

That sounds good, but there are clear regional differences and the researchers warn that tree cover is not synonymous with forests.

In general, it can be said that the tree population in the northern hemisphere increases, while it shrinks in the southern hemisphere, so the scientists. Increases in tree cover were found mainly in subarctic, subtropical and temperate zones - and more often in the mountains.

In arid and semi-arid regions - such as Australia, China and the southwestern US - tree areas declined. Bald areas disappeared mainly in the agricultural areas of Asia.

Clearing for arable land in South America

However, the biggest losses of tree growth were in South America:

  • In Brazil, this area dwindled by 385,000 square kilometers or eight percent,
  • in Argentina by 113,000 square kilometers or 25 percent,
  • in Paraguay by 79,000 square kilometers or 34 percent.

By comparison, Germany has an area of ​​357,000 square kilometers.

In South America, the trees had to yield mainly arable land. In other regions, such as the western US, insects, forest fires, heat waves and droughts were the cause of the decline.

It is questionable whether the increase in tree areas in the northern hemisphere can compensate for the loss of rainforests in South America. Such studies mainly allowed statements about the quantity of land cover, but not about their quality, says Melvin Lippe from the Thünen Institute for International Forestry and Forest Economics, who was not involved in the study.

Overall, the analysis was clean, especially the long investigation period is a great strength. There is only one downside: "The first satellite images had a resolution of one square kilometer, which is rather inaccurate." Today, the resolution could be less than one square meter.

Less primary forests, more man-made plantations

Researchers also stress that 60 percent of land-cover changes are directly related to human activities and 40 percent to indirect impacts such as climate change. The results did not simply mean that the planet would turn green, but rather show a change in human land use. Thus, the scientists write of a now "human-dominated earth system".

Lippe also emphasizes that one can not simply speak of a global greening trend: "Basically, it is true that the earth is getting greener, but this does not say anything about how much biomass has been produced or which wood quality has grown." It can be assumed that there are fewer primary forests and more man-made plantations and degraded forest areas of lower ecological quality.

Is a tree plantation already a forest?

Just recently, biologists argued in the journal "Science" when a forest is actually a forest. A group led by Jean-François Bastin of the Food and Agriculture Organization of the United Nations (FAO) wrote that there are 4.67 million square kilometers more forest on earth than previously recorded.

Criticism from scientists around forest ecologist Daniel Griffith of Oregon State University provoked this: Bastin and his team had inaccurately defined forests, many of the newly classified forests were actually savannahs.

There was a similar dispute between the FAO and environmentalists: the FAO announced in 2015 that the loss of forests was slowing down. Since 1990, the global forest stock has "only" shrunk by three percent. Environmentalists say that the FAO also counts industrial tree plantations as forests. The falsified numbers about the forest cover of the earth and obscure the true extent of forest destruction.