There are more than three trillion trees in the world

Green lungs, air filters, air conditioners – forests are real multi-talents, performing important functions for the climate: they convert carbon dioxide into oxygen, clean the air, and the crowns turn the sun’s energy into steam, which has a cooling effect on the atmosphere. And since forests store vast amounts of carbon, their destruction means that this carbon is released into the atmosphere – mainly as carbon dioxide. Intact, species-rich forests are particularly productive and bind large amounts of carbon. But just how many such forests exist around the globe and how many trees are there in total?

To answer these questions, my colleagues and I investigated a very special forest in Kyrgyzstan. Although there are few forests in the former Soviet Union, a unique ecosystem can be found in the high mountain steppes of the Tien Shan: the wild walnut and fruit forests with around 180 different tree and shrub species, where huge gnarled walnut trees grow alongside wild apples, pears and plums. These forests in the south of the country are the largest of their kind in the world.
 
How are these landscapes responding to climate change?  To what extent are they used by the local population? We’ve investigated these questions and at the same time measured and counted the trees with the help of GPS and conventional tape measures. Our mapping in this remote region is part of a major international study assessing the world’s trees, which has counted over three trillion trees around the globe – seven times more than previously thought.  

To date, estimations have been based solely on satellite images. However, these are often inaccurate since they mainly visualize the trees’ crowns, and the smaller, young trees in the lower layers are not included. Forest inventories, soil data and mapping, like our assessment of the vegetation and land use, help to complete the picture. More than 400,000 data sets from 50 countries were incorporated in the study, which has allowed us to say far more accurately how much land is covered by trees and how dense these forests are.
 
But the surprisingly high number of trees is just one side of the coin; our study has shown that forests around the globe are disappearing at an ever-increasing rate. Currently more than 15 billion trees are being burned or felled every year, which has a two-fold effect on the atmosphere: firstly, burning produces the greenhouse gas carbon dioxide, while at the same time the destroyed trees are no longer available to store carbon. 

The walnut forests in Tien Shan are also in jeopardy, as they have been intensively used since the dissolution of the Soviet Union. Kyrgyzstan is a poor country with high levels of unemployment, and trees are often cut down illegally for firewood. What’s more, sheep and goats graze under the trees – which also puts a strain on the forest since the animals mainly feed on young plants, causing the forests to over-age. To curtail overgrazing, we have suggested steps to promote more sustainable land use. Only then can we better protect these unique forests in the future.


Dr. Peter Borchardt is part of the Biogeography and Landscape Ecology working group at Universität Hamburg’s Institute of Geography. 

More information

• NATURE Article „Mapping tree density at a global scale“ 
• Project „Mensch-Umwelt-Interaktionen in Südkirgistan" (In German) 
• More about Dr. Peter Borchardt