What’s the Tree Population?

Three trillion trees, mapped to the square kilometre.  Source: Crowther et al / Nature

Three trillion trees, mapped to the square kilometre. Source: Crowther et al / Nature

Three trillion trees live on Earth, but there would be twice as many without humans. Each year more than 15 billion trees are lost worldwide, according to a major new study. Previous estimates for the total number of trees on Earth have been much lower. The new study is important not only because it gives a higher number, but how it was produced. As well as using remote sensing data such as images taken by satellites that can classify land type, the research also integrated 429,775 ground-based assessments of tree density.

More from the study published in the journal Nature:

Humans have long used trees as fuel for cooking or smelting, fibres for clothes, timber for construction. However it is the indirect value of trees that may prove to be more important.

A solitary tree can provide a habitat to myriad species in its leaves, branches, bark and roots. But it is the effects trees have on their environments that can affect life across entire landscapes. When alive, trees can stabilise slopes and the course of rivers and streams. When dead their wood debris can form dams and so create ponds and lakes.

As well as changing water on the ground, they can alter it in the air. Transpiration is the name given to the process whereby trees (and other plants) suck up water through their roots, transport it through trunks and branches leaving the tree through tiny holes called stoma in their leaves. Stoma are crucial as they allow carbon dioxide to be absorbed, which along with water and sunlight are the ingredients with which all trees produce their food.

Only a fraction of the water absorbed is consumed during photosynthesis, with the rest evaporating out from leaf stoma. This means some trees act as massive humidifiers. Through sucking up water held in soil and releasing it tens of meters above the ground, forests can be effective cloud machines as that water vapour rises and then condenses. This is one of the reasons why it rains in the rainforest.

More here.

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