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Forest with wood anemones (Anemone nemorosa). Foto: Birgitta Tulin.

Today, the 21st of March, is the United Nations’ International Day for Forests.

The forest is home to about 80 percent of the world's terrestrial species.

The forests comprise over 60,000 known tree species.

Deforestation continues at an alarming rate, causing large emissions of greenhouse gases.

What makes a tree a tree?

What makes a tree a tree? Despite numerous studies, scientists are still struggling to nail down the defining traits of these tall and long-lived woody plants. The world's oldest tree, according to the article's author, is a Pinus longaeva, which is 5,067 years old. The world's largest tree in volume of total wood is Sequoiadendron giganteum, and the tallest is Sequoia sempervirens, and both of them are thousands of years old. A Populus tremuloides clone has been estimated to be 80,000 years old. In Sweden there is Old Tjikko, Picea abies, a clone that is 9,550 years old.

Nevertheless, the scientists cannot point to any particular set of genes that confer tree-ness. The author of the article suggests that it may be time to start thinking of tree as a verb, rather than a noun. Tree-ing, or tree-ifying, like swimming or flying, with no finish in sight until a lightning, a pest, or a forest machine ends the tree-ing.