The measure of a forest

Countries report how much forest they have, and the numbers are compared, published and used to judge policy. Few readers ask what counts as forest, and the answer explains some puzzling figures.

The most widely used definition counts land of at least half a hectare with trees over five metres tall covering more than ten per cent of it. Under that definition, a plantation of a single species planted for timber is forest. So is land that has been completely cleared, provided the owner intends it to regrow. A patch of natural woodland with an unusually open canopy may not be.

The consequences are predictable. A country can clear ancient woodland, plant an equal area of fast-growing timber, and report no change at all. Another can report an increase while losing every old tree it had. The number is not false; it is answering a question about tree cover when readers believe it is answering a question about forests.

This is not an argument for ignoring the figures. Tree cover matters, and a consistent definition applied everywhere is more useful than none. It is an argument for reading the definition before reading the number, which takes two minutes and changes what several national statistics appear to say.

The general version applies to any published statistic. The definition is where the argument actually happens, and it is almost always in a footnote.