Humans are responsible for having wiped out nearly half of the trees on earth, according to new research co-authored by a Kiwi scientist.

A study just published in the major journal Nature estimated deforestation was presently responsible for removing over 15 billion trees each year and the global number of trees has dropped 46 per cent since human civilisation.

The researchers were however also able to calculate that there are more than three trillion trees in the world - about eight times higher than previous estimates - and this equated to about 422 trees-per-person in the world, not 61 as originally thought.

The study, led by researchers from 15 countries, used satellite imagery, forest inventories and supercomputer technologies to generate a map of the world's forests.


More than 420,000 ground-sourced measurements of tree density from 50 countries and every continent, except Antarctica, were used - previous estimates relied solely on satellite imagery.

Among it was data from Landcare Research's National Vegetation Survey (NVS) Databank which contains records of approximately 77,000 vegetation survey plots, provided valuable data about New Zealand forests.

The data was originally collected for the Ministry for the Environment's national carbon monitoring programme.

Landcare Research scientist Dr Susan Wiser said the findings would be invaluable in managing climate change and guiding local, national and international reforestation efforts.

"The study also highlights the importance of our collections and databases, not just in underpinning New Zealand policy but in helping understand global patterns of tree density and how New Zealand fits in this global context."

Landcare Research general manager science Dr Peter Millard said it was important to have accurate numbers of trees across the globe in order to protect the precious resource.

"Trees are very important to us. They're a major sink for carbon dioxide and source of oxygen. If we go on losing forests we're in effect losing the lungs of the planet.

"The study is significant in that they've managed to calculate the rate in which trees have been lost," he said.

Lead author Dr Thomas Crowther, a postdoctoral fellow at the Yale School of Forestry and Environmental Studies, said the study highlighted how much more effort was needed to restore forests worldwide.

"Trees are among the most prominent and critical organisms on earth, yet we are only recently beginning to comprehend their global extent and distribution," he said.

The study found the highest tree densities in the boreal forests in the sub-arctic regions of Russia, Scandinavia and North America.

However, the largest forest areas were in the tropics, which are home to about 42 per cent of the world's trees.

Another study co-authored by Dr Wiser and published in Nature found for the first time that trees' growth did not slow as they grew older and larger - instead, it kept accelerating.

A team of 38 international researchers compiled the growth measurements of more than 673,000 trees belonging to 403 species in different temperature regions across six continents, including more than 45,000 trees across 22 species in New Zealand.

The results showed that for most tree species, mass growth rate increases continuously with tree size - in some cases, large trees appear to be adding the carbon mass equivalent of an entire smaller tree each year.

As was seen in other parts of the world, the proportion of large old trees measured in New Zealand which were found to have a high increasing mass growth rate was more than 95 per cent.

Stand-out New Zealand species included the rimu, kamahi, the silver beech, southern rata and Hall's totara.