One in five of the world's plant species is facing extinction, according to one of the most comprehensive studies ever undertaken to examine the state of Earth's leafy residents.

This marks the first time the true extent of the threat to the world's estimated 380,000 plant species has been revealed.

"This study confirms what we already suspected, that plants are under threatand the main cause is human-induced habitat loss," said Stephen Hopper,director of the Kew Royal Botanical Gardens in the United Kingdom, one of the institutions involved in the study, known as the Sampled Red ListIndex.

Scientists carried out the assessments on a representative sample of the world's plants, in response to the United Nations International Yearof Biodiversity and the 2010 Biodiversity Target.

"For the first time we have a clear global picture of extinction riskto the world's known plants. This report shows the most urgent threats and the most threatened regions," Hopper said.

The report reveals that of the 4,000 plant species most carefully assessed, 22 percent are threatened.

The study took several years to complete; hundreds of scientists fromaround the world examined thousands of species, using a variety of methods from satellite imagery to computer modeling to examining archived plant samples. [See some of the species studied.]

However, about one-third of the examined species were insufficiently known to carry out a conservation assessment, demonstrating the scale of the task still facing scientists.

"In order to answer crucial questions like how fast are we losing species and why, and what we can do about it, we need to establish a baseline so that we have something against which to measure change," Hopper said.

The Sampled Red List Index for Plants is providing that baseline, he said.

Gymnosperms, the plant group that includes conifers and cycads,are the most threatened plants on Earth. Over all, the report reveals that plants are more threatened than birds, and just as vulnerable as the planet's mammals.

"The diversity of plants underpins all life on earth, so it is sobering that our own species is threatening the survival of many thousands of plant species," said Neil Brummitt, of the London Natural History Museum, another partner in the landmark report.

The study revealed that loss of habitat due to the conversion of landfor agriculture or livestock use poses the biggest threat to plant life, and tropical rain forests are the most endangered of plant ecosystems.

The study comes at a pivotal time for policymakers, as governments are gearing up to meet in Nagoya, Japan, next month to set new targets for global environmental efforts at the United Nations Biodiversity Summit.