* Forty million years ago the dawn redwood was among the most abundant tree species growing in the Northern Hemisphere. * Today about 6,000 trees remain in the wild, and all of them are in south-central China. * Dozens of modern plant and animal species share a similar history, once widespread, they are now restricted to the booming Asian country. * China is home to more than 31,500 plant species, about 10 percent of the world's total. Several species, including the dawn redwood and the maidenhair treealso called ginkgo, are as old as the dinosaurs. * But 20 percent of these plants are at risk of extinction due to human pressures.