Earth could have as many as 73,000 tree species, a new first-of-its-kind study has estimated, including some 9,200 that are yet to be discovered. Most of these undiscovered species are likely to be rare, in very low numbers and at threat from human-driven changes in land use and climate, researchers said. South America contains about 43 per cent of the world's tree species and the highest number of rare ones. The findings suggest the continent should be the focus of conservation efforts, along with global tropical and subtropical forests, which also likely harbour many rare, undiscovered species, according to researchers. The study is the outcome of a three-year international project that involved almost 150 scientists and led to the identification of approximately 40 million trees belonging to 64,000 species.