The new study, compiled by 140 researchers in 100 countries, estimates the world to have 73,300 species, with 9,000 not even yet discovered.

But most undiscovered species are rare and live in tropical forests, which are being chipped away at by human activity or its consequences, including the climate crisis, deforestation and fires.

"Tree species are going extinct due to deforestation and climate change," said Professor Peter Reich from Minnesota University, a lead author on the report.

"Understanding the value of that diversity requires us to know what is there, in the first place, before we lose it.

He told Sky News we should be "very alarmed" by the state of the world's trees.

"Both deforestation and climate change - including heat waves, droughts, and wildfires - are going to cause a great many species to fail to thrive all over the world," he added.

"And this is even more important than losing a few species entirely to extinction."

Despite our reliance on trees for things like regulating weather and providing food, timber, medicine and work, we still "lack a fundamental understanding of how many species exist on our planet", researchers said.

Read more: 'It's a crime': Deforestation in Brazilian Amazon reaches highest level since 2006

This knowledge would help us assess which systems can best cope with global change, which need conserving, or how future changes will impact ecosystems, they added.

"It is also difficult to manage or conserve resources without a good understanding of what exists, so our data will help us assess where biodiversity is the most threatened... and thus help us to prioritise conservation efforts," added Professor Reich.

Getting a handle on the number of species and their location can help "identify priorities to protect tropical forests from conversion to cattle pastures, palm oil, or soy farming for animal feed, the three biggest causes of rainforest destruction today," Professor Simon Lewis from University College London, who also worked on the study, told Sky News.

The report is based on a database of more than 35 million trees in more than 100,000 sites around the world.

Researchers combined this data with statistical techniques to estimate the overall number of tree species.

South America is home to almost 17,000 species and an expected 40% of all undiscovered tree species.

Oceania and Eurasia have roughly 4,500 species each, Africa has 3,500 and North America 1,800.

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