Tropical rainforests lost an area the size of a football pitch every six seconds last year.
Global Forest Watch said Tuesday (June 2) the loss of 9.3 million acres of "primary tropical forest" in 2019 was the third-biggest decline since the turn of the century.
"Primary" forests are made up of old-growth trees, and still intact.
Cutting them down makes it harder to fight climate change, since trees absorb about a third of greenhouse gas emissions.
Agricultural expansion, wildfires, logging, mining and population growth all contribute to deforestation.
Brazil accounted for more than a third of all primary forest loss last year - 3.3 million acres.
Although few of last year's wildfires there occurred in primary forest - but instead in areas already logged for farming and cattle.
Democratic Republic of Congo and Indonesia were the other two biggest losers of primary tropical forest.
Catastrophic wildfires also made last year Australia's worst year on record - with a 560% jump in tree loss from the year before.
Researchers called on governments to include forest protection in future economic stimulus plans.