Almost four hundred billion trees belonging to 16,000 different species grow in the Amazon, according to a new estimate.
More than 100 experts analysed data from 1,170 surveys to come up with the figures, highlighting the extraordinary scale and diversity of the Amazon rain forest.
The vast size and difficult terrain of the Amazon Basin has historically restricted studies of tree communities to a local or regional level, making it difficult to see the "big picture". This lack of information about Amazonian flora on a basin-wide scale has hindered science and conservation efforts, according to experts.
"In essence, this means that the largest pool of tropical carbon on Earth has been a black box for ecologists, and conservationists don't know which Amazonian tree species face the most severe threats of extinction," said research author Dr Nigel Pitman, from the Field Museum in Chicago, US.
The new findings, published in the journal Science, provide the first estimates of the abundance, frequency and distribution of many thousands of Amazonian trees. Extrapolating the data, compiled over 10 years, suggests that greater Amazonia harbours around 390 billion individual trees, including Brazil nut, chocolate and acai berry.
The area covered encompasses the Amazon Basin (including parts of Brazil, Peru, Columbia) and the Guiana Shield (Guyana, Suriname, and French Guiana), spanning an area roughly the size of the 48 North American states. In total roughly 16,000 tree species are believed to exist in the Amazon, but half the total number of trees are thought to belong to just 227 species.
"Thus, the most common species of trees in the Amazon now not only have a number, they also have a name," said co-author Dr Hans ter Steege, from the Naturalis Biodiversity Centre in the Netherlands. "This is very valuable information for further research and policymaking."
Almost none of the most dominant species are widespread throughout the Amazon. Instead, most are abundant in a particular region or forest type, such as swamps or uplands.
The study also offers insights into the rarest tree species in the Amazon. According to a mathematical model used in the study, roughly 6,000 Amazonian tree species have populations of fewer than 1,000 individuals. This would automatically qualify them for inclusion in the International Union for Conservation of Nature (IUCN) Red List of Threatened Species - if they could be found and identified.
US ecologist and co-author Professor Miles Silman, from Wake Forest University, said: "Just like physicists' models tell them that dark matter accounts for much of the universe, our models tell us that species too rare to find account for much of the planet's biodiversity. That's a real problem for conservation, because the species at the greatest risk of extinction may disappear before we ever find them."
Why some species are hyperdominant and others are rare remains an unanswered question. Large numbers of hyperdominants, including Brazil nut, chocolate, rubber and acai berry, have been cultivated and used for millennia by human populations, the scientists note.
"There's a really interesting debate shaping up between people who think that hyperdominant trees are common because pre-1492 indigenous groups farmed them, and people who think those trees were dominant long before humans ever arrived in the Americas," said Dr Pitman.