According to a new estimate of tree biodiversity published Monday (January 31) in PNAS, there are about 73,300 species of trees on the planet, thousands of which have yet to be described by scientists. According to the study, the vast majority of these elusive species are likely rare endemics hidden in dense tropical forests that are difficult to access and survey.

“Extensive knowledge of tree richness and diversity is key to preserving the stability and functionality of ecosystems,” lead author Roberto Cazzolla Gatti, a conservation biologist at the University of Bologna in Italy, says in a press release. “Without trees and forests, we would not have clean water, safe mountain slopes, habitat for many animals, fungi and other plants, the most biodiverse terrestrial ecosystems, sinks for our excess of carbon dioxide, depurators of our polluted air, et cetera," he tells Reuters.

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University of Oxford ecosystem scientist Yadvinder Malhi, who was not involved in the work, tells the BBC that the study “shows that tropical forests are even more diverse in their trees than we had previously imagined.”

The estimate relies on a ground-sourced global database generated by compiling data from the Global Forest Biodiversity Initiative (GFBI) and TREECHANGE. In all, the database includes information on approximately 44 million individual trees from 64,100 species in 90 countries. Those records were then used to estimate the number of unknown species using National Tsing Hua University statistician Anne Chao and colleagues’ ecological adaptation of the Good-Turing frequency estimation—an algorithm for discovering unknowns based on rare occurrences developed by codebreakers Irving Good and Alan Turing during the Second World War—reports The Guardian.

The estimated number of tree species and individuals per continent in the GFBI database
R. Cazzolla Gatti et al. / PNAS

Approximately one-third of known tree species are considered rare, meaning just one or two individuals have been documented in the surveys, co–senior author and University of Michigan and University of Minnesota ecologist Peter Reich tells Science. Because of that large proportion of rare species, the code yielded an estimate of 73,300 tree species on Earth, of which 9,200 or so have yet to be described. More than 40 percent of these undocumented species are predicted to occur in South America, a finding the authors note is in line with recent studies that suggest scientists have barely begun to describe the species in hyper-biodiverse forests such as the Amazon.

“This study reminds us how little we know about our own planet and its biosphere,” study co–senior author Jingjing Liang, an ecologist at Purdue University in Indiana, tells Reuters. “There is so much more we need to learn about the Earth so that we can better protect it and conserve natural resources for future generations,” he adds.

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The estimate helps establish a benchmark for conservation efforts, Reich adds, also speaking with Reuters. “Tree species diversity is key to maintaining healthy, productive forests, and important to the global economy and to nature,” he says, noting that “tree species are going extinct due to deforestation and climate change, and understanding the value of that diversity requires us to know what is there in the first place before we lose it.”