9,000 tree species yet to be discovered on Earth

Our planet might be hosting approximately 9,000 tree species yet to be discovered. A third of these are rare species with a population that is restricted both in terms of numbers and areas. This is one of the results of the first-ever estimate of tree species richness at the global level.

The study was published in the PNAS journal and is the outcome of a three-year international project that counted approximately 73,000 tree species currently existing on Earth. This study emphasizes the richness of terrestrial ecosystems and, at the same time, it underlines how forest biodiversity is extremely vulnerable to human-induced changes - from land use to the climate crisis - and rare species are the most at risk.

“Extensive knowledge of tree richness and diversity is key to preserving the stability and functionality of ecosystems”, explains Roberto Cazzolla Gatti who is the first author of this study and a professor at the Department of Biological, Geological and Environmental Sciences of the University of Bologna. “Until today, our data regarding wide areas of the planet was very limited and based on field-observation and lists of species covering different areas. These limitations were detrimental to a global perspective on the issue”.

However, reaching this type of knowledge is no small feat. There are many factors at play, some related to money availability, some to logistics, some to field research, and some to issues regarding the taxonomies. To overcome these hurdles, first researchers collected the most extensive databases of forest tree species. This mapping operation identified approximately 40 million trees belonging to 64,000 species. It involved 150 scientists all over the world and was carried out within the Global Forest Biodiversity Initiative (GFBI).

Based on this preliminary result, researchers performed complex statistical analyses by using artificial intelligence and the supercomputer of the Forest Advanced Computing and Artificial Intelligence (FACAI) Laboratory of Purdue University in Indiana (USA).

The number of tree species and individuals per continent in the GFBI database

Once these analyses and calculations were completed, researchers estimated that our planet has approximately 73,300 tree species, 14% more than those currently known.

“We combined individual datasets, coming from someone going out to a forest stand and measuring every single tree, into one massive global dataset of tree-level data. Counting the number of tree species worldwide is like a puzzle with pieces spreading all over the world. We, the Global Forest Biodiversity Initiative (GFBI), solved it together as a team, each sharing our own piece”, said professor Jingjing Liang, coordinator of the GFBI Purdue-Hub and co-author of the paper.

According to these results, there are still 9,000 unknown species, 40% of them could be in South America, more specifically in the two biomes composed of “grasslands, savannas, and shrublands” and “tropical and subtropical forests” of the Amazon and the Andes. Approximately 3,000 of those species are rare, endemic on the continent and populate tropical and sub-tropical areas.

“To get a reliable estimate of biodiversity, we need to pay attention to the number of rare species that are currently known, those that were found one, two, or three times during the sampling on the field”, explains Cazzolla Gatti. “Indeed, most of the species are quite common and numerous, there are a few rare ones, and even less are those that we don’t know. If many species have been observed only a few times, there will probably be many rare species that have not yet been documented".

Scientists applied this approach to the available databases, both on the continental and global scale, estimated the number of unknown tree species, and identified the areas of the world in which they are likely to be discovered.

“These results highlight the vulnerability of global forest biodiversity to anthropogenic changes, particularly land use and climate because the survival of rare taxa is disproportionately threatened by these pressures”, said Peter B. Reich, regent professor at the University of Minnesota and co-author of the study.

The study was published in the journal PNAS under the title "The number of tree species on Earth". First author of the paper is Roberto Cazzolla Gatti, professor at the Department of Biological, Geological, and Environmental Sciences of the University of Bologna.