How many tree species are on the Earth? Ecologists have been attempting to answer this question for nearly 300 years. The international research team, with Prof. Bogdan Jaroszewicz, the head of the Białowieża Geobotanical Station, University of Warsaw as a member, have estimated the potential tree species richness at biome, continental and global scales.
“The better understanding of a particular group of organisms, the more precise estimation of the species number. “Approximately 2 mln species have been described so far. However, it is expected there are as many as ten times more at least, reaching up to 20 mln species we have not identified yet. Some scientists consider the number of 100 mln species to be discovered and described. This number includes about 400,000 vascular plants. In 2020, according to scientists from the Royal Kew Botanic Gardens, UK, there are 391,000 vascular plants,” said Prof. Bogdan Jaroszewicz from the Białowieża Geobotanical Station, University of Warsaw.
Regarding their size, longevity and lack of mobility, vascular plants are one of the most studied group of plants. “Because of their large size, as well as their economic importance, it is obvious that trees should be one of the most well studied group of plants,” the biologist explained.
Estimated tree richness
We analysed geographical distribution of 38 mln trees that represent 28,192 species recorded on 100,000 researched plots worldwide. “On the basis of these studies we have estimated the potential tree richness at biome, continental and global scales. The ecological modelling approach revealed that the number of tree species on the Earth amounts to 73,300 whereas the literature-based dataset included 64,100 items. This implies that over 9,000 tree species still await to be discovered and described,” explained the scientist. Results of the conducted analyses have been published in “Proceedings of the National Academy of Sciences of the United States of America (PNAS)” https://doi.org/10.1073/pnas.2115329119.
Species across continents
The continent-level scale indicates the highest species richness of trees within South America (app. 43% of the total tree species number). Species richness of some regions of South America may reach 200 tree species per hectare of forest, which is five times more than all native tree species in Poland. As the analysis showed, North and South America share the highest number of common tree species. The second-highest number of shared common species is between Eurasia and Oceania, which had a geological continuity through the Southeast Asian archipelago that is another hot spot of tree diversity. Overall South America is also inhabited by the highest estimated percentage (49%) of continental endemic species, while Eurasia and Africa account together for almost another 32% of unique tree species in the world.
Rarity in forests worldwide
“We also calculated indices of tree species rarity at continental and global scales to help illuminate possible within-sample and among-sample abundance patterns. The highest percentage of species rare within samples is in Africa (38%) and South America (37%) while in North America (17%) and Eurasia (24%) it is the lowest. It is important to notice that our data do not mean that one-third of all species occur only once or twice in nature; instead, their rarity in our dataset suggests their rarity in nature but with unknown distributions of real occurrences,” as Prof. Jaroszewicz explained.
The authors of the article exploited the tree definition agreed on by IUCN’s Global Tree Specialist Group (GTSG): a woody plant with usually a single stem growing to a height of at least two metres, or if multi-stemmed, then at least one vertical stem five centimetres in diameter at breast height. “Such a definition allows for the inclusion of the majority of ligneous plants that are treated as shrubs in Poland, e.g. Corylus avellana (European hazelnut), Prunus padus (bird cherry) Juniperus communis (common juniper), which increased the final number of species,” the biologist remarked.