How many tree species exist in the world? Morton Arboretum partner has a guess
How many tree species exist in the world?
London-based Botanic Gardens Conservation International, the organization that represents the world's botanic gardens and a partner of the Morton Arboretum in Lisle, has an answer to the question.
Based on more than two years of research, working with its partner botanic institutions across the world and consulting more than 500 published references, the answer is 60,065.
A scientific paper published in Journal of Sustainable Forestry, the global tree list highlights the fact that more than half of all tree species occur only in a single country, and many rare species are under threat of extinction.
Brazil has the most tree species, with 8,715 species; followed by Colombia, 5,776; and Indonesia, 5,142. Apart from the Arctic and the Antarctic, which have no trees, the region with the fewest tree species is the Nearctic region of North America, with fewer than 1,400 species.
BGCI's main reason for publishing the list is to provide a tool and baseline for people trying to conserve rare and threatened tree species. Around 10,000 tree species are known to be threatened, largely by deforestation and over-exploitation. This number includes more than 300 species that are critically endangered with fewer than 50 individual trees remaining in the wild. By compiling this information, BGCI has enabled the conservation community to put into context the scale of the task at hand - protecting the world's tree diversity from extinction.
We now know that at least 17 percent of all tree species are threatened with extinction, compared to 13 percent of birds, for example. The list provides the essential starting point for knowing how rich our tree flora is and where the global diversity hot spots are, enabling researchers, land managers, and policymakers to prioritize conservation action.
Working in collaboration since 1992, the Morton Arboretum and BGCI are partners in catalyzing tree conservation, with the arboretum acting as one of two United States hubs for the botanic garden network. In addition to contributing to this effort to ascertain the number of tree species worldwide, a team of five from the arboretum's Global Tree Conservation Program, including Murphy Westwood, director of the arboretum's conservation program, runs several collaborative tree conservation projects with BGCI and other strategic partners around the world.
One endangered tree species the program is targeting for protection is Quercus brandegeei, an evergreen oak native to the tip of southern Baja California in Mexico. Q. brandegeei trees may live up to 800 years, but experts suspect the species is struggling with a lack of seedling establishment, since no trees younger than around 100 years old have been found in the wild. Threats to the species include overgrazing by livestock and climate change. Arboretum scientists and their collaborators are conducting field studies and greenhouse experiments to understand what is preventing regeneration. Seedlings will be propagated in partner gardens in Mexico for eventual reintroduction to suitable habitats in the wild.
"The global tree list is a major achievement and represents the culmination of hundreds of years of botanical effort to catalog and understand the diversity of trees in the world," said arboretum President Gerard T. Donnelly, a member of the BGCI board of directors. "Forests provide much of the framework of the natural world. With this information, we are better able to set clear and realistic goals and prioritize conservation action."
The total number of known tree species in the world is unlikely to remain static, as around 2,000 new plants are discovered and described each year. For this reason, BGCI has published the list on a database called Global Tree Search that will be regularly updated as new tree species are identified.
"Although it seems extraordinary that it has taken us until 2017 to publish the first global, authoritative list of tree species, it is worth remembering that Global Tree Search represents a huge scientific effort encompassing the discovery, collection and describing of tens of thousands of plant species," said Paul Smith, BGCI's secretary-general and one of the paper's authors. "This is 'big science' involving the work of thousands of botanists over a period of centuries."
The Global Tree Specialist Group of the International Union for Conservation of Nature will use Global Tree Search as the baseline to assess the conservation status of all known tree species by 2020 under an initiative called the Global Tree Assessment. The Morton Arboretum is leading the effort to assess all of the world's oak species as part of this initiative, but there are still thousands of other tree species that need evaluation.
"We have the expertise assembled to carry out the survey and inventory work, but not all the funding we need," said Sara Oldfield, chairman of the Global Tree Specialist Group of IUCN and co-author of the global tree list. "We estimate that it will cost in the region of $5 million to do this work, and we will be approaching governments and other funders to support us in this endeavor. Our aim is to ensure that no tree species becomes extinct, but first we need to know which species need urgent conservation action. The Global Tree Assessment will tell us this."