Tree species richness on earth

A new study estimates that tree species richness is ∼73,300 which is 14% higher than the number known today.

The importance and complexity of estimating tree species richness

One of the most fundamental questions in ecology is related to the number of existing species inhabiting the Earth. However, because of limited available data, estimates of tree species diversity at a large scale still presents several technical difficulties. All these limitations have then precluded efforts to generate a global perspective.

Back in 1994, a research supposed that, by 2044, the current number of species on Earth would be known. Half of that time period has already lapsed, and this goal is still far. Even for trees, which are among the largest and most widespread organisms on the planet, we still lack a fundamental understanding of how many species exist on our planet.

Estimating the number of tree species is essential to inform, optimize, and prioritize forest conservation efforts across the globe. This knowledge can be useful to understand the evolutionary mechanisms and how they may play out in the future. It may also provide further information in the assessment of which systems may be most resilient to global change. Considering that undetected species are mostly rare and usually more vulnerable to extinction risk, a better grasp of tree biodiversity is essential to nature conservation efforts. Finally, this may allow quantifying the impacts of regional conservation efforts and planning accordingly.

New Estimates of ree species richness

A new study, based on a ground-sourced global database, estimated the number of tree species at biome, continental, and global scales. The research’s dataset included forest plots worldwide and comprised ∼38 million trees from 28,192 species. From this dataset occurrence-based, values of potential tree species richness were calculated.

The estimated number of unique tree species, adjusted to compensate for overestimations, resulted to be ∼73,300 species. At the global scale, the study highlighted that ∼9,200 tree species are yet to be discovered, which is ∼14% of the ∼64,000 species already encountered and identified.

The estimates at continental scales showed that roughly 43% of all Earth’s tree species occur in South America, followed by Eurasia (22%), Africa (16%), North America (15%), and Oceania (11%). More undiscovered species likely occur in South America than in any other continent. These findings are in general agreement with recent studies of Amazonian plant diversity, which suggested that there are many undiscovered species.

These biome-level estimates of tree species richness provided a more detailed description of the distribution of species richness within continents than previous attempts. As expected, the tropical and subtropical moist forest biome presents the highest estimates of tree species in all continents. Roughly half to two-thirds of all already known species occur in these forests on all five continents. Moreover, the hotspots of undiscovered species may largely occur in this same species-rich and undersampled regions.

Worldwide rarity of tree species

Besides the tree species richness, the study also calculated tree species rarity at continental and global scales. Africa and South America resulted to have the highest percentage of rare species compared to the total amount of estimated species in each continent. Specifically, South America accounts for the highest total number of rare species (∼8,200) followed by Eurasia (∼6,100) and Africa (∼3,900).

Overall, rare species constitutes almost a third of global tree species richness on Earth. Thus, the global number of tree species strongly depends on those rare species. These results highlight the vulnerability of global forest biodiversity to anthropogenic changes, particularly land use and climate. Indeed, these pressures disproportionately threaten the survival of rare taxa.

Comparisons across continents

To better visualise the biogeography of tree biodiversity on Earth, the study combined the data of the five continents to estimate tree species richness across landmasses. The two continents with the highest estimated numbers of tree species are North and South America, which is supported by the fact that they are interconnected by land and both present forests rich in species. Similarly, the second-highest number of shared species is between Eurasia and Oceania, which had a geological continuity through the Southeast Asian archipelago that is another hotspot of tree diversity. Besides the highest number of rare species, South America also shows the highest estimate of continental endemic species. The percentage of shared species estimated among all five continents is lower than 0.1%.

The importance of accurate tree species richness estimates

This study accelerated ecology research relying on a more extensive dataset and more advanced statistical methods than previous attempts. Nevertheless, the research settings were still imperfect. In addition to this, taxonomic identification might have represented an important obstacle to achieving accuracy in estimates of species numbers. In future estimates of tree species richness, increased sample size will provide more accurate results, especially in areas poorly investigated.

These results highlight the vulnerability of global tree species diversity to anthropogenic land-use changes and to future climate. Losing regions of forest that contain these rare species will have direct and potentially long-lasting impacts on the global species diversity and their provisioning of ecosystem services. These results demonstrate the lack of knowledge we still have about the tree species within our global forest systems, which is necessary for providing fundamental insights about the diversity of life on our planet and its needed conservation.

For more information on the study, find the paper of the research below.