The number of tree species on Earth

Abstract

One of the most fundamental questions in ecology is how many species inhabit the Earth. However, due to massive logistical and financial challenges and taxonomic difficulties connected to the species concept definition, the global numbers of species, including those of important and well-studied life forms such as trees, still remain largely unknown. Here, based on global ground-sourced data, we estimate the total tree species richness at global, continental, and biome levels. Our results indicate that there are ∼73,000 tree species globally, among which ∼9,000 tree species are yet to be discovered. Roughly 40% of undiscovered tree species are in South America. Moreover, almost one-third of all tree species to be discovered may be rare, with very low populations and limited spatial distribution (likely in remote tropical lowlands and mountains). These findings highlight the vulnerability of global forest biodiversity to anthropogenic changes in land use and climate, which disproportionately threaten rare species and thus, global tree richness.

Keywords: biodiversity; forests; hyperdominance; rarity; richness.

Fig. 1.

The number of tree species and individuals per continent in the GFBI database. This dataset (blue points in the central map) was used for the parametric estimation and merged with the TREECHANGE occurrence-based data (purple points in the central map) to provide the estimates in this study. Green areas represent the global tree cover. GFBI consists of abundance-based records of ∼38 million trees for 28,192 species. Depicted here are some of the most frequent species recorded in each continent. Some GFBI and TREECHANGE points may overlap in the map.

Fig. 2.

Occurrence-based accumulation curves at global (A) and continental (B) scales. In A, nonparametric (interpolated) and asymptotic (extrapolated) species numbers from Chao2 (upper–lower 95% CI as shaded areas around the means; note that the CI shaded area is narrow because of the high number of sapling units), the Chao2_adj estimate for the true number of singletons (red line) vs. the number of samples (1° grid cell ∼100 × 100 km), and the number of species listed in GlobalTreeSearch (green line) are shown. In B, nonparametric (interpolated) and asymptotic (extr., extrapolated) estimates (upper–lower 95% CI as shaded areas around the means) and Chao2_adj values for the true number of singletons (dashed lines) are displayed vs. the number of samples (1° grid cell ∼100 × 100 km) within continents; the percentage of the global estimated richness in each continent is shown in the cartogram in B, Inset (total richness per continent is reported in Table 1).

Fig. 3.

Biome-level tree species richness estimates. The map shows the number of tree species estimated (S estimated from Chao2_adj) in terrestrial biomes of each continent as a color gradient from low richness (yellow) to high richness (red). More information is provided in SI Appendix, Table S2.

Fig. 4.

Species richness partitioning among continents. Estimates of the percentage of continental endemic (bold percentage values close to each continental map are based on the Chao2_adj estimator) (Materials and Methods) relative to the estimated richness per continent and shared species among continents (numbers in overlapping sets). In the center (bold percentage values at the intersection of all sets), the percentage of shared species among all five continents is shown.