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How many tree species are found in the world’s rainforests?

Rainforest in the Peruvian AmazonRainforest in the Peruvian Amazon. Photos by Rhett A. Butler.

The world’s tropical rainforests are likely home to 40,000 to 53,000 tree species, argues a paper published this week in Proceedings of the National Academy of Sciences (PNAS).

Analyzing abundance data spanning 657,000 individual tress across 11,371 species, Ferry Slik of Universiti Brunei Darusallam and 140 other researchers developed estimates for each of the world’s three major tropical regions: the Indo-Pacific, the Americas, and continental Africa. They conclude that Asia and the Americas are the most speciose when it comes to trees.

The findings come in at the high end of previous estimates for the tropics, which range from 37,000-50,000. The authors chalk up the differences to their inclusion of dry forests.

However unlike previous studies, the new research concludes that Asia is roughly on par with the Americas when it comes to tree diversity.

“Contrary to common assumption, the Indo-Pacific region was found to be as species-rich as the Neotropics, with both regions having a minimum of ∼19,000–25,000 tree species,” write the authors. “Continental Africa is relatively depauperate with a minimum of ∼4,500–6,000 tree species. Very few species are shared among the African, American, and the Indo-Pacific regions.”

CHART / GRAPH showing the Estimated number of tropical tree species according to Slik et al. (2015)
Estimated number of tropical tree species according to Slik et al. (2015)

The study provides an explanation for tree diversity in the regions, noting that topography, geography, and geological history are important factors.

“[Indo-Pacific and the Americas] show similar rates of species turnover for a given increase in geographical distance between locations. This result contradicts the widely held view that the Neotropics are the most diverse and species-rich region for tropical trees,” the write. “This underestimation of Indo-Pacific tree species richness, and our inclusion of dry as well a moist and wet forests, may explain why some of the previous estimates (7, 8) are lower than ours. Nevertheless, the high species richness in the Indo-Pacific is understandable given the highly variable topography, complex geological history, steep environmental gradients, past and ongoing merging of several contrasting floras from Madagascar, India, Southeast Asia, and New Guinea–Australia, as well as the large current and time-integrated forest area.”

“[Africa] shows comparatively low species turnover,” they continue. “The differences in species richness and spatial turnover, when comparing continental Africa with the other tropical regions, cannot be explained solely by Africa’s smaller forest area or lower environmental variability. Rather, these disparities further support the hypothesis that African forests have experienced severe extinction events due to repeated shrinkage of forest area during the Pleistocene. When these forests expanded to their present size, they could only be repopulated by a severely depleted species pool derived from a limited number of refugia. In contrast, tropical America retained considerable forest cover and equatorial forests of the Indo-Pacific may even have expanded during the same period.”

But while Africa lags the other regions in tree diversity, it still far outpaces Europe, whose temperate forests have only 124 species, and North America, which has less than 1,000 species. Some rainforests may have more than 400 tree species per hectare.

However diversity doesn’t correlate to abundance, say the authors.

“Our study shows that most tree species are extremely rare, meaning that they may be under serious risk of extinction at current deforestation rates.”

CITATION: J.W. Ferry Slik et al. (2015) An estimate of the number of tropical tree species. PNAS. 10.1073/pnas.1423147112

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