1 June 2015, Mongabay news - 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.

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

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."

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."

Source:  http://news.mongabay.com/2015/0601-rainforest-tree-diversity.html#ixzz3dNWxUTNv