Biogeographic History and the High Beta-Diversity of Rainforest Trees in Panama

In a recent study examining the degree to which tree species composition differs among rainforest sites (

i.e., β

-diversity), Condit

et al.

(2002) found that plots in the Panama Canal Watershed separated by 50 km were more highly differentiated than plots in western Amazonia separated by nearly 1400 km. The high

β

-diversity of trees in Panama was attributed to sharp environmental gradients between the Atlantic and Pacific coasts. However, the pattern may also result from Panama's history as a land bridge over which floras from Central America and South America mixed during the Great American Biotic Interchange (GABI) roughly 3 million years ago. Under this scenario, it would be expected that wetter Panamanian forests would contain more trees of South American origin, whereas drier Panamanian forests would have more trees of Mesoamerican origin due to the historical prevalence of dry habitats in Mesoamerica. This idea was tested by quantifying the geographic distributions of 714 tree species found in three sites in the Panama Canal Watershed, which represent a gradient in annual rainfall. Nearly identical proportions of geographic representation of trees among the three sites, with species distributions of

ca

. 15% Mesoamerican, 17% South American, 9% Panama endemic, and 59% widespread. These data do not support the biotic interchange hypothesis. However, this analysis found that 433 of the 714 tree species (61%) have a cross-Andean distribution, which suggests that these tree species may be sufficiently old to have participated in the GABI.