PARENT SESSION
Poster Session #67: Conservation Ecology.
Friday, August 9. Presentation from 8:00 AM to 9:30 AM. Exhibit Hall B & C, TCC

Floristic diversity of Mexican seasonally dry tropical forests.

Trejo, Irma^*,1, Dirzo, Rodolfo², ¹ Instituto de Geografia, UNAM, Mexico, D.F., UNAM² Instituto de Ecologia, UNAM, Mexico, D.F., Mexico

ABSTRACT- Studies of the variation in tropical plant species diversity and its relationship with environmental factors are largely based on research in tropical moist/wet forests. Seasonally dry tropical forests (SDTF), in contrast, have been poorly investigated. In this paper we present data from 20 Mexican SDTF sites sampled to describe the magnitude of floristic diversity in these forests and to address the following questions: i) To what extent is species diversity related to rainfall? ii) Are there other climatic variables that explain species variation diversity in SDTFs? iii) How does species identity vary spatially (species turnover) within the country? We found that species diversity was consistently greater (a ca. 2-fold difference) than would be expected according the sites precipitation. Rainfall did not significantly explain the variation in species diversity. Likewise, the number of dry and wet months per year was unrelated to species diversity. In contrast, a simple measure of potential evapotranspiration (Thornthwaite s Index) significantly explained the variation in species diversity. In addition to the great diversity of species per site (local diversity), species turnover was considerable: of 917 sampled species, 72% were present only in a single site and the average similarity (Sorensen's Index) among sites was only 9%. These aspects of floristic diversity and the high deforestation rates of these forests in Mexico indicate that conservation efforts should be directed to tropical forests growing in locations of low and seasonal rainfall

KEY WORDS: Seasonally tropical dry forest, species diversity, turnover, evapotranspiration