Floristic diversity of Mexican seasonally dry tropical forests

Studies of the variation in tropical plant species diversity and itsrelationship with environmental factors are largely based on research intropical moist/wet forests. Seasonally dry tropical forests (SDTFs), incontrast, have been poorly investigated. In this paper we present data from 20Mexican SDTF sites sampled to describe the magnitude of floristic diversity inthese forests and to address the following questions: (i) to what extent isspecies diversity related to rainfall? (ii) Are there other climatic variablesthat explain variation in species diversity in SDTFs? (iii) How does speciesidentity vary spatially (species turnover) within the country? We found thatspecies diversity was consistently greater (a ca. twofold difference) than wouldbe expected according to the sites' precipitation. Rainfall did notsignificantly explain the variation in species diversity. Likewise, the numberof 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 thegreat diversity of species per site (local diversity), species turnover wasconsiderable: of a total of 917 sampled species, 72% were present only in asingle site and the average similarity (Sorensen's index) among sites wasonly 9%. These aspects of floristic diversity and the high deforestation ratesof these forests in Mexico indicate that conservation efforts should be directedto tropical forests growing in locations of low and seasonal rainfall.