Patterns in species richness and distribution of vascular epiphytes in Chiapas, Mexico

Aim We aim to assess regional patterns in the distribution and species richness of vascular epiphytes with an emphasis on forests that differ in altitude and the amount of rainfall. Location Tropical America, in particularly the 75,000 km(2) large state of Chiapas in southern Mexico at 14.5-18.0degreesN. Chiapas is diverse in habitats with forests from sea-level to the tree-line at c. 3800 m altitude and with annual amounts of rainfall ranging from 800 to over 5000 mm. It is also one of the botanical best-explored regions in the tropics. Methods First we give an overview of epiphyte inventories to date. Such epiphyte surveys were mostly carried out on the basis of surface area or individual trees and we discuss their problematic comparison. Applying a different methodological approach, we then used 12,276 unique vascular epiphyte plant collections from Chiapas that are deposited in various botanical collections. The locality data were georeferenced and compiled in a relational data base that was analysed using a geographical information system. To compare the number of species between inventories that differed in the numbers of records, we estimated the total richness, S-Chao, at each. Results We recorded 1173 vascular epiphyte species in thirty-nine families (twenty-three angiosperms), comprising c. 14% of all confirmed plant species in the state. About half of all species were orchids (568). Ferns and bromeliads were the next species-rich groups with 244 and 101 species, respectively. Most species were found in the Montane Rain Forest and in the Central Plateau. Trees of different forest formations, rainfall regimes, altitudes and physiographical regions supported a characteristic epiphyte flora. Main conclusions We were able to confirm the presumed presence of a belt of high diversity at mid-elevations (500-2000 m) in neotropical mountains. In contrast to predictions, however, we observed a decrease in diversity when the annual amount of rainfall exceeded 2500 mm. The decrease is attributed to wind-dispersed orchids, bromeliads and Pteridophyta that may find establishment problematical under frequent downpours. In the wet but seasonal forests in Chiapas, this decrease is not compensated by plants in the animal-dispersed Araceae that are abundant elsewhere. We presume that in addition to the annual amount of rainfall, its distribution in time determines the composition of the epiphyte community.