Elevational gradients in species richness and endemism of selected plant groups in the central Bolivian Andes

I analyzed the distribution of Acanthaceae, Araceae, Bromeliaceae, Cactacea e, Melastomataceae, and Pteridophyta in 62 vegetation plots of 400 m(2) alo ng an elevational transect between 500 m and 2450 m, and at a nearby lowlan d site in western Santa Cruz department, Bolivia. These groups were selecte d because they are physiognomically distinctive, have high species numbers, are comparatively easy to identify, adequately reflect overall floristic r elationships, include a wide range of life forms, and are small. The transe ct was located in the Tucumano-Boliviano biogeographic zone and included dr ought-deciduous (< 850-1000 m), mixed evergreen (850-1000 m to 1800 m), and evergreen Podocarpus-dominated (> 1800 m) forests. Elevational patterns of species richness were group-specific and probably related to the ecophysio logical properties of each group. Species richness in Pteridophyta and Mela stomataceae was correlated with moss cover (i.e., humidity), with elevation (i.e., temperatures) in Acanthaceae and epiphytic Bromeliaceae, with poten tial evapotranspiration (i.e., ecosystem productivity) in Araceae, and with light availability at ground level in terrestrial Bromeliaceae and Cactace ae. Community endemism generally increased with elevation, but showed a max imum at 1700 m for terrestrial Pteridophyta, and a nonsignificant decline f or epiphytic Bromeliaceae and Cactaceae. Endemism was higher for terrestria l than for epiphytic taxa, and was lower among Pteridophyta compared to all other groups, reflecting different dispersal ability among taxonomic and e cological groups. Elevational zonation, tested against a null-model of rand om distribution of elevational limits, revealed a significant accumulation of upper and lower elevational range boundaries at 900-1050 m and at 1500-1 850 m, corresponding to the elevational limits of the main physiognomic veg etation types.