Research interests My research is centered on the biotic and abiotic factors determining patterns of species diversity and the distribution of species, especially in the tropics. Specific research topics include: Fern flora of Bolivia (with A.R. Smith, Berkeley, CA, USA) Systematics and biogeography of Andean Acanthaceae, Araceae, Bromeliaceae, Cactaceae, and Palmae. Ecology and biogeography of ferns in Costa Rica, Ecuador, Bolivia, Africa and the Mascarene Islands, Indonesia, and the Philippines. Causes determining the lower elevational limits of plants in mountains. Causes determining the upper timberline. Tropical deciduous forest ecology. GIS modelling of the relationship of plant distributions and climate. Influence of plant species richness on ecosystem function and stability.