Abstract: This dissertation examines patterns of plant biodiversity and distribution of understory shrubs in the pantropical and specious family, Rubiaceae, in montane rainforest in Costa Rica. Conducting vegetation surveys between 100 and 2800 m along a continuous elevational gradient in old growth tropical rainforest, I tested the predictions of competing hypotheses for species richness patterns across various environmental gradients. These hypotheses included mid domain effect, area effect, climatic variables, and environmental variables such as soil nutrients. Results indicated a peak in species richness in the mid-elevations, with rainfall best predicting richness. A test of species propagation methods and a common garden transplant design enabled an investigation into the vulnerability of understory species to increasing temperatures associated with global climate change. A factorial design identified the most effective propagation methods for cuttings across substrate, chemical applications of rooting hormone, cutting type and life form within six species of the genus Psychotria (Rubiaceae). Cutting survival and biomass accumulation were not influenced by use of rooting hormone. The effects of substrate, cutting type and growth form produced significantly different results with the greatest survival in forest soil treatments in shadehouses, using stem cuttings and small shruby growth forms. Physiological examinations of leaf temperature tolerance for species in the genus Psychotria, specifically investigating the performance of the photosystem II, revealed complex patterns of response to predicted warming. Within naturally occurring individuals along the elevational gradient and cuttings transplanted to a common garden, there were little direct effects of temperature on photosystem II, due to broad temperature tolerances. However, biomass accumulation was reduced with increased warming demonstrating an important direct effect on productivity. Species responses to key factors such as temperature and rainfall are complex and species-specific but overall, as shown here, they influence the distribution of biodiversity in montane tropical ecosystems. Accurately predicting the effects of global climate change on the future distributions of tropical species enables us to assess the vulnerability of montane rainforests to global warming and better strive towards its protection for future generations, particularly where biodiversity is high in the mid-elevations, which are poorly protected in existing reserves.