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Latitudinal and elevational richness gradients have received much attention from ecologists but there is little consensus on underlying causes. One possible proximate cause is increased levels of species turnover, or beta diversity, in the tropics compared to temperate regions. Here, we leverage a large botanical dataset to map taxonomic and phylogenetic beta diversity, as mean turnover between neighboring 100 x 100 km cells, across the Americas and determine key climatic drivers. We find taxonomic and tip-weighted phylogenetic beta diversity is higher in the tropics, but that basal-weighted phylogenetic beta diversity is highest in temperate regions. Supporting Janzen's 'mountain passes' hypothesis, tropical mountainous regions had higher beta diversity than temperate regions for taxonomic and tip-weighted metrics. The strongest climatic predictors of turnover were average temperature and temperature seasonality. Taken together, these results suggest beta diversity is coupled to latitudinal richness gradients and that temperature is a major driver of plant community composition and change.