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Tropical savannas are hypothesized to be hot spots of nitrogen-fixer diversity and activity because of the high disturbance and low nitrogen characteristic of savanna landscapes. Here we compare the abundances of nitrogen-fixing and non-fixing trees in both tropical savannas and tropical forests under climatically equivalent conditions, using plant inventory studies across 566 plots in South America and Africa. A single factor, aridity, explained 19-54% of the variance in fixer abundance, and unexpectedly was more important than fire frequency, biome, and continent. Nitrogen fixers were more abundant in arid environments; as a result, African savannas, which tend to be drier, were richer in nitrogen fixers than South American savannas. Fixer abundance converged on similar levels in forests in both continents. We conclude that climate plays a greater role than fire in determining the distribution of nitrogen fixers across tropical savanna and forest biomes.