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The distribution of southern Africa’s woody flora (N=1372 species) describes a west-to-east pattern of increasing species richness, being lowest in arid to semi-arid areas and highest in mesic to humid areas. Climate accounts for 77.8% (R2; P<0.0001) of the variation: species richness is greatest where the amount and duration of energy is optimized (not too much, not too little) and moisture maximized, and decreases as the amount or duration of energy moves above or below optimal conditions, or as moisture decreases. Given the perpetual and necessary relationship between climate and plant photosynthesis, climate provides a first-order, albeit partial, explanation for the persistence of pattern (especially latitudinal and elevational gradients) in the distribution of woody plant species richness over space and time.