The radiant energy income of a slope influences its ambient temperatures and water movements, both of which are important controls on the growth behaviour, species composition and structure of its vegetation cover. Therefore, information about the radiation environments of topographically diverse areas should provide a basis for predicting the likelihood of local variations in vegetation composition and structure. From a simple model of the annual shortwave energy load of slopes of different angle and compass orientation we predict that aspect effects should be greatest at 45⚬ N/S and least in equatorial and polar regions. Other predictions concern likely physiological responses of plants to varying slope angle over the range of latitude. A literature review shows good agreement between these physically based predictions and observations of vegetation patterns in a geographically wide range of countries.