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Scaling relationships that describe variation in population density with body size in ecological communities, such as the thinning law in plant ecology^,,, can be explained in terms of how individuals use resources as a function of their size. Data for rates of xylem transport as a function of stem diameter show that rates of resource use in individual plants scale as approximately the 3/4 power of body mass, which is the same as metabolic rates of animals^,,,. Here we use this relationship to develop a mechanistic model for relationships between density and mass in resource-limited plants. It predicts that average plant size should scale as the −4/3 power of maximum population density, in agreement with empirical evidence and comparable relationships in animals,,, but significantly less than the −3/2 power predicted by geometric models. Our model implies that fundamental constraints on metabolic rate are reflected in …