Summary: Metabolic scaling theory predicts that the rate of resource use per unit area is independent of the average mass per individual and that the slope of the log mass–log density relationship should be −4/3. Data were obtained from plant communities along a natural gradient of moisture and latitude in north‐west China to test the generality of this theory. The allometric exponents (slopes of the log mass–log density relationship) for above‐ground biomass decreased with natural moisture levels and plant cover, deviating from the predictions of the energy equivalence theory. Allometric exponents for below‐ground and total biomass were similar among the three sites and were much closer to the predicted value of −4/3. Metabolic scaling theory may be applicable under many growth conditions, but not when restricted to above‐ground biomass under drought stress. The rate of supply of the limiting resource per unit area determines which plant parts behave according to theory.