Plant mass-density relationship along a moisture gradient in north-west China

1 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. 2 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. 3 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. 4 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.