Abstract
Debate continues in theoretical ecology over whether and why the scaling exponent of biomass-density (M-N) relationship varies along environmental gradients. By developing a novel geometric model with assumptions of allometric growth at the individual level and open canopy at the stand level, we propose that plant height-crown radius and canopy coverage-density relationships determine the above-ground M-N relationship in stressful environments. Results from field investigation along an aridity gradient (from eastern to western China) confirmed our model prediction and showed that the above-ground M-N scaling exponent increased with drought stress. Therefore, the 'universal' scaling exponents (-3/2 or -4/3) of the M-N relationship predicted by previous models may not hold for above-ground parts in stressful environments.