Plant geometry and self-thinning.

Monocultures of Agrostemma githago, Cichorium endivium and Festuca pratensis were grown at high densities. Whilst populations of all three species followed the -3/2 power law when weight was plotted against density on logarithmic scales, the intercepts were different and fell in the order Festuca > Agrostemma > Cichorium. These differences are related to plant geometry. The mean volume of canopy available to each plant was also related to density by the -3/2 power law. However, the intercepts for a number of species on a volume-density plot on logarithmic scales all fell within one order of magnitude in comparison with 2 orders of magnitude on a weight-density plot. The biomass which could be packed into a given volume of space generally increased with canopy height, but the range was nevertheless relatively narrow (1.5-5.2 kg m-3) for the 3 species. Only part of the variation in intercept between species on a weight-density plot could be related to differences in biomass packing. Variation in the biomass per unit volume through time means that the gradient of the thinning line on a weight-density plot may be different to that on a volume plot. The -3/2 power law might be better stated in terms of canopy volume rather than weight.-from Authors