LIGHT AND SELF-THINNING.

Monocultures of Lolium perenne were grown at high densities under various shade regimes to investigate the effects of shade on self-thinning. Unshaded populations conformed to the minus 3/2 power law when dead matter was taken into account in the calculation of mean weight per plant. In a plot of live weight per plant, populations reached a common minus 3/2 line but then deflected from it. Adherence to the minus 3/2 power law is a characteristic of shoots but not of whole plants of L. perenne. Populations increased in shoot:root ratio as they developed so that the thinning line for shoot plus root weight per plant was shallower than the thinning line for shoot weight per plant. Shoot: root ratios were generally higher in shaded than in unshaded populations. Tiller weights and numbers in experimental populations of L. perenne also conformed to the minus 3/2 power law. Populations grown in deep shade thinned along a line of slope minus 1 when sown at 'low' densities but those sown at very high densities underwent an initial period of thinning along a slope of minus 3/2, followed by a switch to a slope of minus 1. Populations at intermediate levels of shade showed a decrease in the intercept of the thinning line with increasing shade but no change in gradient. A schematic diagram is presented to help explain the effects of shade on self-thinning. [ABSTRACT FROM AUTHOR]