Self-Thinning: The Effect of Shading on Glasshouse Populations of Silver Beet (Beta Vulgaris)

Abstract

The self-thinning rule describes the mortality of plants undergoing competitive thinning in a crowded even-aged stand as a function of accumulation of biomass or weight without reference to the passage of time. An important exception to the rule occurs at low light flux, with the slope of the thinning line changing from -1/2 (biomass basis) to 0. Silverbeet populations were grown in a glasshouse in full light and under various shade treatments. Some plants were grown during summer and others during winter. Full daylight in winter was about 55% of that in summer. The winter treatments self-thinned as if they were summer treatments subjected to that amount of extra shade. The effect of shading down to 18% of summer daylight was progressively to decrease the intercept of the thinning line on the biomass axis, without changing its slope. At 9% of summer daylight mortality was high while biomass decreased, i.e., the slope was positive; presumably this light flux was below the compensation point. Apparently, a zero slope could only be obtained as a transitional phenomenon, at a light flux immediately above the compensation point. The difference between full-daylight treatments in summer and in winter suggests that some of the reported variation between stands in the level of the thinning line may be a function of the different light climates in which the stands were grown.