Do Stands Self-Thin Through a Common Point? An Additional Concept for the Self-Thinning Rule

Once the steady and ongoing mortality of individuals in a plant population has started, the widely accepted ‘self-thinning rule’ holds; this maintains that the set of progressive average sizes of the plants and their residual stocking densities bear a constant relationship to each other. This work aimed to quantify the self-thinning processes of monospecific Eucalyptus obliqua L’Hér. forest using data from each of a set of plots in Tasmania, Australia. Unexpectedly, it was found that, with or without extrapolation, the self-thinning lines of all the plots tended to intersect near a common plant average size and stocking density. This finding was extended using previously published data for monospecific forests of seven other tree species spread widely across the world, five hardwood and two softwood. All five showed the same phenomenon. Whilst this adds a new concept to the existing self-thinning rule, no immediate explanation could be offered as to why it occurs. It was concluded that it will require studying the various mechanisms that determine tree growth behavior, the competitive interactions between individuals for the resources each requires from the site (light, water, and mineral nutrients), and how these lead to the mortality of smaller, less competitively successful individuals.