self-thinning: one construct, many systems

In plant and forest ecology, self-thinning is the density-dependent mortality that occurs as a crowded even-aged cohort grows: individuals compete, the weakest die, and the surviving stand tracks a predictable boundary relating mean plant size to density, classically the "-3/2 power law" of the self-thinning line. White's 1981 The allometric interpretation of the self-thinning rule grounded that slope in how plant geometry scales, and Westoby's 1984 The self-thinning rule reviewed its form and limits across stands. For silviculture it is the basis of stand-density management, and its referent is a trajectory in size-density space traced by a thinning population, the same law found in packed animals.[1][2]

Unit

The slope and position of the mean-size-versus-density trajectory of a crowding cohort

Recovery means

Not applicable

Measured by

Estimated by plotting mean plant mass or size against surviving density over time on log axes and fitting the self-thinning boundary

The same construct governs dense cohorts of sessile marine animals: as barnacles or mussels settle crowdedly and grow, they too suffer density-dependent mortality and the surviving population moves along a size-density line set by the geometry of packing bodies into limited space. Hughes and Griffiths' 1988 Self-thinning in barnacles and mussels: the geometry of packing showed that these animal cohorts follow the self-thinning relation, with the slope arising from how the organisms fill area or volume as they enlarge. It is the identical quantitative law as in a plant stand, realised in an animal system, which is why self-thinning counts as a genuine invariant rather than a coincidence of words.[3]

Unit

The slope and position of the mean-size-versus-density trajectory of packed sessile animals

Recovery means

Not applicable

Measured by

Assessed by tracking mean body size against surviving density in settling cohorts and comparing the fitted slope to the plant self-thinning line