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Ellison Abstract- 1989 Ellison

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Ellison, A. M. 1989. Morphological determinants of self-thinning in plant monocultures and a proposal concerning the role of self-thinning in plant evolution. Oikos 55: 349-355.


Understanding density-dependent dynamics in plant populations has been limited by the lack of a clear mechanism responsible for self-thinning in plant monocultures. Although many plant populations exhibit predictable thinning dynamics based on the self-thinning rule, experimental support for possible mechanisms generating the self-thinning rule is lacking. Here, I review the formulation of the self-thinning rule based on plant geometry, and describe an experimental test of the geometrical hypothesis of self-thinning. Geometric theory predicts that plants of disparate morphologies should thin along lines with slopes different from -3/2. I examined the thinning dynamics of three species in the family Chenopodiaceae. Salicornia europuea, Suaeda maritima, and Atriplex triangularis, whose morphologies range from leafless, sparsely-branched and short (Salicornia) to bushy (Atriplex). As predicted by the geometric theory of self-thinning, Salicornia did not self-thin. Atriplex thinned along a line with slope -1.82, and Suaeda thinned along a line of intermediate slope, -2.50. These results strongly support a gemoetric basis for self-thinning. Morphologically, Salicornia is similar to the earliest land plants, and by analogy I propose that self-thinning was not a selective force in early land plant evolution. Competition for light among plants would result in selection for increased height and canopy spread, and the widespread occurrence of the self-thinning rule in contemporary natural plant populations is hypothesized to be a result of these morphological changes.

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