Validity of Constant-stress and Elastic-instability Principles of Stem Formation in Pinus contorta and Trifolium pratense

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Abstract
Independent studies have shown that both the constant-stress and the elastic instability models describe the form of a plant's central stem. Teleologically, the constant stress model states that a stem tapers to equalize stress produced by wind pressure along the stem. The elastic instability model states that stems and branches taper to maintain similar elasticity throughout the tree and that total tree height is limited to about one-quarter of the height at which the stem would buckle under its own weight. We investigated the ability of these two models to describe the central stems of both mature and sapling Pinus contorta var. latifolia Engelm. We also tested whether these two models are active in petiole formation by systematically defoliating Trifolium pratense L. The constant stress model adequately described stem taper in both age classes of P. contorta and the petiole form in T. pratense regardless of defoliation treatment. The elastic instability model was valid only for the mature P. contorta and for the control and perhaps for the moderately defoliated T. pratense. The constant stress model appears to be valid for both P. contorta and T. pratense while the elastic instability model is valid only for larger plants.