Structural Stability and Architecture of Vines vs. Shrubs of Poison Oak, Toxicodendron Diversilobum

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Abstract
To learn whether vine stems are less structurally stable than shrub stems, I studied the architecture, anatomy, and mechanics of western poison oak (Toxicodendron diversilobum), a plant that grows as a vine when provided with external support but otherwise as a shrub. I assessed the relative structural stabilities of vines and shrubs, and the relative importance of stem morphological vs. material properties for this structural stability using the following types of information: abovegound architecture and biomass distribution, and geometric, anatomical, and material properties of the stems. Shoots that were supported by a fence (vines) had a lower proportion of their dry mass in primary stem and had more aboveground dry mass, more leaves, and a greater total length of stem than did unsupported shoots (shrubs). Both growth forms had about the same proportion of their biomass at a given relative height above the ground. Supported shoots were taller and had lower stem taper (change in radius per unit length of stem) than did unsupported shoots, and, in a common garden, had longer internodes. Xylem of supported shoots had a higher proportion of vessel lumen and lower maximum thicknesses of fiber cell walls than did xylem of unsupported shoots. Supported stems were less stiff (had lower apparent material stiffness in bending) than unsupported stems at diameter ≤3.5 cm. A simple model showed that the lower taper and lower material stiffness of vines yielded much lower flexural stiffness in vine than in shrub stems. A second model showed that simplified shrub stems could reach almost 11 m before becoming structurally unstable. Actual shrubs from the modelled environment, however never exceeded 3.2 m in height. A simplified vine modelled for the same environment would become unstable at 1.2 m, whereas vines in the field can be over 30 m tall. These findings suggest that for poison oak (1) shrubs are overbuilt and vines are underbuilt for self—support, and (2) the higher structural stability of shrubs than vines is due mainly to differences in stem geometry, not to differences in stem material properties.

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