The Scaling of Plant Height: A Comparison Among Major Plant Clades and Anatomical Grades

The scaling plant height h (m) with respect to stem diameter d (m) was determined for a total 610 species (mosses, n = 40; pteridophytes, n = 16; dicotyledonous herbs, n = 117; palms, n = 17; gymnosperms, n = 105; dicotyledonous trees, n = 315); axial length or mass vs. d was determined for the pteridophyte Psilotum nudum ; and the scaling of critical buckling height hcrit of gymnosperm and dicotyledonous trees was calculated based on the record trunk d and average Young's modulus E and density p of 33 wood species. The scaling exponents (based on least squares and reduced major axis regressions; α LS and α RMA , respectively) of these relations were compared to expected values for three scaling models: elastic similarity, α = 0·666; stress similarity, α = 0·500; and geometric similarity, α = 1·00. Least squares regression of the data from all species yielded h = 22 d0·91 ( r2 = 0·955, n = 610; α RMA = 0·93). The scaling exponent of this formula complied best with the geometric similarity model (α ≈ 1·00). However, α differed among plant clades and anatomical grades: mosses, α LS = 1·10 ( r2 = 0·974; α RMA = 1·12); pteridophytes, α LS = 1·69 ( r2 = 0·847; α RMA = 1·83; herbaceous dicotyledons, α LS = 1·26 ( r2 = 0·742; α RMA = 1·46); palms, α LS = 1·76 ( r2 = 0·940; α RMA = 1·82); gymnosperm trees, α LS = 0·430 ( r2 = 0·247; α RMA = 0·87); dicotyledonous trees, α LS = 0·488 ( r2 = 0·515; α RMA = 0·69); woody species (gymnosperm and dicotyledonous trees, α LS = 0·538 ( r2 = 0·541; α RMA = 0·73); and non-woody species, α LS = 1·29 ( r2 = 0·949; α RMA = 1·32). Based on α RMA , the interspecific scaling of woody species complied neither with the stress or elastic similarity model. Regression of hcrit yielded the formula hcrit = 97·7 d0·689 ( r2 = 0·969; α RMA = 0·70), which was interpreted to support the assumption that E/p ≈ a constant among gymnosperm and dicotyledonous woods. Intra- and interspecific variations in α RMA caution against using any of the three scaling models to predict h based on d across a broad taxonomic spectrum of species, although, on the average, α RMA < 1·0 and α RMA > 1·0 for woody and non-woody species, respectively.