The allometric relationships for plant annualized biomass production (“growth”) rates, different measures of body size (dry weight and length), and photosynthetic biomass (or pigment concentration) per plant (or cell) are reported for multicellular and unicellular plants representing three algal phyla; aquatic ferns; aquatic and terrestrial herbaceous dicots; and arborescent monocots, dicots, and conifers. Annualized rates of growth G scale as the 3/4-power of body mass M over 20 orders of magnitude of M (i.e., G ∝ M ^3/4 ); plant body length L (i.e., cell length or plant height) scales, on average, as the 1/4-power of M over 22 orders of magnitude of M (i.e., L ∝ M ^1/4 ); and photosynthetic biomass M _p scales as the 3/4-power of nonphotosynthetic biomass M _n (i.e., M _p ∝ M _n ^3/4 ). Because these scaling relationships are indifferent to phylogenetic affiliation and habitat, they have far-reaching ecological and evolutionary implications (e.g., net primary productivity is predicted to be largely insensitive to community species composition or geological age).

The study of size and its biological consequences – called allometry – has fascinated biologists for centuries. Recent advances in this area of study have stimulated a renewed interest in these scaling phenomena, especially in terms of the search for mechanistic explanations that transcend mere descriptive phenomenology. These advances are reviewed in the context of plant biology. Allometric derivations are presented that predict how annual growth in total body biomass is partitioned to construct new leaf, stem, and root tissues at the level of an individual plant. Derivations are also presented to predict annual reproductive effort and to predict how the biomass of body parts changes as a function of the number of plants per unit area in communities. The predictions emerging from these derivations are then examined empirically by comparing predicted and observed scaling exponents for each relationship using a world‐wide data compendium gathered from the primary literature. These comparisons provide strong statistical support for each of the allometric predictions. This support is taken as evidence that a general unifying allometric theory for plant biology is near at hand. Nevertheless, the validation of this theory requires much additional work and raises a number of procedural and conceptual concerns that must be resolved before a single‘global’theory is accepted.

More than 5,000 measurements from 1,943 plant species were used to explore the scaling relationships among the foliar surface area and the dry, water, and nitrogen/phosphorus mass of mature individual leaves. Although they differed statistically, the exponents for the relationships among these variables were numerically similar among six species groups (ferns, graminoids, forbs, shrubs, trees, and vines) and within 19 individual species. In general, at least one among the many scaling exponents was <1.0, such that increases in one or more features influencing foliar function (e.g., surface area or living leaf mass) failed to keep pace with increases in mature leaf size. Thus, a general set of scaling relationships exists that negatively affects increases in leaf size. We argue that this set reflects a fundamental property of all plants and helps to explain why annual growth fails to keep pace with increases in total body mass across species.

The size-dependent variations of plant height L and mass M with respect to basal stem diameter D are important to the analysis of a broad range of ecological and evolutionary phenomena. Prior examination of some of the world's largest trees suggests that the scaling relationships L α D ^2/3 and M α D ^8/3 hold true, ostensibly as functional adaptations for mechanical stability. This concept remains engrained in the literature in the form of null hypotheses (or predictive models), despite numerous examples showing that the 2/3 and 8/3 rules are violated by small and intermediate-sized plants. Here, we present a growth-hydraulic model that provides more accurate and biologically realistic predictions of L and M . This model also sheds light on why L , D , and M scale differently across species and habitats as a result of differences in absolute size.