Does the exception prove the rule? (Reply)

Abstract

In my News & Views article¹, I argued for the need to include factors other than body size to create a truly universal theory of plant scaling. I based my expectations for the metabolic scaling theory on Enquist's own conclusion that “unlike animal clades...all plants comply with a single allometric formula that spans 20 orders of magnitude in body mass”². Because in this recent analysis the authors applied a ¾ scaling slope across plants ranging in size from unicellular algae (< 10⁻⁷ g body mass), to duckweed (10⁻⁵ to 10⁻² g), to forest herbs and trees including giant Sequoia (10⁻¹ to 10⁷ g), I found the comparison to the data of Reich et al.³ entirely reasonable. Nevertheless, I explicitly discussed my concern about whether and how the findings of Reich et al. could extend to mature trees¹.

Main

I am glad to see the more nuanced statement of metabolic scaling theory by Enquist et al.⁴, which now explicitly introduces the idea of scale dependence in scaling slope between smaller and larger plants. This seems to be an important improvement, especially as many of Earth's plant species are smaller than adult trees, and within the size range considered by Reich et al. (< 10⁴ g). What is less clear, however, is whether the proposed change in slope is abrupt or gradual, and across what size range it takes place. However, this revision by Enquist et al. does not address the effect of nitrogen on plant respiration⁵ shown by Reich et al., an effect that is well documented theoretically and experimentally at the scale of individual leaves as well as of whole plants^5,6. It is interesting that there is even recent evidence that plant hydraulic architecture varies as a function of nitrogen supply⁷. Future investigations should seek to resolve this vexing interaction of body size, nutrient status and metabolic scaling slopes.