Enquist et al. (2007)
make some modifications to their original WBE model and restate that scaling exponents are close to 1.0 for seedlings owing to the violation of WBE assumptions in seedlings and shift to three-quarters in larger plants. However, our results indicate that scaling exponents of RA
relationships for larger trees () differ significantly from three-quarters (p
= 0.0075 and 0.0074, respectively). Exponents (0.82–0.85) and confidence intervals for respiration–mass scaling using data for larger trees from the current study closely agree with the values reported by Reich et al. (2006)
(i.e. 0.81–0.84) and Mori et al. (2010)
(i.e. 0.838–0.844) across all observations in their studies, although the latter study uses fresh biomass. Furthermore, across all datasets, scaling exponents of these relationships remain significantly different from three-quarters (p
< 0.0001 for both). Thus, our results cast doubt on the validity of the R M3/4
scaling for larger plants. In addition, there is evidence that field-grown tree saplings have a metabolic scaling slope of nearly 1.0 (Reich et al. 2006
), and that the metabolic scaling exponents of seedlings can vary during a short-term developmental period (Peng et al. in press
). Together, we conclude that no ‘canonical metabolic scaling’ exists in plants and that the scaling exponent is nearly 1.0 for saplings, and smaller than 1.0 (but still significantly larger than three-quarters) as trees age. This is consistent with the previous findings (Niklas 2004
; Cheng et al. 2009
) that the scaling exponents between tree production rates and biomass vary from 1.0 to less than 1.0 with increasing biomass and stand age.
We argue that shifting of plant scaling exponents may partially result from the increasing accumulation of necromass as plants grow. That is, metabolic rates scale isometrically with the biomass of metabolic tissue (e.g. sapwood, leaf) and accumulation of non-metabolic tissue (e.g. heartwood) would therefore lead to a decline in the mass-specific metabolic rate and the scaling slope. Such an argument is reasonably supported by the previous findings that: (i) respiration allometry shifts over ontogeny (Mori et al. 2010
; Peng et al. in press
); (ii) scaling exponent for leaf mass and total mass shifts from 1.0 to approximately three-quarters as plants grow (Enquist et al. 2007
); (iii) metabolism scales more closely with nitrogen content (expected to be more related to live tissue mass) than total mass (Reich et al. 2006
). Our data do not enable us to directly quantify the scaling relationship between plant metabolism and nitrogen content. However, some insight is gained from the scaling of forest nitrogen content and biomass. Nitrogen content scales isometrically with biomass in plants of similar age, but scales as the 0.85 power of biomass (95% CIs = 0.82–0.87) across the entire data (D. L. Cheng & K. J. Niklas 2009, unpublished data). This, combined with the result that RT
scaled as 0.85 power with MT
, yields a nearly isometrical relationship between R
(i.e. RT N0.85/0.85 = 1.00
). Therefore, our results provide some support for the notion (e.g. Ryan 1991
; Reich et al. 2006
) that plant metabolism is more restricted by nitrogen content.
Respiration rates are important to plant performance and the ecosystem carbon cycle. Progress into understanding the scaling of plant respiration rates requires additional data to explore the relationships among plant respiration rate, nitrogen content, metabolic, and non-metabolic biomass allocation patterns. It also needs theoretical interpretation of how and when plant metabolic patterns shift from isometry to allometry.