Moving on from Metabolic Scaling Theory: hierarchical models of tree growth and asymmetric competition for light

Summary

1. Coomes & Allen (2009) showed that the Metabolic Scaling Theory of Plant Growth (MST-PG) has no empirical support, because the only piece of confirmatory evidence was an analysis of a small and noisy data set from which it was impossible to draw strong inferences. Our re-analyses showed that MST-PG predictions were contained within very broad 95% confidence intervals, creating an illusion of close adherence to theoretical predictions. In a response to our paper, Stark, Bentley & Enquist (2010) acknowledge these shortcomings.

2.  We reasoned that MST-PG makes inaccurate predictions because asymmetric competition for light is not included in the theoretical model. We argued that asymmetric competition has its greatest impact on small trees within populations, such that the mean size-scaling relationship of the population has a greater exponent than that of trees growing without competitors (i.e. the slope predicted by MST-PG). Stark, Bentley & Enquist (2010) appear to dispute this logic and criticize the statistical approach we developed to test our hypothesis.

3.  Here we use Bayesian hierarchical models (BHMs) to re-examine three forest data sets, and find no support for MST-PG. Based on a large data set from New Zealand, the majority of tall tree species have scaling exponents greater than predicted by MST-PG, suggesting that asymmetric competition may have influenced the scaling relationships. Parameters for many small tree and shrub species were very poorly estimated, in part because their size ranges were narrow, so these species cannot be used to argue for, or against, MST-PG.

4.  We hypothesize that scaling relationships are most affected by asymmetric competition in high-productivity forests because, as many ecologists argue, competition for light is intense in such forests (the productivity-dependent scaling hypothesis). We estimate growth scaling exponents for Nothofagus forest sites which differed in above-ground biomass (AGB) production. As predicted, scaling exponents were greater in sites with high AGB production and close to MST-PG predictions in sites with low AGB production.

5. Synthesis. Competition from taller neighbours has strong effects on tree growth, particularly in forests growing in productive locations. We continue to advocate for the inclusion of asymmetric competition into general models of tree growth, mortality, recruitment and size structure.