Testing predictions of the energetic equivalence rule in forest communities

Abstract

As growth rate is a reasonable proxy measure of the rate of resource use per plant individual, the ‘energetic equivalence rule’ predicts that net primary productivity (the rate of biomass production per unit area, NPP) will be independent of plant biomass and maximum population density in plant communities. However, only a few studies have tested these relationships in plant communities. In this study, we investigated allometric scaling of net primary productivity (NPP) to tree biomass (M) and density (N) across a range of tree-dominated communities in China. The aim was to test the universality of the ‘energetic equivalence rule’ (i.e. whether the exponents of these relationships take a universal value of 0) in forest communities. We used both ordinary least square (OLS) and standardized major axis (SMA) regression for selected boundary points, and quantile regression (QR) to estimate the slopes of regression lines. QR, OLS and SMA regression all showed that four NPP–M and two NPP–N exponents were different from 0 across the 8 forest types. In addition, when we combined all the data to determine a larger pattern that typifies Chinese forests, five out of the six exponents of NPP–M and NPP–N relationships deviated strongly from 0. Therefore the universality of the ‘energetic equivalence rule’ does not hold for forest communities at both the regional and the national scale of China. However, the “zero” exponent seems to be a central tendency for NPP–M and NPP–N relationships in 7 out of 8 forest types. Deviation from the energetic equivalence possibly reflects multiple, unsound assumptions for “an average idealized forest” by metabolic scaling theory, as well as unaccounted-for variations of site factors (e.g. stand age and stand conditions) within forest communities. In addition, our study suggested that statistical methods should be subject to strict scrutiny in testing the ‘energetic equivalence rule’.