Ideally, net primary production (NPP) scales allometrically with plant biomass (B) as a power law. But complications may ensue, including competition by other plants and metabolic limits by abiotic conditions. Plant adaptations may thus deflect ideal NPP scaling to nonlinear allometric relationships, leading to nonlinear curves. Importantly, nonlinear functions can have tipping points, with implications for carbon cycling, biodiversity, and human appropriation of NPP. Three alternative hypotheses for NPP scaling were tested here: ideal metabolic theory (linear logNPP-logB); competition (quadratic), and Universal Adaptive Strategy Theory (sigmoidal), which recognizes stress-tolerant, ruderal, and competitive strategies. Above-ground NPP and B data were extracted from published literature for woody and nonwoody vascular plants. Data were organized for reported levels of hierarchical organization (species' populations, assemblages, ecosystems, biomes; total N = 709). Alternative regression models were compared with the corrected Akaike information criterion (AICc). Overall pattern was evaluated with mixed effect models (where organization levels had random slopes and intercepts). Tipping points were evaluated with segmented regression.
Results/Conclusions
UAST and/or competition were supported in 5 of 6 data subsets. Overall, the UAST-based sigmoidal model was easily the most plausible (AICc weight = 1.0; residual standard error = 0.29). The general sigmoidal model was independently supported by tipping points at 38 and 360 g/m2. Adaptive strategies (and corresponding environmental conditions) appear to constrain terrestrial NPP scaling relative to the ideal predicted by metabolic theory. This conclusion represents a "maturation" of NPP allometric scaling that parallels analogous refinement of species-area relationships. Beyond theory, results predict thresholds and alternative states in NPP scaling that are important to biodiversity conservation, human resource acquisition and terrestrial responses to climate change. Diverse systems, including some grasslands, wetlands, and forests, exceed 360 g/m2; high-NPP systems extend beyond rainforests. Given the interaction of impending climate change and human population growth (~11 billion by 2100), these systems need to be sustained. Systems between 38 and 360 g/m2 will respond strongly in NPP to climate change and human land use. Results here may help refine conservation priorities.