Age-related relationship between annual productivity and body size of trees: testing the metabolic theory

Metabolic theory of ecology predicts a 3/4 power relationship between annual productivity PT and body size MT (i.e., P ∞ M3/4), which has important implications to estimates of carbon fluxes, ecosystem health, global carbon budgets, and a variety of other phenomena. To test this prediction, we examined a large dataset for Chinese forests. Such dataset covers six major forest biomes and a total of 17 forest types grown across a range of annual temperature (–6.6 to 25.2ºC), mean annual rainfall (27 to 2989 mm), elevation (10 to 4240 m a.s.l.), and stand age (3 to 350 yrs.). Reduced major axis (RMA) regression analyses were used to compare the PT versus MT scaling exponents and normalization constants (i.e., slopes and Y-intercepts of log-log linear relationships, respectively). Comparisons were made for ten different age-sequences (stand age ranges from 20 to 200 yrs). When stand age was less than 100 yrs, relationship of PT versus MT had similar scaling exponents (αRMA » 1.0), while the Y-intercepts decreased systematically. When stand age exceeded 140 yrs, scaling exponents decreased (αRMA <0.86). Both the aboveground annual productivity and aboveground body size per individual tree (PA and MA, respectively) showed the same behavior. We therefore conclude that the relationship of PT versus MT systematically declined with the stand age, and was inconsistent with the predictions of metabolic theory