NPP Grassland: Consistent Worldwide Site Estimates, 1954-1990

In many grasslands, aboveground net primary productivity (ANPP) is commonly estimated by measuring peak aboveground biomass. Estimates of belowground net primary productivity (BNPP), and consequently, total net primary productivity (NPP), are more difficult. We addressed one of the three main objectives of the Global Primary Productivity Data Initiative for grassland systems - to develop simple models or algorithms to estimate missing components of total system NPP. Any estimate of BNPP requires an accounting of total root biomass, the percentage of living biomass, and annual turnover of live roots. We derived a relationship using aboveground peak biomass and mean annual temperature as predictors of belowground biomass (r2 = 0.54; P=0.01). The percentage of live material was 0.6, based on published values. We used three different functions to describe root turnover: constant, a direct function of aboveground biomass, or as a positive exponential relationship with mean annual temperature. We tested the various models against a large database of global grassland NPP and the constant turnover and direct function models were approximately equally descriptive (r2 =0.31 and 0.37), while the exponential function had a stronger correlation with the measured values (r2=0.40) and had a better fit than the other two models at the productive end of the BNPP gradient. When applied to extensive data we assembled from two grassland sites with reliable estimates of total NPP, the direct function was most effective, especially at lower productivity sites. We provide some caveats for its use in systems that lie at the extremes of the grassland gradient and stress that there are large uncertainties associated with measured and modeled estimates of BNPP.