Net primary production in tropical forests: An evaluation and synthesis ofexisting field data

Information on net primary production in tropical forests is needed for the development of realistic global carbon budgets, for projecting how these e cosystems will be affected by climatic and atmospheric changes, and for eva luating eddy covariance measurements of tropical forest carbon flux. Howeve r, a review of the database commonly used to address these issues shows tha t it has serious flaws. In this paper we synthesize the data in the primary literature on NPP in old-growth tropical forests to produce a consistent d ata set on NPP for these forests. Studies in this biome have addressed only a few NPP components, all aboveground. Given the limited scope of the dire ct field measurements, we sought relationships in the existing data that al low estimation of unmeasured aspects of production from those that are more easily assessed. We found a predictive relationship between annual litterf all and aboveground biomass increment. For 39 diverse tropical forest sites , we then developed consistent, documented estimates of the upper and lower bounds around total NPP to serve as benchmarks for calibrating and validat ing biogeochemical models with respect to this biome. We developed these es timates based on existing field measurements, current understanding of abov eground consumption and biogenic volatile organic carbon emissions, and our judgment that belowground production is bounded by the range 0.2-1.2 x ANP P (aboveground NPP). Across this broad spectrum of tropical forests (dry to wet, lowland to montane, nutrient-rich to nutrient-poor soils), our estima tes of lower and upper bounds on total NPP range from 1.7 to 11.8 Mg C.ha(- 1).yr(-1) (lower bounds) and from 3.1 to 21.7 Mg C.ha(-1).yr(-1) (upper bou nds). We also showed that two relationships that have been used for estimat ing NPP (the Bray-Gorham relationship based on leaf litterfall and the Miam i model based on temperature or precipitation) are not valid for the tropic al forest biome.