Estimating Aboveground Net Primary Productivity in Forest-Dominated Ecosystems

T he measurement of net primary productivity (NPP) in forest ecosystems presents a variety of challenges because of the large and complex dimensions of trees and the difficulties of quantifying several components of NPP. As summarized by Clark et al. (2001a), these methodological challenges can be overcome, and more reliable spatial and temporal comparisons can be provided, only if greater conceptual clarity and more standardized approaches to the problem are achieved. The objective of this chapter is to contribute to correction of these limitations in forest NPP measurement. Because Clark et al. (2001a) did an exemplary job with this topic, our task is made somewhat easier. We focus our attention on a variety of practical matters concerning field measurements and calculations for aboveground NPP in broadleaf deciduous, evergreen coniferous, and tropical forest biomes. We evaluate the advantages and disadvantages of contrasting approaches to key measurements and provide recommendations that should aid researchers in designing field campaigns. In general, field measurement of NPP involves quantifying two distinct sets of organic matter: (1) that which was added and retained by the plants through the measurement interval (net biomass increment) and (2) that which was produced, but lost by the plants during the same interval: