Net primary productivity
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The Net Primary Productivity (NPP) of an ecosystem is the rate at which it accumulates energy or biomass, excluding the energy it uses for the process of respiration. This typically corresponds to the rate of photosynthesis, minus respiration by the photosynthesizers.
This concept plays into what ecologists call trophic levels, or levels of energy transference that can be depicted as an inverted pyramid. Net primary productivity is at the base of the trophic pyramid, followed by herbivores and then primary and secondary carnivores. The second law of thermodynamics then dictates that each departure to a successive trophic level causes some of the prior useful energy to be lost in the way of heat.
The net primary productivity should not be confused with the Gross primary productivity (GPP). This refers to the rate at which an ecosystem accumulates biomass including the energy used for the process of respiration.
Both the Net Primary Productivity and Gross Primary Productivity are often measured in dry Carbon kg/m²/year. (see Table at Biomass (ecology) for NPP for differing biomes).
Measurement of NPP: A variety of field methods are used to estimate NPP. Although its definition is straightforward, field measurements used to estimate productivity vary according to investigator and biome. Field estimates rarely account for belowground productivity, herbivory, decomposition, turnover, litterfall, volatile organic compounds, root exudates, and allocation to symbiotic microorganisms. As discussed (Clark et al 2002, Olsen 2002), biomass based NPP estimates result in underestimation of NPP due to incomplete accounting. However, many field measurements correlate well to NPP. Comprehensive reviews of field methods used to estimate NPP can be found (Olson et al 2002, Clark et al 2002, Lieth and Whittaker 1975).
Most frequently, peak standing biomass is assumed to measure NPP. In systems with persistent standing litter, live biomass is commonly reported. Measures of peak biomass are more reliable in if the system is predominantly annuals, or when perennial, if there was a synchronous phenology driven by a strong seasonal climate. These methods may underestimate ANPP in grasslands by as much as 2 (temperate) to 4 (tropical) fold (Olsen et al 2002). Wetland productivity, eg of marshes and fens, is similarly measured. In Europe, annual mowing makes the annual biomass increment is evident. Repeated measures of standing live and dead biomass provide more accurate estimates of all grasslands, particularly those with large turnover, rapid decomposition, and interspecific variation in timing of peak biomass.
Measurements of forest productivity are more diverse. Biomass increment based on stand specific allometry and litterfall is considered a suitable if incomplete accounting of aboveground net primary production (ANPP) (Clark et al 2002). Field measurements used as a proxy for ANPP include annual litterfall, diameter or basal area increment (DBH or BAI), and volume increment.
Clark D A, Brown S, Kicklighter D W, Chambers J Q, Thomlinson J R and Ni J 2001 Measuring net primary production in forests: Concepts and field methods. Ecological Applications 11, 356-370.
Lieth H and Whittaker R 1975 Primary Productivity of the Biosphere. Springer-Verlag.
