Net Primary Productivity

Plants use some of the energy they fix in photosynthesis for their own respiration, to power processes such as active transport and protein synthesis. This energy is eventually lost as heat. What remains is stored in new plant biomass.Net primary productivity (NPP) is the rate at which energy is stored in plant biomass, per unit area per unit time. It is the energy available to primary consumers and decomposers.NPP = GPP − Rwhere R is the energy lost in plant respiration. All three values are in kJ m⁻² year⁻¹.Worked example: a grassland has a GPP of 30 000 kJ m⁻² year⁻¹ and loses 18 000 kJ m⁻² year⁻¹ in respiration.NPP = 30 000 − 18 000 = 12 000 kJ m⁻² year⁻¹The percentage of GPP used in respiration is 18 000 ÷ 30 000 × 100 = 60%. In many ecosystems, plants use roughly half or more of their GPP in respiration.NPP varies with the age of the vegetation. A young, fast-growing forest has a high NPP, because much of what it fixes goes into new growth. In a mature forest, the large mass of living tissue respires a lot, so a much greater proportion of GPP is used up and NPP is lower.NPP is important in agriculture: crop yield depends on the NPP stored in the harvested parts. The efficiency of transfer from producers to primary consumers is usually calculated using NPP, because only the energy in plant biomass can be eaten.