Fixed carbon, be it from photosynthesis or chemosynthesis, is the currency used to measure primary production. The total amount of carbon fixed by primary producers in an ecosystem is called gross primary production (GPP). Not all GPP becomes living plant matter, or biomass, because primary producers use some of that energy for their own respiration and other metabolic processes. Thus net primary production (NPP), the amount of biomass incorporated into the tissues of primary producers after respiration, is the most common metric used to describe an ecosystem’s carbon fixation. This relationship is represented mathematically as

NPP = GPP – respiration

The carbon not used in respiration is then allocated to such important processes as growth, reproduction, and survival in the face of physical stress, herbivory, or diseases (see Chapter 38).