Gross vs Net Primary Productivity
Primary production is the rate of accumulation of biomass. Gross primary production (GPP) is the total amount of energy produced by vegetation; some of that energy is used for cellular respiration i.e. for the growth and development of the plant. What is left over is called net primary production (NPP) and that represents the total available energy in an ecosystem the form of dry plant biomass.
Let me explain this with the concept of a “Standing Crop”, which is the total amount of biomass of living photosynthetic organisms in an ecosystem (if you think about it, there are thousands of plants doing this). Productivity refers to increases in this standing crop; it is not so much about the mass of primary producers as it is about increases in this mass. In order to measure the productivity of an ecosystem, increases in the mass of primary producers (i.e. the net amount of biomass) has to be measured. For example, if there is an old, fully grown tree, it has a set amount of biomass but its productivity is zero because no new biomass is being created. Productivity is about new matter, new energy entering the ecosystem. Additionally, as mentioned in the preceding paragraph, when measuring plant biomass, the amount of water in the vegetation is disregarded because it does not represent any energy, and hence the dry weight of the plant is measured.
NPP an important variable in the terrestrial biosphere because it is an indicator of incoming energy to the biosphere and a measure of terrestrial carbon dioxide assimilation. Hence it represents the fundamental source of energy for all organisms in an ecosystem and is a driver of the most essential of ecosystem services necessary for your, and the rest of humanity's welfare
Let me explain this with the concept of a “Standing Crop”, which is the total amount of biomass of living photosynthetic organisms in an ecosystem (if you think about it, there are thousands of plants doing this). Productivity refers to increases in this standing crop; it is not so much about the mass of primary producers as it is about increases in this mass. In order to measure the productivity of an ecosystem, increases in the mass of primary producers (i.e. the net amount of biomass) has to be measured. For example, if there is an old, fully grown tree, it has a set amount of biomass but its productivity is zero because no new biomass is being created. Productivity is about new matter, new energy entering the ecosystem. Additionally, as mentioned in the preceding paragraph, when measuring plant biomass, the amount of water in the vegetation is disregarded because it does not represent any energy, and hence the dry weight of the plant is measured.
NPP an important variable in the terrestrial biosphere because it is an indicator of incoming energy to the biosphere and a measure of terrestrial carbon dioxide assimilation. Hence it represents the fundamental source of energy for all organisms in an ecosystem and is a driver of the most essential of ecosystem services necessary for your, and the rest of humanity's welfare