In the equation GPR - R = NPP, R represents:
Productivity refers to the rate of biomass production.
It is expressed in terms of gm–2 yr –1 or (kcal m–2) yr–1 to compare the productivity of different ecosystems.
It can be divided into gross primary productivity (GPP) and net primary productivity (NPP).
Gross primary productivity of an ecosystem refers to the rate of production of organic matter by producers during photosynthesis. Plants utilise a considerable amount of GPP for respiration (R).
Net primary productivity refers to the biomass that is available for the consumption to heterotrophs (herbiviores and decomposers).
GPP – R = NPP