NPP = Net Primary Productivity (amount of stored chemical energy the plant has to use for growth. This is directly proportional to biomass)
GPP = Gross Primary Productivity (amount of stored chemical energy the plant earns through photosynthesis)
R = Respiration (amount of energy lost through respiration, i.e. heat, lost as CO2 etc)
Best analogy is a salary. GPP is the amount of stored chemical energy the plant earns through photosynthesis. R is like income tax. The plant has to pay “respiration tax” because it can’t photosynthesis at night & not all parts of the plant are capable of photosynthesis. NPP = disposable income: what the plant has to spend after paying tax.