ABSTRACT
In both unmanaged ecosystems and agro-ecosystems biological productivity is expressed in terms of the rate of plant and/ or animal biomass accumulated per unit land area within a specified time period. In both it is a function of the same basic process – photosynthesis – whereby simple inorganic elements (carbon, oxygen, hydrogen, nitrogen, potassium, phosphorus) derived from the atmosphere and the soil are converted, by chlorophyll-carrying plant cells using light energy, into complex organic compounds (carbohydrates, proteins, fats). In both types of ecosystem the rate of plant growth (net primary produaivity; NPP) is dependent, on the one hand, on the efficiency with which the available solar radiation is intercepted and used; and, on the other hand, on the difference between the rate of photosynthesis (gross primary produaivity; GPP) and the rate of respiration (R) during which the energy used in plant metabolism is dissipated as heat, i.e.
NPP = GPP- R
