Net Primary Productivity (NPP) is the net amount of energy a plant accumulates during a specific time period. The unit in the accompanying map is kg carbon per square meter per year. Since the mass of a plant is directly related to the energy accumulated by the plant, the NPP can also be considered as the amount of mass a plant gains (or grows) over some period of time. NPP is (or better yet, how much the plant grows over a given time frame). NPP is calculated by taking the gross primary productivity (the total amount of energy/mass taken in by the plant) minus the total amount of energy/mass lost by the plant as it respires.