Gross primary production (GPP) is the chemical energy store in plant biomass, in a given area or volume. It is the total chemical energy that producers take in by photosynthesis, before any is used.GPP is usually expressed as a rate, called gross primary productivity, in units such as kJ m−2 yr−1 or kJ ha−1 yr−1. Giving a unit area and a unit time lets different ecosystems be compared fairly.Only a small percentage of the light energy that reaches plants ends up in GPP, because:• some light is reflected from leaves or passes straight through them;
• some light is the wrong wavelength to be absorbed by chlorophyll;
• some light misses chloroplasts altogether;
• photosynthesis may be limited by another factor, such as carbon dioxide concentration or temperature.Worked example: A field receives 2 000 000 kJ m−2 yr−1 of light energy, and the crop has a GPP of 40 000 kJ m−2 yr−1.
Percentage of light energy converted = 40 000 / 2 000 000 × 100 = 2%.GPP is not all available to the rest of the ecosystem. Plants use a large part of it in their own respiration, and that energy is eventually lost as heat. What remains is net primary production.GPP is highest in warm, wet, bright ecosystems such as tropical rainforests, where limiting factors restrict photosynthesis least.
• some light is the wrong wavelength to be absorbed by chlorophyll;
• some light misses chloroplasts altogether;
• photosynthesis may be limited by another factor, such as carbon dioxide concentration or temperature.Worked example: A field receives 2 000 000 kJ m−2 yr−1 of light energy, and the crop has a GPP of 40 000 kJ m−2 yr−1.
Percentage of light energy converted = 40 000 / 2 000 000 × 100 = 2%.GPP is not all available to the rest of the ecosystem. Plants use a large part of it in their own respiration, and that energy is eventually lost as heat. What remains is net primary production.GPP is highest in warm, wet, bright ecosystems such as tropical rainforests, where limiting factors restrict photosynthesis least.