One of the most important properties of an ecosystem is its productivity, which is a measure of the rate of incorporation of energy by plants per unit area per unit time. In terrestrial ecosystems, ecologists usually estimate plant production as the total annual growth—the increase in plant biomass over a year. Since productivity reflects plant growth, it is often used loosely as a measure of the organic fertility of a given area.

The flow of energy through an ecosystem starts with the fixation of sunlight by green plants during photosynthesis. Photosynthesis supplies both the energy (in the form of chemical bonds) and the organic molecules (glucose) that plants use to make other products in a process known as biosynthesis. During biosynthesis, glucose molecules are rearranged and joined together to become complex carbohydrates (such as cellulose and starch) and lipids (such as fats and plant oils...