Suppose you take a walk in the forest one day and you look around you at all the trees and think, "That's a lot of wood, I wonder how much all these trees weigh?" Biologists ask the same question. Biomass is the total mass of all the trees (after the water has been taken out).

The term "biomass" is actually applied to three related but slightly different concepts. The total dry weight of all living organisms that can be supported at each trophic level in a food chain is known as the biomass. The term is also applied to the dry weight of all living organic matter in an entire ecosystem. Finally, people interested in alternative sources of energy use the term to apply to the total mass of all plant materials and animal wastes that can be used as fuel.

Plants use a complex chemical process known...