Biomass refers to the mass of living organisms, including plants, animals, and microorganisms, or, from a biochemical perspective, cellulose, lignin, sugars, fats, and proteins. Biomass includes both the above- and belowground tissues of plants, for example, leaves, twigs, branches, boles, as well as roots of trees and rhizomes of grasses. Biomass is often reported as a mass per unit area (gm~2 or Mgha~ ) and usually as dry weight (water removed by drying). Unless otherwise specified, biomass usually includes only living material. For example, neither dead-wood nor the organic matter of soils is considered biomass, although soils do contain biomass in the form of bacteria, fungi, and meiofauna. Generally, the biomass of soils (living and dead microbes) is <5% of soil organic matter.
This article deals almost exclusively with terrestrial biomass and particularly with the biomass of plants. Forests account for 70-90% of terrestrial biomass, most of this biomass in trees. The distribution of terrestrial biomass among producers, consumers, and microbes is c. 0.90%, 0.001%, and 0.10%, globally, with considerable variation among ecosystems. Producers and microbes, for example, may account for similar fractions of biomass in some nonforest ecosystems. The estimates given here refer to the biomass of plants, or producers, only.
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