Dry Matter Content
Biomass is usually determined on a dry matter basis, which is the weight of plant material after the moisture within the plant material has been extracted. Moisture content varies among species and during the year, according to the stage of growth, growth form (herbaceous, woody, succulent), soil moisture levels, and atmospheric humidity. Standardizing weights on a dry matter basis facilitates comparisons of biomass among sites and over time by eliminating these other confounding factors.
Dry matter content is determined by drying a sample in an oven, usually at 60° C, until a constant weight is obtained. Since moisture levels differ among species, samples of individual species may have to be dried.
Methods to determine biomass usually demand a subjective correction for moisture content in the field during data collection. With training and experience, observers are mostly able to adequately correct green plant weight to a dry matter basis. However, some situations including the presence of succulents (eg., Opuntia sp.), fresh growth, or quadrats containing mixtures of woody and herbaceous plants always present a challenge to accurately estimate for dry matter content.
References and Further Reading
Bonham, C.D. 1989. Measurements of terrestrial vegetation. John Wiley Sons, New York, NY. p 201.