Dry mass is the mass of an organism's tissue after all the water has been removed. It is a better measure of biomass than fresh mass, because water content varies and water is not a chemical energy store.Method:
1. Collect a sample, for example all the plants in a quadrat of known area.
2. Weigh the sample in a crucible, then place it in an oven at a low temperature, around 80 °C. This is hot enough to evaporate the water but not hot enough to burn the organic material.
3. Remove and reweigh the sample at regular intervals.
4. Continue until the mass stays the same on two successive weighings. This constant mass shows that all the water has gone.The dry mass is then scaled up to give biomass per unit area of the habitat.Worked example: A 0.5 m2 quadrat of grass has a fresh mass of 210 g. After drying to constant mass, its mass is 63 g.
Biomass = 63 / 0.5 = 126 g m−2.
Percentage of the fresh mass that was water = (210 − 63) / 210 × 100 = 70%.Limitations: the method kills the organisms, so only small samples can be taken. One sample may not be representative, so several random quadrats are used and a mean is calculated. For large organisms such as trees, dry mass is estimated from measurements rather than by drying whole trees.
1. Collect a sample, for example all the plants in a quadrat of known area.
2. Weigh the sample in a crucible, then place it in an oven at a low temperature, around 80 °C. This is hot enough to evaporate the water but not hot enough to burn the organic material.
3. Remove and reweigh the sample at regular intervals.
4. Continue until the mass stays the same on two successive weighings. This constant mass shows that all the water has gone.The dry mass is then scaled up to give biomass per unit area of the habitat.Worked example: A 0.5 m2 quadrat of grass has a fresh mass of 210 g. After drying to constant mass, its mass is 63 g.
Biomass = 63 / 0.5 = 126 g m−2.
Percentage of the fresh mass that was water = (210 − 63) / 210 × 100 = 70%.Limitations: the method kills the organisms, so only small samples can be taken. One sample may not be representative, so several random quadrats are used and a mean is calculated. For large organisms such as trees, dry mass is estimated from measurements rather than by drying whole trees.