Environmental Engineering Reference
In-Depth Information
pyramid of energy (a spring). C, carnivores; H, herbivores;
P, plants.
the trees (e.g. diameter of the trunk at breast height, DBH) is measured and biomass
calculated using yield tables or appropriate formulas. Underground biomass (i.e. root
biomass) is more difficult to measure, but an estimate can be arrived at by washing the
plant material from a volume of soil taken beneath the sampling plot.
Harvested material is dried in the laboratory at 80° C until it reaches a constant dry
weight, which it usually does in about twenty-four hours. Dry weight provides the best
estimate of biomass because fresh weight (or wet weight ) includes the water content,
which varies widely among plant species and even between fresh samples from the same
species.
The energy content of different plant species or plant parts also varies. Dry weight can
be converted to energy content if the calorific value of the material is known. The units
are calories per gram (cal g −1 ) or joules per gram (J g −1 ). This value can be determined in
the laboratory, using a calorimeter. Energy contents have been determined for many
species and foods; Table 22.2 gives examples of some of them.
The increase in biomass (∆B, delta B) with time (∆ t , delta t ) is a measure of net
ecosystem production (NEP). In order to use data collected by the harvest of dry matter
for calculating net primary productivity (NPP), the losses of biomass to herbivores
(grazers) and detritus must be taken into account (Figure 22.3). In equation form:
where G = grazing and D = decomposition. In controlled experimental conditions, say in
a laboratory or greenhouse, the flows D and G can usually be made very small so that:
However, under field conditions, especially in natural ecosystems, the measurement of
NPP is much more difficult because flows D and G must also be measured.
ENERGY FLOWS IN DECIDUOUS WOODLAND
The study of energy flows in ecosystems is a long-term investigation. Populations of all
plants and all animals (both vertebrate and invertebrate) are studied by sampling methods
for many years. This involves counting their
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