Basics of Renewable Energy Supply - Renewable Energy: Technology, Economics and Environment - page 83

sidering the seasonal and locally changing assimilation conditions, the photosyn-
thetic efficiency of different plant populations fluctuates between 0.04 % in desert
areas to up to 1.5 % in rainforests. The efficiency of agricultural crops during
their growth period is between 1 and 3 % /2-26/.
CO 2 has a lower energy content compared to organic molecules. This energy
difference is used by the plants when breathing, i.e. when decomposing the carbo-
hydrates produced during photosynthesis (dissimilation). The energy is used for
metabolic processes, and building up different components of the plant mass, such
as proteins, fats and cellulose. Respiration, which increases with an increasing
temperature level, leads to a loss in substance. Normally the gain in substance
during photosynthesis, which can only take place when light is present, is larger
than the loss in substance caused by respiration, which can take place during the
day (photorespiration) as well as during the night (dark respiration). The net pho-
tosynthesis is the result of the gross photosynthesis minus the respiration losses.
For C 3 -plants it is up to 30 and for C 4 -plants between 50 to 90 mg CO 2 per
100 cm 2 leaf surface and hour /2-27/. One reason for the higher production of
organic matter by the C 4 -plants is the lower level of photorespiration in compari-
son to the C 3 -plants. This is caused by a more effective binding of CO 2 and the
described separation of the different types of chloroplasts. Further energy losses
are caused by water evaporation leading to long-wave reflection and heat release
in order to keep the temperature at a physiologically acceptable level.

Solar irradiaton 37 TJ/(ha a)

Gross gain 24 t/(ha a)

Plant respiration

12 t/(ha a)

Herbivores 0.3 t/(ha a)

Decomposition
of foliage

0.8 t/(ha a)

Decomposition
of humus

2.8 t/(ha a)

Subterranean
storage

Of which: humus
roots

0.4 t/(ha a)

2.4 t/(ha a)

5.7 t/(ha a)

2.0 t/(ha a)

Storage above
ground

Fig. 2.48 Materials balance of a plant population for the example of a hornbeam forest of
temperate climate (see /2-26/)

Fig. 2.48 shows the gain in net biomass of an ecosystem using the example of a
hornbeam forest. By utilising 1 % of the incident solar energy, 24 t/(ha a) of bio-
mass (dry matter) are produced. Half of that amount is lost by plant respiration.
One part of the remaining biomass is added to the soil as leaves fall and is decom-