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Fig. 10.1. Seasonal dynamics of exergy, radiation (energy) balance, exergy and radiation efficiency coefficients (Hex and hr) and Kullback's measure (K) in the research area near Kiel. "Forest" is a 100-year-old beech forest. "Crop field" is a crop rotation field. The results shown are averaged values for the years 1991-1996.

Jan Mrz Mai Jul Sep Nov Jan Mrz Mai Jul Sep Nov

Fig. 10.1. Seasonal dynamics of exergy, radiation (energy) balance, exergy and radiation efficiency coefficients (Hex and hr) and Kullback's measure (K) in the research area near Kiel. "Forest" is a 100-year-old beech forest. "Crop field" is a crop rotation field. The results shown are averaged values for the years 1991-1996.

In principle, on both sites all parameters follow the annual dynamics of radiation balance. But there are differences, which will be used below to characterise the specific behaviour of different sites. Usually, exergy is higher than the radiation balance, but during summer, when productivity of plants is the highest, this difference becomes smaller. Correspondingly, exergy efficiency, vEx = Ex/Eln, is higher than energy efficiency, HR = R/Eln, but during summer their values almost coincide. Note that the absolute values of exergy are the result of our calculation with only two spectral frequency intervals. A calculation with more intervals could result in slightly different values.

For the crop field the values of all parameters are slightly lower than for the forest. Looking at exergy and energy balance it can be seen that the difference between the two parameters is larger in the forest. This means that the forest not only absorbs more energy from solar radiation, but it also uses this energy, more efficiently to perform work. During summer this trend becomes clearer. Lower values of the crop field efficiency (see middle part of Fig. 10.1) may be due to higher emission of long wave radiation, resulting from a higher temperature. A thin layer of vegetation usually evaporates less and therefore has a lower cooling ability (Herbst, 1997).

Consequently, incoming energy cannot be dissipated to the same extent as in the forest. Another reason for the lower exergy on the field is that the albedo of crops is higher, which results in a lower amount of available energy, i.e. energy balance. Therefore it can be stated that forests can use the incoming solar radiation more efficiently and more effectively: exergy is higher per unit of absorbed energy, whereas the total amount of absorbed energy is also higher.

All these results allow us to formulate the following teleological hypotheses:

1. Vegetation works as an information machine (Ex > R and vEx > VR in the course of almost the whole year).

2. Exergy (Ex), energy balance R) and the increment of information K) achieve maxima when the productivity of vegetation is also maximal (in the Northern Hemisphere it is usually maximal in June-July). The same statement is also true for the exergy and radiation efficiency coefficients (vEx and Vr). In this case Ex 0 R and Vex 0 VR •

Before formulating the third hypothesis, it is necessary to keep in mind that the exergy efficiency coefficient vEx is a function of two independent variables, vr and K• The total increase of vEx with growing productivity means that it is a result of the summation of two independent and opposite processes. The first is an increase of vEx with increasing K, the second is a decrease of vEx with increasing v since vEx ! VR (see Fig. 5.4). Using the terminology of the game theory we can say that in order to maximise productivity (under corresponding constraints) the vegetation uses a minimax strategy, so that

Vr K

Note that if there is the natural constraint for vr: Vr # 1, then the constraint for K is defined by arguments lying outside our formalism. The reasons for this may be related to the structure of vegetation cover. For instance, K-values (1-year averaging, see Table 10.2) for grassland and crops are lower than for forests.

Table 10.2

Mean annual values of exergy and energy balance of different sites

Table 10.2

Mean annual values of exergy and energy balance of different sites

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