The ecosystem exergy theory is an ecosystem succession theory based on thermodynamics and hypothesizes that energydissipationincreases with ecosystem maturity. It was developed along with a number of specially designed dissipation indicators, derived from thermal remote sensing. The theory provides an interesting method for the rapid evaluation of the degree of naturalness and/or the maturity of an ecosystem, e.g. in land use impact assessment studies. However, lack of proof of the validity of the ecosystem exergy theory has limited its application. In addition, it remains unsolved whether the dissipation indicators are influenced by meteorological conditions, how they are related with each other and which dissipation indicator has the largest discriminative power.
In this study, the ecosystem exergy theory and the dissipation indicators have been evaluated using theoretical simulations and observational data. A theoretical model was applied to assess the influence of ecosystem properties and of meteorological conditions on the dissipation indicators. With respect to the observations, the dissipation indicators were calculated with two series of DAIS-imagery, obtained during the summers of 1998 and 2001 in the sandy and sandy loam regions of Flanders (Belgium), respectively. For all dissipation indicators, differences between major land use types, between different forest tree species and age classes and between agricultural crop types were analysed.
Simulations and observations showed that the dissipation indicators are highly correlated with each other. Solar exergy dissipation (SED) had the highest discriminative power and can be recommended when all land uses or ecosystems are measured simultaneously, possibly complemented with the standard deviation of surface temperature within the considered land unit. If data are not measured simultaneously, the Evaporative fraction (EF), which we suggest as a new dissipation indicator in this study, can be recommended because of its robustness to meteorological conditions and its high discriminative power. The simulations and observations elicited that dissipation indicators are very suitable for the detection of differences in energydissipation between agricultural crops, less so for the detection of differences in forest management.
The simulations supported to a large extent the main hypothesis of the ecosystem exergy theory. Higher energydissipation is obtained through increased evapotranspiration or increased surface roughness, two aspects generally related with succession in terrestrial ecosystems from non-vegetated land to forest. This was confirmed by the observations. The energydissipation increased with an increasing degree of naturalness of the major land use and with increasing forest stand age. However, the ecosystem exergy theory was not confirmed for forest succession from the pioneer to the climax phase. Poplar plantations, which are equivalent to the early successional aggradation phase, showed higher energydissipation than forests equivalent to late successional stages, in contrast with the ecosystem exergy theory.