Carbon sequestration has an additional component, namely, the biology of the species involved. There has been an ongoing discussion about the role of biodiversity in the carbon sequestration process and this is not captured by a classification of species into functional types. In a pan-European field experiment it was shown (Hector et al., 1999) that the aboveground biomass of grasslands during a growing season increased linearly with species richness and richness of functional types (Fig. 8a,b). A closer inspection of the data, however, shows (Fig. 8c) two important points. First, the data show that monocultures of certain species may reach a biomass that is not different from the species richest plots. Second, the trend of biomass with species richness was quite different in various countries. While Greece, England, and Ireland showed no trend, the response was weak in Portugal, Switzerland, and Sweden, and exponential in Germany.

Revisiting the German experiment (Fig. 9), we find two types of diversity plots: one group containing Trifolium pratensc and the other group containing no Trifolium. If other legumes were present, they had a smaller effect than Trifolium. Each individual group of plots showed only a minor or no trend of biomass production with species richness. The overall average trend is exponential mainly due to the fact that there was no high-diversity plot without Trifolium.

This result indicates, that functional groups (Cramer et al., 1998) and species richness (Hector et al., 1999) may not be driving biomass production, but the presence of certain genera or species, we may call them keystone species (Bond, 1994), determines the NPP of a system.

equilibrated by import, then the change in average biomass would be equivalent to the average growth rate of this forest (after 10 years) in that period. Flowever, it cannot be expected that this forest will last for the next 100 years, because that would lead to exploitation of the timber reservoirs of other countries while this forest would reach the life expectancy of some of the species in European climate. If the mean residence time in the organic layer of soils is indeed of the same order of magnitude as the life

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