First, plant biomass may be determined in the same plot every year without imposing an extra disturbing factor, since the biomass removal (mowing, grazing) is a regular part of the grasslands ecological regime. Moreover, this biomass, eventually sorted into components, can be quantified very easily. Second, there is a clear methodical advantage in working in grasslands, which are usually composed of species of similar sizes and of a limited number of life forms. Third, grasslands react sooner after introduction of an experimental regime. Finally, recent changes in grassland use (abandonment, overfertilization) together with global problems such as eutrophication have resulted in the endangerment of many species, whose preservation thus becomes a priority for conservation. Understanding the mechanisms of species coexistence should provide some clues for the preservation of these rare species. There are a number of explanations of high species number in grasslands (RABOTNOV 1984) and an array of potential mechanisms for their coexistence (WILSON 1990, PALMER 1994). There are also several groups of researchers interested in problems of grassland species coexistence and this community of scientists has begun to meet semi-regularly to exchange