Developmental responses of subdominant grassland species to current weather conditions and their relevance for annual vegetation changes

Year-to-year vegetation changes have been observed many times in temperate grasslands. Ordinarily, variation in annual weather patterns is considered responsible for these changes. However, the exact mechanisms of vegetation dynamics have seldom been studied. In order to analyse the direct response of plants to distinct weather situations, the shoot growth rates of three subdominant grassland species were determined during three growing seasons and tested for significant relationships to meteorological variables measured simultaneously in the study site. Half of the shoots grew in the natural community with competition from neighbouring plants, For the other half, above-ground interference was avoided by regularly clipping the surrounding vegetation. The results lead to the distinction of three different impact patterns of current weather conditions on the growth of subdominant grassland species: (i) As a consequence of extraordinary weather conditions, e.g. lasting periods of drought in the summer, plants die completely or partially or pass into secondary dormancy. Such weather situations may cause quantitative or even qualitative changes in species composition by altering the density and frequency of the species involved. (ii) Major divergences from average weather conditions, such as unusually warm or cold periods in the spring, affect the growth of subdominant species and may therefore lead to quantitative annual variation of the species involved in terms of cover or biomass. (iii) Average weather conditions with slight deviations from the long-term means of the weather variables do not produce detectable direct growth responses and therefore average weather conditions are not key factors for year-to-year variations in the quantitative or qualitative performance of subdominant species