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

Arabis alpina and Arabidopsis thaliana have different stomatal development strategies in response to high altitude pressure conditions

The altitudinal gradient involves changes of the partial pressures of atmospheric gases such as CO2. This omnipresent phenomenon likely represents an evolutionary selective agent. We asked whether high altitude plant species had evolved specific response strategies to cope with high altitude pressure conditions. Plants of the high altitude species Arabis alpina and the low altitude species Arabidopsis thaliana were cultivated in growth chambers with high altitude pressure conditions (corresponding to 3000 m a.s.l.) and low altitude conditions (560 m). In both species, high altitude conditions resulted in the narrowing of stomatal aperture as well as a decrease in leaf area and weight. A. alpina produced significantly more stomata under high altitude conditions compared to low altitude conditions, while A. thaliana did not. Under low altitude conditions, however, stomatal density of A. alpina was smaller compared to A. thaliana. The increase in stomatal density of A. alpina was strongly related to the decrease in the partial pressure of CO2 under high altitude conditions. Thus, the adaptation of the high altitude plant A. alpina to high altitude pressure conditions does not consist in a genetically fixed elevated stomatal density but in a different response strategy of stomatal development to environmental factors compared to the lowland plant A. thaliana. A. alpina developed stomata largely uncoupled from other environmental factors than CO2. The increased stomatal density of A. alpina may ensure an optimal CO2 supply during the periods of favourable weather conditions for photosynthesis that are relatively rare and short in the alpine life zone