Responses of rare calcareous grassland plants to elevated CO2: a field experiment with Gentianella germanica and Gentiana cruciata

1 Endangered plant species may be particularly vulnerable to global change. We investigated differences in the behaviour of the rare calcareous grassland species Gentiana cruciata and Gentianella germanica under ambient (360 mu l l(-1)) and elevated CO2 (600 mu l l(-1)) in a field experiment. 2 Rosettes of G. germanica and C. cruciata were planted into grassland plots with 29 other plant species. Each of the 30 rosettes of G. germanica in a plot represented a different maternal seed family, whereas G. cruciata was grown from a mixture of seeds from one field site. After overwintering, eight of the 12 plots were equipped with open-top chambers, four of which were run at ambient and four at elevated CO2 concentrations; the remaining four plots were left without chambers. 3 CO2 concentration did not significantly affect growth and survival of G. cruciata. Rosette diameter increased by 70% over 1 year. 4 Overall only 13.6% of transplanted G. germanica survived for 1 year. Elevated CO2 reduced survival by 57% (this reduction was only marginally significant due to large variation between plots) and seed set by 46%. Both these effects appeared to be mediated by competition from other species since survival and seed set were negatively correlated with total plot biomass at the time of fastest growth in June 1994 and at the time of fruit set in October, respectively. Compared with plots under ambient CO2, population growth rate (based on survival and reproduction) was reduced by 56% under elevated CO2. 5 There were no significant effects of elevated CO2 on leaf characters in either species. 6 The sugar concentration of the nectar of G. germanica was increased by 36% under elevated CO2 but its composition remained unchanged. 7 Significant interactions between the effects of seed family and CO2 concentration on demographic parameters in G. germanica indicated large genetic variation in the response to elevated CO2, which represents evolutionary potential. Although predictions based on mean responses are therefore unreliable, the majority of genotypes reacted negatively to elevated CO2, suggesting that competitive exclusion and extinction of G. germanica would occur at many sites before populations could adapt to increased concentrations of CO2.