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 ch ange. We investigated differences in the behaviour of the rare calcare ous grassland species Gentiana cruciata and Gentianella germanica unde r ambient (360 mu l l(-1)) and elevated CO2 (600 mu l l(-1)) in a fiel d experiment. 2 Rosettes of G. germanica and C. cruciata were planted into grassland plots with 29 other plant species. Each of the 30 roset tes of G. germanica in a plot represented a different maternal seed fa mily, whereas G. cruciata was grown from a mixture of seeds from one f ield site. After overwintering, eight of the 12 plots were equipped wi th open-top chambers, four of which were run at ambient and four at el evated 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 yea r. Elevated CO2 reduced survival by 57% (this reduction was only margi nally significant due to large variation between plots) and seed set b y 46%. Both these effects appeared to be mediated by competition from other species since survival and seed set were negatively correlated w ith 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 repr oduction) was reduced by 56% under elevated CO2. 5 There were no signi ficant effects of elevated CO2 on leaf characters in either species. 6 The sugar concentration of the nectar of G. germanica was increased b y 36% under elevated CO2 but its composition remained unchanged. 7 Sig nificant interactions between the effects of seed family and CO2 conce ntration on demographic parameters in G. germanica indicated large gen etic variation in the response to elevated CO2, which represents evolu tionary potential. Although predictions based on mean responses are th erefore unreliable, the majority of genotypes reacted negatively to el evated CO2, suggesting that competitive exclusion and extinction of G. germanica would occur at many sites before populations could adapt to increased concentrations of CO2.