Relationships between population and habitat characteristics and reproduction of the rare Gentiana pneumonanthe L

We investigated the relationships among a number of population and site characteristics and the ovule and seed production and seed set in 17 populations of the rare, self-compatible, bumblebee-pollinated perennial Gentiana pneumonanthe L. in the Netherlands. Population size and offspring heterozygosity were used as population properties; grass and heath cover, soil concentrations of various ions, soil acidity (pH), and soil moisture were used as habitat characteristics. Our aim was to find out how important population size and genetic variation are for the seed production of a rare plant, when accounting for a number of habitat parameters that might also be important for successful reproduction. Correlation coefficients between population and habitat characteristics showed that sites with low soil moisture and higher concentrations of most nutrients have a higher cover of grasses and a low cover of heath species. At such sites the population size of G. pneumonanthe was small and offspring heterozygosity was low. The number of ovules per fruit (a component of reproductive effort) was negatively associated with heterozygosity, phosphorus, and pH and positively associated with calcium, potassium, SO4, NH4, and electric conductivity. Total vegetation cover correlated positively with the variation in ovule production among individuals. Seed set and number of viable seeds per fruit, indicators of the success of fertilization and seed maturation, correlated positively with population size, the cover of co-flowering heath species, and the same group of nutrients that (independently) had a positive effect on ovule production. A high NO2 concentration might be toxic because it had a negative effect on the number of viable seeds. Seed weight seemed to be independent of any population or habitat parameter. Besides lower mean values, we also found more variation in reproductive success among individuals in smaller populations. We conclude that plants in small populations of G. pneumonanthe have a similar reproductive potential per flower but have lower seed production and a higher uncertainty of successful reproduction than plants in large populations. Genetic variation was apparently not very important. In addition to population size, habitat characteristics also seemed to be important for seed production and seed set. The most important factors were the facilitating effect of the presence of other insect-pollinated species and the concentrations of specific nutrients