EFFECTS OF POPULATION-SIZE ON PERFORMANCE IN THE RARE PLANT GENTIANELLA-GERMANICA

1 We studied the relationships between population size, reproduction a nd population growth rate in 23 populations of the rare, short-lived p lant Gentianella germanica. We also investigated a possible correlatio n of population size effects with environmental variation in climate, topography, soil, vegetation, and management. To ascertain whether the differences between populations have a genetic basis, we grew 20 seed families from each of the populations in a common garden experiment. 2 In 1993, population sizes ranged from 40 to 5000 flowering plants in the field populations (geometric mean 386). Plants in small populatio ns had fewer seeds per fruit and fewer seeds per plant than plants in larger populations, whereas seed mass was independent of population si ze. From 1993 to 1995 population size decreased in most populations (m ean annual population growth rate 0.876), and this decrease was larger in small populations than in large populations.3 In the common garden the positive correlation between the number of surviving plants per p lanted seed and original population size increased steadily over time. After 17 months there were significantly more flowering plants and mo re flowers per planted seed for seeds from large populations than for those from small populations. 4 Although environmental variables accou nted for significant variation in population growth rate and plant per formance both in the field and in the common garden, effects of popula tion size detected in stepwise multiple regressions were not confounde d with environmental effects. 5 We conclude that the reduced performan ce observed in plants from smaller populations was explained by geneti c effects rather than by habitat quality, although pollinator limitati on may have contributed to fitness reductions in the held. The observe d patterns suggest further reductions of population sizes in Gentianel la germanica in the future, especially in already small populations. O ur study lends support to the view that genetic problems are a major c oncern in plant conservation.