Increasing Population Density and Seed Production with Altitude in Eritrichium nanum (Boraginaceae)—an Arctic Alpine Obligatory Seeder

Abstract

At 17 sites in the European Alps (2l70–3310 m), the population density and seed production of the high alpine cushion plant Eritrichium nanum were investigated. Recruitment in this non-clonal species relies exclusively on seeds. The population density rose significantly with increasing altitude. A mean number of 368 seeds per m2 was observed. Whereas the quantity of the generative rosettes (metamers) was relatively constant, the annual seed production strongly varied and was found vulnerable, particularly in 1996, when it was suppressed by unfavorable weather conditions. In 1997 seed production again reached the usual level, when about the same amounts of seeds were recorded as in previous years, indicating the strong resistance of elevated E. nanum populations against the severe climate at high altitudes. With rising population density, E. nanum produces distinctly higher seed numbers. This increasing amount of seeds seems essential for the persistence of its uppermost populations. Our results indicate that it is caused by the rising number of rosettes/surface and not a higher reproduction by single rosettes. Compared to other alpine species, the seed weight of E. nanum (average 0.76 mg) is high. Most of its neighboring species are clonal, producing much lighter seeds. The strategy of producing heavy seeds favors successful recruitment and appears to be decisive for an obligatory seeder such as E. nanum.