Effects of slug herbivory on the seedling establishment of two montane Asteraceae species

Seedling establishment as the life stage transition most sensitive to herbivory might be impossible even if the herbivory losses suffered by adult plants of the same species are tolerable. We tested the hypothesis that herbivory impedes seedling establishment of two montane Asteraceae species on their lower altitudinal distribution border.In a submontane meadow in the Harz Mountains, Germany, the montane grassland perennials Arnica montana and Centaurea pseudophrygia (Asteraceae) were sown into artificially created gaps of two different sizes, half of which were protected from slug grazing by molluscicide application in the initial phase of the experiments. The Arnica experiments started in spring and late summer of 1999 and in spring of 2000; whereas Centaurea was only sown in the spring of 2000. Seedling survival was monitored until the autumn of 2000.Ten weeks after sowing, mollusc exclusion resulted in significantly higher Arnica or Centaurea seedling numbers in all experiments; whereas grasses and dicotyledons that emerged from the seedbank were promoted by slug exclosure in only one experiment. Species diversity was not affected by molluscicide application. By the end of the second growing season, the survivorship of Arnica sown in the spring of 1999 was reduced to less than 1% of the number of sown seeds in the gaps with natural slug grazing, compared to 7–15% in the gaps with molluscicide treatment. Plant density of most species was higher in the small than in the large gaps, indicating a microclimatic shelter effect of the surrounding vegetation.The results show that slug herbivory might exert large effects on species composition in grassland; it does strongly influence the seedling establishment of the highly palatable, slow-growing A. montana, while the seedlings of C. pseudophrygia, although of similar palatability, are more able to tolerate slug feeding because of their higher growth rate.