-1 Research article Effects of mammalian herbivores on revegetation of disturbed areas in the forest-tundra ecotone in northern Fennoscandia Johan Olofsson 1, *, Philip E. Hulme 2 , Lauri Oksanen 1 and Otso Suominen 3 1 Department of Ecology and Environmental Science, Umea ˚ University, S-901 87 Umea ˚, Sweden; 2 Centre for Ecology and Hydrology – Banchory, Hill of Brathens, Glassel, Banchory, Kincardineshire, AB31 4BW, Scotland; 3 Section of Ecology, Department of Biology, University of Turku, FIN-20014 Turku, Finland; *Author for correspondence (e-mail: johan.olofsson@emg.umu.se; phone: +46-90-786-7704; fax: +46-90- 786-6705) Key words: Disturbance, Gap, Germination, Grey-sided vole, Herbivory, Norwegian lemming, Reindeer, Seedling establishment, Species richness Abstract Herbivores influence the structure of plant communities in arctic-alpine ecosystems. However, little is known of the effect of herbivores on plant colonisation following disturbance, and on its variability depending on the identity of herbivores and the characteristics of the habitats. To quantify the role of large and small vertebrate herbivores, we established exclosures of two different mesh sizes around disturbed subplots in forest and nearby tundra habitats in four contrasting locations in the forest-tundra ecotone in northernmost Sweden and Norway. The study revealed that herbivores influenced the abundance but not the species composition of regenerating vegetation. Gaps were colonised by the dominant species in the surrounding vegetation. The only exception to this expectation was Empetrum nigrum, which failed to colonise gaps even though it dominated undisturbed vegetation. Significant effects of herbivory were only detected when both small and large herbivores were excluded. Herbivores decreased the abundance of three of the most common species Vaccinium myrtillus, Vaccinium vitis idaea, and Deschampsia flexuosa. The effect of herbivory on the abundance of these three species did not differ between habitats and locations. However, the composition of the regenerating vegetation differed between habitats and locations. The disturbance treatment increased the species richness on the scale of plots, habitats, and sites. However, on the scale of whole locations, all species found in disturbed areas were also found in undisturbed areas, suggesting that the natural disturbance regime in arctic landscapes is high enough to sustain colonising species. Introduction During the last decades, numerous studies have shown herbivores to have substantial impacts upon arctic plant communities (Jefferies et al. 1994; Mulder 1999). Herbivores can influence plant community structure directly by reducing the abundance of preferred forage species (Bazely and Jefferies 1986; Virtanen et al. 1997; Bra˚then and Oksanen 2001; Olofsson et al. 2001; Grellmann 2002), and indirectly by modifying competitive interactions between plants (Mulder and Ruess 1998; Olofsson et al. 2002) and/or altering nutrient availability (Bazely and Jefferies 1985, 1989; Olofsson et al. 2001; Olofsson and Oksanen 2002; Olofsson et al. 2004a; Stark and Grellmann 2002). Nevertheless, most plant species in the arctic are long-lived perennials and responses to the direct Landscape Ecology (2005) 20:351–359 Ó Springer 2005 DOI 10.1007/s10980-005-3166-2