SECONDARY SUCCESSION AND SUMMER HERBIVORY IN A SUB-ARCTIC GRASSLAND -COMMUNITY STRUCTURE AND DIVERSITY

A field experiment was established in a subarctic grassland in the Fin nish Lapland to study the role of summer herbivory in plant community succession. Perennial vegetation and moss cover were removed in an are a of 324 m(2). The site was divided into four blocks, of which two wer e fenced to prevent herbivory by large mammals (reindeer, hare). Early successional changes in the vegetation were assessed. Mean species ri chness per 3 x 3 m plot was consistently higher in the fenced area, in dicating that herbivory can suppress small-scale diversity. Herbivory affected the height of several plant species. However, there was no co rrelation between frequency and height of individual species. There wa s a weak indication that taller species were more successful in early succession when grazed. Light competition is apparently not a key proc ess determining successional change. Thus, in early stage of successio n, summer herbivory has little effect on diversity by limiting light c ompetition, and most species are equally successful in grazed and ungr azed plots. There was some indirect evidence about competitive interac tions in the developing community. However, unlike temperate grassland s, large mammal herbivory and competition for light seem not to be imp ortant determinants of community change in this subarctic grassland (a t least what concernes early successional stages). This may be explain ed by the harshness of local climate, and abundance of light due to th e polar day.