Species diversity, niche metrics and species associations in harvested andundisturbed bogs

Competition is considered an important force in structuring plant communiti es and in governing niche relations, but communities recovering from distur bance, may be less governed by species interactions and less orderly organi zed. To address this issue, we studied species richness, abundance and patt erns of association between plant species at three spatial scales (1 m(2), 1/25 m(2), 1/625 m(2)) in two ombrotrophic mires in east-central Sweden. On e was at a secondary successional stage following peat extraction 50 yr ago and the other was undisturbed. Feat extraction leads to a change in hydrol ogy which is slowly restored by the formation of new peat. Niche breadth an d niche overlap along the gradient of height above the water table were cal culated for the five common Sphagnum species occurring at both mires in an attempt to better understand differences in species co-occurrence at each m ire. Species cover differed between the mires, and the number of species per plo t was higher in the undisturbed community at all scales, suggesting that th e degree of species intermingling was greater than at the harvested site. A t all scales, the number of non-random associations was higher, and niche o verlap lower among ecologically similar species (e.g. hollow Sphagnum speci es) in the undisturbed mire. These differences indicate that random events are important in colonization, and that biotic interactions between neighbo urs later result in a higher degree of non-randomness. In addition, we surveyed a number of abandoned peat pit sites to test the e ffect of disturbance for species composition at a regional scale. Ombrotrop hic peat pits contained several Sphagnum species normally associated with m inerotrophic mires, and species of wooded mires occurred frequently in pear pits, making them more species-rich than undisturbed bogs. There were also Sphagnum species new to, or rare in, this part of Sweden which indicates e ffective long-distance dispersal. Even 50 yr after peat extraction had ceas ed, the vegetation had not recovered to its original composition.