Invasion of raised bogs by the Scots pine; a review of vegetation history and case study in selected peat-bogs in Bieszczady Mts. (Eastern Carpathians)

The objective of this study was to analyse the diversity of pine and spruce bog forests on the domes of Carpathian raised bogs located in the Upper San River Valley. In order to determine the present species composition, 59 phytosociological relevés were performed (32 within spruce forests, and 27 in pine forests). For all relevés, the Shannon-Wiener index, evenness index and mean Ellenberg indicator values were calculated. Detrended correspondence analysis was conducted in order to determine the degree of similarity between distinct units of vegetation and to identify the main environmental gradients that have an impact on species composition. The results revealed considerable similarities between the ground vegetation of both spruce and pine forests growing on peat substrate. Within pine forests there was a greater proportion of certain plant species characteristic of raised bogs, and plants typical of bog coniferous forests. The ground flora of spruce forests was more diverse. Although spruce forests predominantly contained species typical of raised bogs, there was also a group in which Molinia caerulea predominated. Ellenberg indicator values typical of pine forests lay within ranges that were characteristic of spruce forests, but pine forests occupy more humid, more acidic and poorer habitats than most patches of spruce forests. Both spruce and pine bog forests were developed as a result of the colonization of open bogs, but the time-scale and rate of intensity were different. Palaeobotanical and historical data indicate that the Scots pine first reappeared on the domes of raised bogs in the first half of the 20th century. The concept that the spruce forests are older than those of pine is supported by their greater habitat diversity. Conversely, the younger age of pine forests is confirmed by their low habitat diversity, and greater proportion of photophilous raised-bog species.