One of the main problems associated with small natural reserves is their progressive loss of ecological integrity owing to the influence of surrounding human activities. In southern Quebec (Bas-Saint-Laurent, Canada), peatlands are extensively mined to extract peat for the production of horticultural compost and are isolated within agricultural lands. Government environmental agencies have proposed that peat industries set aside 5-10% of a bog's area as a natural refuge for peatland plants and animals. Do these fragments constitute reliable refuges? Do they maintain their ecological characteristics over a long period? We studied the recent evolution of plant communities in peatland fragments using paleoecological techniques had a geographical information system. In the study area, some treeless fragments dominated by Sphagnum species have recently (since 1940) converted to forest sites. Macrofossil and dendrochronological analyses suggest that peat-mining activities were not the main factors responsible for the afforestation of peatland fragments. On the other hand, the isolation of the Bas-Saint-Laurent peatlands within an agricultural plain for more than 100 years may explain the afforestation process (drainage activities). Furthermore, fires may have accelerated afforestation by facilitating the spread of seeds of tree species with serotinous cones. Because most peatlands of the Bas-Saint-Laurent region are still affected by drainage and fires, it is probable that several open bog fragments will not maintain their treeless vegetation structure over a long period. Consequently, peatland fragments should not be considered as a solution to long-term conservation needs in southern Quebec, at least not for plant and animal species of open bogs. This study also shows that even ecosystems known to be resistant to invasions by exotic species (such as peatlands) can be strongly affected by fragmentation and by their surrounding environment on a long-term basis. © 2000 NRC Canada.