| In this thesis, the recent dynamics of several peatlands isolated within an agricultural landscape was reconstructed in order to determine which factors are responsible for changes in their vegetation. First of all, a general statement of human activities disturbing the peatlands located between the municipalities of Riviere-du-Loup and L'Isle-Verte (Bas-Saint-Laurent) is presented. In this region, 60% of the peatland area has been disturbed by human activities since 1929. Peat extraction for horticultural compost, logging and farming are the main human activities disturbing the peatlands. Secondly, the recent evolution of plant communities in peatland fragments was studied using paleoecological and aerial photograph analyses. In the study area, some treeless fragments dominated by Sphagnum species have recently been converted to forest stands. Fires and the isolation of the Bas-Saint-Laurent peatlands within an agricultural plain for more than 100 years may have facilitated the afforestation process. Thirdly, jack pine colonization and expansion in ombrotrophic peatlands was analyzed using macrofossil and dendrochronological analyses. During the last 80 years, and mainly between 1920 and 1945, several open peatlands dominated by Sphagnum species were invaded by jack pine following a fire event. Fires triggered pine establishment and a drier-than-average climatic period during the first part of the 20th century may have facilitated the phenomenon. Finally, the recent vegetation dynamics of all ombrotrophic peatlands of the region was studied through paleoecological (dendrochronology, plant macrofossil and pollen analyses) and historical (aerial photograph analyses) techniques. Plant macrofossil analyses show that prior to European colonization, peatlands were predominately open environments dominated by Sphagnum mosses. Several sites subsequently became forested with little or no Sphagnum. A dry climatic period during the first part of the 20th century, drainage resulting from human activities, and fire events seem to be the main causal factors of vegetation changes. Globally, the results indicate that peatlands isolated within an agricultural landscape are not resilient. |
