Long-term stand-scale dynamics of a boreal mixed forest in Québec, Canada

We conducted a high-stratigraphic resolution palaeoecological study (fossil pollen, plant-macrofossils, charcoal, stomata, sediment stratigraphy) of two organic sedimentary deposits in a mixed balsam fir/white birch forest in the Laurentian Highlands in eastern Québec (Canada) to elucidate the long-term stand development of this ecosystem (species invasions, succession), and to examine the role and effects of natural disturbances on the vegetation dynamics. The two sites studied, consisting of a forest hollow (Laflamme site) and a peatland margin/forest interface (DesAulnaies site), were dated 10 200 and 5930 cal. yr BP, respectively. The constitution of the balsam fir/white birch forest occurred 9500 cal. yr ago, after an afforestation period (> 10 200–9500 cal. yr BP) corresponding to an open coniferous woodland characterized by black spruce. Our results allowed a major revision of the chronology of the vegetation dynamics during the early Holocene for the study area: spread of the first tree species and onset of the constitution of the first open forests occurred at least 2500 yr earlier than previously estimated. Macroscopic wood charcoals were found during the afforestation period, but were very scarce thereafter. Only four fires were recorded on the two sites since the first appearance of firs in the region, 9500 yr ago. This suggests that forest fires have been rare events since the establishment of fir, and that fires played a very minor role in the holocene dynamics of pristine balsam fir/white birch forests in eastern North America. The comparison of the pollen diagrams of the two study sites with that of a nearby peatland which was used previously to reconstruct the postglacial regional vegetation history shows that the pollen rain registered by the forest hollow and the peatland margin/forest interface is primarily composed of local and extra-local pollen inputs, delivering the long-term vegetation dynamics at the local stands.