Pollen and macrofossil analyses from central peat cores along with 23 radiocarbon dates were used in palaeoecological reconstructions for three peatlands (LG1, LG2 and LG3) within the lower La Grande Rivière watershed in northern boreal Québec. Basal ages from LG3 and LG2 indicate up to an 1100 years later and possibly more abrupt Tyrrell Sea retreat in the LG3 area compared with the timeline for the region. Both autogenic and allogenic factors were found to have influenced local vegetation succession and rates of peat accumulation. Internal autogenic factors such as peat accumulation were key elements for the general peatland developmental pathway that followed the classic hydrosere sequence (pond-fen-bog). Regional climate and hydrography are the main external factors associated with changes in vegetation assemblages, surface wetness and consequently rates of peat accumulation. The LG2 and LG3 peatlands began developing shortly after 7000 cal. BP as shallow ponds with herbaceous freshwater aquatic and emergent taxa. Both sites rapidly evolved into fens with brown mosses. The autogenic transition from fen to bog occurred at both sites between 6000 and 5500 cal. BP. A long-term decrease in peat accumulation rates corresponding to a gradual densification of the local tree and shrub cover occurred at the LG2 and LG3 sites between 5000 and 1500 cal. BP. These changes were simultaneous at the two sites and therefore suggest the influence of external factors such as a shift to cooler and drier climatic conditions from the middle to late Holocene (Neoglacial period). Development of the LG1 site was delayed by a much later Tyrrell Sea retreat and started with a relatively long eutrophic aquatic phase. Both internal factors (minerotrophic conditions) and external factors (local topography and climate) contributed to the somewhat slower pace of vegetation succession. Synchronous increased peat accumulation rates in the last 1500 years at the three sites are attributable to regional vegetational shifts possibly due to the influence of recent climatic conditions and lower peat compaction of the acrotelm.
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