Local versus regional processes: can soil characteristics overcome climate and fire regimes by modifying vegetation trajectories?

We analysed charcoal and pollen from sediments obtained from two lakes in the northwestern mixed-wood Canadian boreal forest in order to reconstruct fire-return intervals and vegetation dynamics over the last 8000 years. Sites were selected with contrasting soil properties (mesic versus dry-sandy soils), allowing an estimation of the potential influence of soils on long-term vegetation and fire dynamics. The sites likely experienced fewer fires during the period extending from 8000 to 4000 cal. a BP than over the last 4000 years. At both sites, eastern white pine (Pinus strobus) populations were most extensive shortly after deglaciation, with vegetation later shifting towards mixed woodlands with less P. strobus and more extensive Picea and Pinus banksiana populations. This gradual vegetation shift was probably induced by the establishment of colder and moister conditions along with a fire-regime change. In spite of the parallel long-term vegetation trajectories, vegetation composition differed between the two sites in both the past and present. Whereas Picea was more abundant at the mesic site, the fire-adapted P. banksiana populations were more extensive at the sandy-soil site. These differences in vegetation composition indicate that, in addition to climate changes and fire occurrence, soil properties also influenced vegetation dynamics. A likely increase in fire frequency in the Canadian boreal forest during the 21st century might therefore favour the expansion of these two disturbance-adapted trees with spatial heterogeneity in the populations due to varying soil types. Copyright (C) 2012 John Wiley & Sons, Ltd.