Wildfire size alters long‐term vegetation trajectories in boreal forests of eastern North America

Abstract

Aim Wildfire activity is projected to increase under global warming in many parts of the world. Knowledge of the role of these disturbances in shaping the composition of boreal forests is needed to better anticipate their future impacts. Here, we investigate the incidence of wildfire activity (burned biomass, frequency and size) on multi‐millennia vegetation trajectories in two coniferous boreal forest regions that display different types of vegetation composition and relief. We hypothesize that this difference in vegetation results from dissimilar wildfire activity during the Holocene. Location Conifer‐dominated boreal forests in Quebec‐Labrador, eastern North America. Methods Fire and vegetation histories during the last 8000 years were reconstructed and compared through analyses of charcoal and pollen records extracted from nine lacustrine deposits located in two spruce‐moss forests: the western region, co‐dominated by Pinus banksiana, and the eastern region, co‐dominated by Abies balsamea. Results Between 7000 and 2000 cal. yr bp, the western region experienced fewer fires than the eastern region, but they were larger in size. The main species adapted to fire, P. banksiana and Alnus viridis ssp. crispa, progressively co‐dominated with Picea sp.. Conversely, in the eastern region, P. banksiana and A. viridis ssp. crispa were very rare, and Picea sp. co‐dominated with non‐fire‐adapted A. balsamea and Betula sp.. Then, around 2000 cal. yr bp, fires decreased in frequency but were larger in size in the eastern region than in the western one, thus allowing densification of P. banksiana and A. viridis ssp. crispa in these landscapes. Main conclusions In the coniferous boreal forests of eastern North America, fire size was relatively more important in determining the long‐term vegetation trajectories in comparison with fire frequency. Changes in the rate of occurrence of large‐fire episodes will have significant impacts on vegetation dynamics over the next decades under continuing warming.

Fires of the Last Millennium Led to Landscapes Dominated by Early Successional Species in Québec’s Clay Belt Boreal Forest, Canada

“…During the last millennium, the pollen assemblages of Aspen and Shadow are dominated by Pinus banksiana and are at their highest level on the scale of the last 8000 years. An increase of Pinus banksiana in the two last centuries is also recorded elsewhere, notably at Schön Lake located close to the studied peatlands [15]. This recent dominance of early successional species is summarized in Figure 8, where positive scores of the first ordination axis are reported.…”

“…However, some lakes indicate an increase in fire frequency during this period in the western Picea mariana-moss domain. This is the case of Schön Lake, located close to the studied peatlands, to the north, in an environment dominated by clay [15,34] (Figure S1). Remy et al [15] suggest that fire frequency increased since 2000 BP in the western Picea mariana moss (lakes Garot, Schön, Figure S1) and Picea mariana lichen (lakes Loup, Nano, Trèfle, Marie-Ève) domains.…”

“…Our results concerning the fires differ from those based on lake sediments which show a relatively high fire frequency (expressed by a number of fires per millenium). However, there is a wide variation in the maximum frequency of fires and the period during which this rate was maintained [10,15,27,31,34].…”

“…These classifications consider the physical features, such as the abundance of lakes, the area covered by peatlands or sandy soils and the relief. All these physical features have a strong influence on the long-term history of fire and vegetation [5][6][7][8][9][10][11][12][13][14][15]. Knowing the long-term natural variability of landscapes, it will become easier to define forest strategies in regard to ecosystem management and climatic changes [16][17][18][19][20].…”

Suppression of forest fuel thermal decomposition under the influence of liquid aerosol and water droplets with additives

“…Especially it is necessary to distinguish brush fires, because the dry forest litter is often ignited. Huge areas of the forest burn down during brush fires [1,2]. The steady spread of ignition along the lower tiers of forest vegetation and litter is often the cause crown fire.…”