Fire history of the central boreal forest in eastern North America reveals stability since the mid-Holocene

Abstract

Past and present fire frequencies are key factors to evaluate fire-mediated changes in climate, but this metric is difficult to evaluate realistically in paleoecological and climatic reconstructions. Here, we applied charcoal analysis of forest soils to test from direct evidence the stability and resilience of the eastern North-American boreal forest at the Holocene timescale. Current knowledge indicates that the boreal forest is not so stable and resilient in several parts of its range, particularly at its northern and southern edges where it is converted to tundra and woodland communities, respectively. However, it is not known to what degree the structure and composition of the boreal forest situated at the central core of its range (FCR), far from the climate-sensitive edges, have been modified during the Holocene. To address the long-term status of the boreal forest vis-a-vis the Holocene climate, we have used a large dataset composed of 14 C-dated and botanically identified charcoal. Long-term fire frequencies in several sites of the FCR were calculated to assess the stability of the main forest ecosystems. The mean fire interval over the last 5000 years of the two principal ecosystems composed of black spruce-jack pine forests and black spruce-balsam fir forests was ~200 and >300 years, respectively. Fires occurred repetitively during this period without fire gaps longer than 400 years (ecosystem 1) and 600 years (ecosystem 2), an indication that the fire regime of the FCR remained stable since the mid-Holocene. Unchanged forest ecosystems during this period also suggest the maintenance of both fire-prone and less fire-prone tree species in their respective sites and the ability of the boreal biome to maintain its structure and function in a repetitive fire-disturbance regime.

Fire History of Appalachian Forests of the Lower St-Lawrence Region (Southern Quebec)

“…F(max) is the factor controlling the flattening of the negative exponential curve that is considered as the estimate of the real number of fires (asymptote) [28,29,43]. k is the constant controlling the shape of the curve.…”

Have some landscapes in the eastern Canadian boreal forest moved beyond their natural range of variability?

“…The forest dynamics presented in this study suggest a clear dominance of successional dynamics (Frégeau et al 2015). Abies-Betula and Abies-Picea potential vegetation types are mainly characterized by successional forest dynamics in regions with a long fire cycle (Bergeron 2000;Lesieur et al 2002;Lecomte & Bergeron 2005;Couillard et al 2012).…”

“…Some stands originated from fires older than any individual tree still standing. Considering the abundance of wood charcoals found throughout the forest soils of our study area (Frégeau et al 2015) and the forest origin maps available (Fig. 2), we estimated that these forests originated mainly from fires that occurred in the first part of the 19th century.…”

“…For example, Betula papyrifera do not develop toward Abies balsamea stands, and early successional stands define a cyclical dynamic. Mixed stands containing Picea mariana and Pinus banksiana and belonging to the Picea-mosses potential vegetation type are also generally characterized by a cyclical dynamic (Cogbill 1985;Couillard et al 2012;Frégeau et al 2015). These stands belong theoretically to the early successional vegetation but, according to the forest classification of successional stands in terms of shade tolerance, they are considered late successional stands (S4-S5) (Saucier et al 1994).…”

Detecting Local Drivers of Fire Cycle Heterogeneity in Boreal Forests: A Scale Issue

“…When large fires occur, jack pine may spread outside short FC zones, helped by post-fire colonization adaptations such as seed abundance and dispersal, fast growth and serotinous cones [38,85,86]. Paleo-ecological researchers [33,87] observed a stable jack pine and black spruce local co-occurrence during the Holocene. Their relative abundance depends on time elapsed since last fire.…”