by Marie-Claude Boileau | 14 November 2022
Published in Forests: 8, 120.
Sugar maple (Acer saccharum) forests are among the main forest types of eastern North America. Sugar maple stands growing on Appalachian soils of the Lower St-Lawrence region are located at the northeastern limit of the northern hardwood forest zone. Given the biogeographical position of these forests at the edge of the boreal biome, we aimed to reconstruct the fire history and document the occurrence of temperate and boreal trees in sugar maple sites during the Holocene based on soil macro-charcoal analysis. Despite having experienced different numbers of fire events, the fire history of the maple sites was broadly similar, with two main periods of fire activity, i.e., early- to mid-Holocene and late-Holocene. A long fire-free interval of at least 3500 years separated the two periods from the mid-Holocene up until 2000 years ago. The maple sites differ with respect to fire frequency and synchronicity of the last millennia. According to the botanical composition of charcoal, forest vegetation remained relatively homogenous during the Holocene until recently. Conifer and broadleaf species coexisted in mixed forests during the Holocene, in phase with fire events promoting the regeneration of boreal and temperate tree assemblages including balsam fir (Abies balsamea) and sugar maple.
by Marie-Claude Boileau | 14 November 2022
Published in Forest Ecology and Management, 485: 118954.
The dynamics of the boreal forest have followed several successional trajectories during the Holocene caused by recurrent stand-scale, abiotic and biotic disturbances. Under stable environmental conditions, site disturbances should favour the regeneration of forest communities enabling the recovery process to produce forests similar to the pre-disturbance states. However, a failure in the post-disturbance recovery process can also occur to shift the forest communities to alternative states. Although fire is the main disturbance factor affecting the dynamics of the boreal forest, long-term resilience of most forest ecosystems remains poorly understood because of lack of stand-scale paleoecological evidence.
To evaluate the fire resilience of boreal forest ecosystems, we reconstructed their Holocene fire histories and documented their successional pathways at the stand-scale in two climatically contrasted regions (western continental versus eastern humid climates of boreal Québec, Canada) based on botanically identified and ¹⁴C-dated soil charcoal remains. Since the mid-Holocene, western closed-crown conifer forests were resilient to fire disturbance, but hardwood trees declined substantially. In the eastern region, closed-crown conifer forests were similar to those that are still observed today. In both regions, moderate fire frequency leads to recurrence dynamics that favour renewal of forest stands with the same composition. Yet, balsam fir–paper birch stands follow a successional pattern characterized by the establishment of paper birch, with subsequent increasing abundance of balsam fir. This allows the maintenance of forests composed of these species, the dominance of which fluctuates according to the time elapsed since the last fire. The succession from black spruce to balsam fir stands seems to be an uncommon process that requires a prolonged fire-free period. Although most closed-crown forests are resilient post-fire ecosystems that have persisted over several millennia, their resilience is precarious as evidenced by the transformation of some forests into lichen woodlands after fire.
by Marie-Claude Boileau | 11 November 2022
Published in Canadian Journal of Forest Research, 46: 822-831.
In the St-Lawrence Lowlands, sugar maple (Acer saccharum Marsh.) is considered the dominant species of old-growth deciduous forests, whereas red maple (Acer rubrum L.) tends to dominate sites recently disturbed by logging and agricultural practices. Considering that the long-term influence of fire is not documented for such stands, we reconstructed the postglacial tree composition (as deduced from charcoal species) and fire history of a sugar maple stand (Ste-Françoise area) and a red maple stand (Villeroy area) located southwest of Québec City, Canada. The sites are 10 km apart and show contrasting soil and landform features. Using botanical identification and 14C dating of soil macro-charcoal, we found that fire struck both maple stands 14–20 times since deglaciation. Most fires occurred in the early Holocene and over the last 2000 years, with the mid Holocene being a period with low fire frequency or no fires. During the last 1600 years, the Villeroy stand shifted from a Tsuga canadensis (L.) Carrière – conifer forest to a mixed forest and, most recently, to a red maple stand as fire became more frequent, possibly due to human activities over the last 400 years. This study confirms the influence of fire on the development of maple forests. Fire should be considered as an important disturbance factor in the dynamics of temperate deciduous and mixed forests.
by Marie-Claude Boileau | 11 November 2022
Published in The Holocene, 25: 1246-1256. https://doi.org/10.1177/0959683615580863
Eastern hemlock (Tsuga canadensis) is a shade-tolerant tree species of the temperate conifer-hardwood forests of northeastern North America, the northern distribution limit of which coincides with the St. Lawrence River around Québec City (Canada). We analyzed the structure and dynamics of one of the very few old-growth hemlock stands in this area to evaluate its successional status at the Holocene scale. To document the origin and long-term development of the hemlock site, we used conventional forest surveys and macrofossil analysis of woody debris and charcoal pieces at the soil surface and buried in the mineral soil. ‘Rivière-du-Moulin hemlock forest’ is an old-growth forest, at least 1000-years-old, the structure of which has been rejuvenated by recurrent surface fires killing most plants of the shaded forest floor and facilitating hemlock regeneration. According to the number of fires and the corresponding fire intervals, the hemlock site has experienced a sustained fire regime since the mid-Holocene. It first developed as a hardwood forest where beech (Fagus), butternut (Juglans) and birch (Betula) grew, and then for the last 2400–2100 years as a conifer forest where hemlock prevailed during a large part of the period. Our data highlight the influence of fire on the dynamics of hemlock-hardwood stands, a forest ecosystem generally viewed as being controlled by local light and medium canopy-gap disturbances. Soil charcoal analysis of conifer-hardwood forests may be used concurrently with canopy-gap analysis to decipher the influence of stand-scale disturbances and to calculate better forest turnover at several time scales.
by Marie-Claude Boileau | 11 November 2022
Published in Forest Ecology and Management, 433: 376-385.
Analysis and ¹⁴C dating of charcoal fragments ≥ 2 mm buried in mineral soils make it possible to obtain a stand-scale portrait of Holocene fires that occurred in well-drained, fire-prone environments, as well as changes in forest stand composition over time, based on botanical identification of charcoals. Yet, it is not always possible to reconstruct all fire events, due to disturbances that have altered soil stratigraphy. To evaluate the efficacy of this approach, we conducted a comparative analysis with a proximal environment that presents an a priori continuous stratigraphy of charcoal fragments. For two sites in the coniferous boreal forest of eastern North America, the charcoal record of a forest soil was compared with that of an adjacent peatland margin situated at a distance of 20 m. For both types of sedimentary environments, a similar fire history was reconstructed for a most of the Holocene. The greatest differences were for the early Holocene period, for which a smaller number of fires were detected in the forest soil compared to the peatland soil. Retracing the oldest fires using mineral soils in a fire-prone environment is more difficult, given charcoal decay that results from repeated fire events. Yet, forest soils reveal a relatively accurate fire history for subsequent millennia if the number of charcoals being dated is sufficiently large. Any accurate reconstruction of the fire history of proximal peatland environments is strongly dependent on continuous stratigraphic units of peat and charcoal. Indeed, the age of charcoal fragments in peat may be different from that of the sedimentary layer in which they are buried due to allogenic disturbances such as erosion events caused by deep burning of the organic horizon and other mass-wasting events. Despite the large number of ¹⁴C dates it requires, analysis of soil macro-charcoal yields a realistic picture of the fire history at the stand scale. The concurrent analysis of macro-charcoal from adjacent peatland deposits may be used as a complement to more accurately record the oldest fire events.
