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.

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 macrocharcoal, we found that fire struck both maple stands 14–20 times since deglaciation. Most fires occurred in the early Holocene and during 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 of 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.

Botany D r a f t 2 Abstract: The long-standing hypothesis that sugar maple (Acer saccharum) communities are maintained at equilibrium by present climate and small-scale disturbances is questioned as empirical evidence is accumulating about the ability of the species to withstand several stand-scale disturbances. The fire history of a sugar maple site at the northeastern range limit of the species (Gaspé Peninsula, eastern Canada) was documented to test the hypothesis that this forest type is resilient to fire disturbance. The fire history was reconstructed using radiocarbon-dated soil macrocharcoals. Two main fire periods were recorded during the Holocene. The oldest period occurred between 9055 and 8265 cal. years BP, and was characterized by the presence of conifers, including spruce. After 6900 years of fire-free activities, the second period covered the last 1335 years, and was characterized by the presence of sugar maple in the charcoal assemblage. The dominance of sugar maple after more than 1000 years of recurrent fires underlines the species resilience to frequent site disturbances. The soil of the forest stand was heavily disturbed by earthworms. However, the dense seedling and sapling bank of sugar maple suggests that earthworms do not affect negatively the regeneration and survival of the species.

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Eastern hemlock (Tsuga canadensis) is a shade-tolerant tree of the temperate conifer-hardwood forests of northeastern North America whose northern limit of distribution coincides with the St Lawrence River around Québec City (Canada). We have 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 have used conventional forest surveys and macrofossil analysis of woody debris and charcoal pieces at the soil surface and buried in the mineral soil. The 'Rivière-du-Moulin hemlock forest' is an old-growth forest, at least 1000 years old, whose structure 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 experienced a sustained fire regime since the mid-Holocene, first in a developmental context of hardwood forests where beech (Fagus), butternut (Juglans), and birch (Betula) were growing, and then for the last 2400-2100 years as conifer forests where hemlock prevailed throughout or during a large part of the period. Our data highlight the influence of fire on the dynamics of hemlockhardwood 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.

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Les chablis et les épidémies d’insectes sont les principales perturbations qui régissent la dynamique des sapinières des régions maritimes. Bien que peu fréquents, des feux peuvent aussi survenir à l’occasion, mais on ne connaît pas leur historique à long terme (sur plusieurs centaines et milliers d’années). Nous avons reconstitué l’histoire des feux naturels ayant eu lieu sur l’île Bonaventure à l’aide d’une analyse pédoanthracologique. Située à l’extrémité est de la péninsule gaspésienne, il s’agit d’une petite île dont le couvert forestier consiste en une sapinière à épinette blanche. Au sein d’une place-échantillon de 500 m2, les charbons de bois de taille macroscopique (< 2 mm) de 50 échantillons de sol minéral ont été dénombrés, identifiés et datés. Au total, seulement 7 charbons ont été trouvés. Les identifications botaniques indiquent qu’il s’agissait de feuillus (5 charbons) et d’un conifère (1 charbon), tandis que l’identification n’a pas été possible pour le dernier charbon. Les datations au carbone 14 (14C) ont livré des âges s’échelonnant entre 7 780 et 8 400 ans avant aujourd’hui (années étalonnées). Ces résultats suggèrent que des feux naturels sont survenus sur l’île Bonaventure lors des premiers millénaires ayant suivi la déglaciation. L’île semble avoir été épargnée du feu depuis 7 700 ans, ce qui marquerait l’époque de formation de la véritable sapinière.