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Stand-scale gap-phase dynamics is generally viewed as the main driver of development in mesic deciduous forests of the temperate biome. Soil charcoal of temperate forests in eastern North America are unnoticed in most surveys, thus explaining why fire is undervalued as a driver of forest succession. The extent to which gap-phase, fire, or other processes are responsible for the regeneration and maintenance of mesic deciduous forests is unknown because paleoecological evidence is lacking. We tested the fire-driven succession hypothesis on the development of this major forest type. Based on charcoal 14C dates of two sites, 44 and 55 fires occurred since early Holocene, with a mean interval of 170 to 215 years. The vegetation of both sites followed comparable post-glacial trajectories consisting of three distinct periods. Conifers dominated the two first periods during 5200–6000 years and were replaced by hardwoods–conifers over the last 3500 years. The first period was represented by boreal conifers, whereas the second period, dominated by white pine (Pinus strobus) forests, persisted during 3000–4300 years. The third period marked the development of hardwood (sugar maple, Acer saccharum) forests. Fires occurred continuously on the sites since early Holocene likely under dry conditions during the conifer periods and cooler and moister conditions during the hardwood–conifer period. Recurrent fires appear with climate as key drivers of the long-term dynamics of several temperate forests in eastern North America. Similar studies on other temperate forests should be pursued to test the hypothesis of climate–fire interactions influencing tree composition change. [ABSTRACT FROM AUTHOR]

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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 14C-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. [ABSTRACT FROM AUTHOR]

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 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. [ABSTRACT FROM AUTHOR]