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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]

Highlights • Closed boreal forests at their northern range limit. • Reconstruction of postfire succession of conifer forest sites. • Migration of jack pine based on macrofossil analysis. • Estimating fire frequency at the stand scale and Holocene time scale. • Tree composition change during succession based on identified macrocharcoals. Abstract The distribution of vegetation zones in northeastern North America forms a gradual transition from closed to open forests, and to tundra. Closed forests near the open forest/tundra boundary generally correspond to residual patches of a once larger forest community. Whether these forest patches have survived as untouched, fire-free communities or as resilient communities during the Holocene remains unknown. To answer to this question, we reconstructed the successional history of two black spruce closed-crown forests and one jack pine forest near the northern limit of closed forest ecosystems. Stand-scale postfire succession was analysed based on charcoal macrofossils. Extensive 14C dating and botanical identification of charcoal fragments located at the organic soil surface and buried in the mineral podzolic solum were used to reconstruct the successional pathways of the sites. The forest sites developed in a context of continuous fire disturbance, except for a short interval of several centuries. The fire history of the three sites is similar with a fire frequency of 34–37 fires over the last 5000–5800 years, which corresponds to a mean fire interval of 150–180 years. Based on parametric and non-parametric estimators, the regional fire regime of the Laforge area (combined fire data of two sites located about 7 km from each other) was most likely characterized by 38 fires over the last 5000–5800 years, i.e., at an interval of about 150–160 years. Birch (Betula papyrifera) was present in the spruce sites between 5800 and 3000 cal. years BP. Jack pine arrived at the pine site about 3000 cal. years BP, at the time when birch was extirpated from the spruce forests. The closed-crown forests near the open forest/tundra boundary are resilient postfire ecosystems which regenerated successfully since mid-Holocene. Compositional change of these forests, which led to the decline of birch but maintained the dominance of conifers, appears related to cooler and wetter conditions of late Holocene. Climatic conditions also slowed the eastward migration of jack pine, which explains the late arrival of this species at its northeastern range. Our data suggest that fire disturbance in these sites of the boreal biome has been a continuous and positive regenerative process since mid-Holocene despite the influence of climatic change on the tree flora. [ABSTRACT FROM AUTHOR]

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]

• Boreal forest is resilient in the face of repeated disturbances during the Holocene. • Moderate fire frequency leads to a recurrence dynamics. • Resilience remains precarious as shown by lichen woodlands formation. • Climate change is expected to amplify the forest/woodland phenomenon. • Charcoals in mineral soils allow reconstruction of past local tree assemblages. The dynamics of the boreal forest has followed several successional trajectories during the Holocene caused by recurrent stand-scale, abiotic and biotic disturbances. Under stable environmental conditions, site disturbances should favor 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 known because of lack of stand-scale paleoecological evidence. To evaluate the resilience ability of boreal forest ecosystems, we reconstructed their Holocene fire history 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 mid-Holocene, western closed-crown conifer forests were resilient to fire disturbance, but hardwood trees declined significantly. Yet in the eastern region, closed-crown conifer forests similar to that observed today seems to have remained in place since its origin. In both regions, moderate fire frequency leads to a recurrence dynamics that favors renewal of forests stands with the same composition. However, 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, whose dominance 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 postfire ecosystems which have persisted over several millennia, their resilience is precarious as evidenced by the transformation of some forests into lichen woodlands after fire. [ABSTRACT FROM AUTHOR]