The hemi-boreal zone, marking North America’s southern boreal forest boundary, has evolved post-glaciation, hosting diverse ecosystems including mixed forests with savannas, grasslands, and wetlands. While human, climate, and fire interactions shape vegetation dynamics therein, specific influences remain unclear. Here we unveil 12,000 years of hemi-boreal zone dynamics, exploring wildfire, vegetation, climate, and human population size interactions at such long time scales. Postglacial biomass burning exhibited episodes of persistent elevated activity, and a pivotal shift around 7000 years ago saw the boreal forest transition to an oak-pine barren ecosystem for about 2000 years before reverting. This mid-Holocene shift occurred during a period of more frequent burning and a sudden uptick in mean annual temperatures. Population size of Indigenous peoples mirrored wildfire fluctuations, decreasing with more frequent burning. Anticipated increases of fire activity with climate change are expected to echo transformations observed 7000 years ago, reducing boreal forest extent, and impacting land use.

An isolated sugar maple (Acer saccharum Marsh.) stand is located in the boreal forest of Abitibi, about 75?km beyond its present northern range limit. When did this relatively thermophilous tree species establish after ice retreat? Were its populations more abundant than now sometimes in the past? If so, when and how did they expand then retracted? How did the species persist in boreal forest over time? What could have been the role of fire on this stand? To answer those questions, we reconstructed the postglacial fire and vegetation history from three lacustrine sediment sequences distributed along a c. 180?km latitudinal transect from southern boreal forests to the northern portion of deciduous forests. From north to south, those are lakes Labelle, Chasseur and Fur. We explored a procedure based on pollen accumulation rates in order to detect the probable presence of sugar maple within the lakes? watershed. The procedure successfully indicates a sugar maple establishment c. 7800?5100 cal. BP at Fur, 5500?4400 cal. BP at Chasseur and c. 4000?2700 cal. BP at Labelle, in the north. At Fur, the subsequent sugar maple expansion happened 1 to 2 thousand years after establishment, during colder and moister climatic conditions favoring Pinus strobus L. replacement by Betula spp. c. 6000?5000 cal. BP. Sugar maple establishment, persistence or expansion is apparently not linked to a change in fire activity at Fur and Chasseur, but at Labelle, the species was more abundant during periods of shorter fire return intervals from 2000 to 500 years ago. Our study suggests that northern (Chasseur and Labelle) sugar maple establishment and possible expansion was probably more controlled by a complex interaction of inhibition and facilitation dynamics than by climate alone, a process reliant on the dominant vegetation?s composition and structure.

Sedimentary charcoal records are widely used to reconstruct regional changes in fire regimes through time in the geological past. Existing global compilations are not geographically comprehensive and do not provide consistent metadata for all sites. Furthermore, the age models provided for these records are not harmonised and many are based on older calibrations of the radiocarbon ages. These issues limit the use of existing compilations for research into past fire regimes. Here, we present an expanded database of charcoal records, accompanied by new age models based on recalibration of radiocarbon ages using IntCal20 and Bayesian age-modelling software. We document the structure and contents of the database, the construction of the age models, and the quality control measures applied. We also record the expansion of geographical coverage relative to previous charcoal compilations and the expansion of metadata that can be used to inform analyses. This first version of the Reading Palaeofire Database contains 1676 records (entities) from 1480 sites worldwide. The database (RPDv1b – Harrison et al., 2021) is available at https://doi.org/10.17864/1947.000345.

Climate changes are expected to progressively increase extreme wildfire frequency in forests. Finding past analogs for periods of extreme biomass burning would provide valuable insights regarding what the effects of warming might be for tree species distribution, ecosystem integrity, atmospheric greenhouse gas balance, and human safety. Here, we used a network of 42 lake-sediment charcoal records across a ~2000 km transect in eastern boreal North America to infer widespread periods of wildfire activity in association with past climate conditions. The reconstructed fluctuations in biomass burning are broadly consistent with variations in ethane concentration in Greenland polar ice cores. Biomass burning fluctuations also significantly co-varied with Greenland temperatures estimated from ice cores, at least for the past 6000 years. Our retrospective analysis of past fire activity allowed us to identify two fire periods centered around 4800 and 1100 BP, coinciding with large-scale warming in northern latitudes and having respectively affected an estimated ~71% and ~57% of the study area. These two periods co-occurred with widespread decreases in mean fire-return intervals. The two periods are likely the best analogs for what could be anticipated in terms of impacts of fire on ecosystem services provided by these forests in coming decades.

Aux abords du lac Labelle, dans la réserve écologique projetée du Ruisseau Clinchamp, se trouve une des érablières les plus isolées de l’aire de répartition actuelle. Relique d’une aire de répartition autrefois plus vaste ou établissement récent ? Aucune étude paléoécologique locale ne permet actuellement de répondre à cette question. En outre, alors que le maintien des érablières dans la forêt tempérée est principalement régi par une dynamique de trouée, un régime de perturbation dominé par les feux de forêt dans le domaine bioclimatique de la sapinière à bouleau blanc où se trouve l’érablière à l’étude ne semble ni affecter ni expliquer la présence d'érable à sucre. L'objectif de cette étude est donc d'identifier l'origine de l’érablière du lac Labelle ainsi que le régime de feux associé à la présence de l’érable à sucre. Pour répondre à ces deux objectifs, ce projet porte sur trois sites le long d'un transect latitudinal dans la région Abitibi-Témiscamingue, dont un site à 75 km au nord de la limite d’aire de répartition (lac Labelle), un site en limite d’aire de répartition (lac Chasseur) et un site au coeur de l’aire de répartition (lac Fur). Une analyse palynologique et anthracologique des sédiments lacustres de ces trois sites a été utilisée afin de reconstruire l’histoire de la végétation et des feux. Les résultats nous permettent de constater que l’érable à sucre (Acer saccharum Marsh.) est présent régionalement depuis au moins 9000 cal. BP dans la partie sud (lac Fur) de la région d’étude et depuis 8000 cal. BP au nord (Lacs Chasseur et Labelle). L’établissement local de l’espèce est cependant attesté qu’à partir de 7800 cal. BP au site Fur, 4000 cal. BP au site Chasseur et 2500 cal. BP au site Labelle. L’analyse des données palynologiques couplées aux analyses anthracologiques ont permis de caractériser la sévérité des feux pour chacun des sites au cours de l’holocène. D’après nos interprétations, l’augmentation de la proportion de feuillues dans la forêt boréale mixte (Betula cf. papyrifera) à partir de 6000 cal. BP aurait entraînée une baisse de la sévérité des feux et ainsi favorisé l’établissement d’Acer saccharum. Une baisse de la fréquence des feux fut néanmoins nécessaire pour permettre l’établissement d’Acer saccharum aux sites Chasseur et Labelle.