Most black spruce (Picea mariana [Mill.] B.S.P.) forests at tree line in northern Québec burned over the last 1000 years were transformed into shrub tundra because of postfire seed regeneration failure in response to colder climatic conditions. Only a small part of the northernmost spruce forests have escaped fire during this period, and they are now forming old-growth lichen-spruce woodlands (figure 1). The structure of an old-growth spruce woodland located at the arctic tree line in the Boniface River area (57° 45’ N; 76° 20’ W, northern Québec, Canada) was analyzed in order to evaluate its structure and spatiotemporal dynamics.

Vegetation and soil characteristics were used to reconstruct the development of the forest cover over the last 1000 years and to evaluate its stability in a context of changing climate. Soil developed under the main plant covers (lichens and spruce) were compared. Fire history was reconstructed using charcoal located at the soil surface and buried in the mineral soil. Stand structure was described using size and age structure based on tree-ring analysis.

The lichen woodland studied was growing on acidic, well-drained eluviated brunisols and humo-ferric podzols (figure 2). Soils developed under the lichen cover were less developed than forest soils. One black spruce stomata found in a paleosol indicates the presence of the species at this site at about 4200 cal. yr BP, several thousand years after the first recording of the species in this region (i.e., 6500 cal. yr BP according to Bhiry et al. 2007). Radiocarbon-dated charcoal fragments indicate that the site burned several times between 3000 and 1000 cal. yr BP (figure 3). No evidence of fire was found before and after this period despite the presence of spruce at this site since 4200 cal. yr BP. According to charcoal dates, two major changes in fire frequency occurred during the last thousand years. Fire frequency was relatively high between 3000 and 1000 cal. yr BP, whereas the lack of fire evidence over the last 1000 years suggests more humid conditions. Tree establishment and mortality occurred regularly since the last fire event 1000 years ago. The oldest subfossil tree sampled established at 1313 AD, which indicates again the old-growth character of the woodland. The inverted J-shaped size structures (height and diameter) of black spruce, characteristic of old-growth stands, show that the forest is at equilibrium with ambient conditions in absence of fire. The forest structure and development are currently maintained by spruce layering and occasional seedling establishment.