A 250-year postfire plant chronosequence in well-drained sites at the northern limit of the Boreal Forest in the Grande rivière de la Baleine area, northern Quebec, was reconstructed from nine sites associated with the development of the lichen–spruce woodland. Most species recorded along the chronosequence reinvaded burned sites within 15 years after fire, whereas important vegetational changes occurred during the first 100 years of postfire recovery, corresponding to full development of the Cladina stellaris – spruce woodland. No vascular plant species replacement was observed during succession, whereas by contrast a well-defined lichen–bryophyte sequence occurred along five successional stages. Species diversity (Shannon index) was low 4 years after fire, but it has reached a maximum about 25 years after fire and then dropped and stabilized at a low value in old-growth woodlands dominated by black spruce (Picea mariana (Mill.) BSP) and C. stellaris. The active period of black spruce sexual regeneration spans about 20–25 years, with maximum regeneration occurring between 5 and 14 years after fire. After this period, seed regeneration is mostly sporadic and sustained layering becomes fairly common at all sites. From 100 to 250 years after fire, no significant changes were observed in vegetation structure, floristic composition, species diversity, and spruce regeneration, suggesting that lichen woodlands are selfperpetuating in absence of fire. In limiting spruce regeneration at the ground surface, the lichen cover seems to be the most important factor controlling the open structure of the lichen–spruce woodland.

The recent fire history of northern Quebec biomes (54 000 km2), including the northern Boreal Forest, the southern and northern Forest—Tundra, and the Shrub Tundra, was documented by examining size and dates of 20th century wildfires using tree ring techniques. Results showed that pronounced south—north differences in fire properties existed, corresponding to climate and vegetation gradients. Fire frequency per biome decreased south—north from closed forest (0.7 fire/yr) to Shrub Tundra (0.4 fire/yr). Average fire size decreased south—north by 100—fold from °8000 ha in the northern Boreal Forest to 80 ha in the Shrub Tundra, while modal fire size was <50 ha in each of the four biomes. Most fires (>80%) of the northern Forest—Tundra and the Shrub Tundra were <100 ha, and fires >100 000 ha occurred only in the northern Boreal Forest and the southern Forest—Tundra. Less than 35% of all mapped fires in the Boreal Forest were <50 ha, but >30% were >1000 ha. From south to north, the fire—free interval per biome was, respectively, °2.6, 0.6, 0.6, and 2.2 yr, the Boreal Forest data being overestimated. The largest burned areas were recorded in the 1950s throughout the biomes, most likely associated with longlasting drier and warmer conditions. The fire rotation period per biome, based on the percentage of burned areas during the 1920—1984 period (or 1930—1984 in Tundra), increased south—north by 100—fold from 100 yr in the northern Boreal Forest to 9320 yr in the Shrub Tundra. The fire rotation period around the tree line, i.e., 20 km south and north of the present tree line, was estimated to be >7800 yr. Biome boundaries have developed and are maintained in response to fire by the ability of spruce to seed and regenerate. Stability of northernmost conifer sites is maintained by (1) the inability of patchy shrub and conifer cover in the northern Forest—Tundra and Shrub Tundra to carry fire and (2) failure of trees to produce viable seeds in these two biomes. Present data suggest that the area is characterized by a much higher fire frequency than expected from the fire weather index and from calculated frequencies typical of vegetation—type studies. It is concluded that size of the study area is a key element in the determination of regional fire regimes. Finally, the ecological significance of the natural fire rotation and postfire regeneration in northern environments is discussed in a paleoecological perspective.

We used tree-ring chronologies from sugar maple (Acersaccharum Marsh.) stands showing various degrees of dieback (i.e., 16 sugar maple chronologies from healthy trees and 11 from damaged trees), distributed throughout the species range in southern Quebec, to analyze the spatial extent and timing of the recent sugar maple decline. Furthermore, six tree-ring chronologies of American beech (Fagusgrandifolia Ehrh.) from six damaged sugar maple stands were used to compare for differential responses associated with factors such as insect defoliation (from the forest tent caterpillar, Malacosomadisstria Hbn., for which American beech is a nonpreferred species), drought, and other climatic events. It was found that several small-scale drought-induced disturbances occurred repetitively over the last 100 years in the western part of the species range in southern Quebec. Most sugar maple chronologies from stands located west, north, and south of Québec City also show extreme narrow tree rings, indicating the incidence of three large and deep growth depressions from the early to mid-1910s, mid-1950s, and early 1980s. The factors explaining the large growth depression of dominant sugar maple of the early 1980s, in the region where the 1980s maple decline was the most severe, are likely associated with the synergistic influence of drought and insect defoliators. The recovery of sugar maple stands from the 1980s growth decline emphasizes the positive responsiveness of the robust native trees to frequent natural disturbances. The 1980s maple decline corresponds to the category of natural disturbances affecting stand dynamics by the combination of events such as drought and insect infestations, and possibly (but to a minor extent) winter thaw-frost, which has yet to be demonstrated, rather than by anthropogenic pollution.

The first tree health decline symptoms usually observed are foliar deficiency symptoms, foliage loss, and dieback. To improve the subjective nature and unspecificity of these assessments, we examined sugar maple (Acer saccharum Marsh.) radial growth and health to develop an indicator of sugar maple tree health status based on radial growth pattern. We used the basal area increment (BAI) of 328 tree-ring collections from 16 sites located in southern Quebec, throughout the sugarbush natural range, that were categorized by defoliation class. BAI of trees with decline symptoms was significantly lower than that of healthy trees in 9 of the 16 stands. BAI trends since 1955 showed an inverse relationship with tree decline class measured in 1989, irrespective of tree age. The results indicate that declining trees in these stands have not recovered based on BAI. They also suggest that the decrease in slope of BAI predated the observed symptoms of sugar maple decline by at least one decade. Results suggest that sugar maple vigor and health can be assessed by measuring tree's BAI trend, an indicator that may be useful for the diagnosis of sugar maple health and status years before the appearance of visible canopy symptoms.

We present direct evidence for the relict nature of lichenspruce woodlands thriving at treeline, using 14C-dated stands growing in the humid climate of eastern Hudson Bay in northern Québec. Black spruce, Picea mariana (Mill.) BSP, forms two groups of relict stands: a Neoglacial group of postfire origin dating between 2000 and 900 yr B.P. and a pre-Neoglacial group most likely established between 4500 and 3200 yr B.P. The latter group shows no evidence of fire (absence of charcoal under topsoil organic horizon). The stands are exceptional because they probably are the direct lineages of the primeval forests that colonized the land sometime after deglaciation. The structure of the relict lichen-spruce communities gives strong support to the current thesis that considers lichen-spruce woodland as a self-perpetuating open forest, typical of the subarctic environment.