Luc Sirois

RECENT FIRE HISTORY OF THE NORTHERN QUEBEC BIOMES

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 de- creased 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 80%) of the northern Forest-Tundra and the Shrub Tundra were 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 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 1 950s throughout the biomes, most likely associated with long- lasting 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), in- creased 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.

Conifer seedling recruitment in a southeastern Canadian boreal forest: the importance of substrate

In order to explain conifer species recruitment in Canadas southeastern boreal forest, we characterized conifer regeneration microsites and determined how these microsites vary in abundance during succession. Microsite abundance was evaluated in deciduous, mixed and coniferous stands along a 234-yr postfire chronosequence. Conifers were most often found in relatively well-illuminated microsites, devoid of litter, especially broad-leaf litter, and with a reduced cover of lower vegetation (< 50 cm tall). Although associated with moss-rich forest floor substrates, Abies balsamea was the most ubiquitously distributed species. Picea glauca and es- pecially Thuja occidentalis seedlings were frequently found on rotten logs. Light measurements did not show differences among seedling species nor between stand types. The per- centage cover of broad-leaf litter decreased significantly during succession. Also, rotten logs covered with moss occu- pied a significantly larger area in the mid-successionnal stands than in early successional deciduous or late succes- sional coniferous stands. The results suggest that the pres- ence of specific forest floor substrate types is a factor ex- plaining low conifer recruitment under deciduous stands, conifer codominance in the mid-successional stage, and de- layed Thuja recolonization after fire. Results also suggest that some facilitation mechanism is responsible for the ob- served directional succession.

Vegetation changes caused by recent fires in the northern boreal forest of eastern Canada

From 1980-1989, fires burned 32 440 km2 of boreal forest, 200 km south of the forest-tundra border in northern Quebec, Canada. An assessment of the impact of fire on tree population densities was carried out by comparing the number of Pinus banksiana and Picea mariana in 83 sites before and after the sites burned in 1981, 1983, 1988 or 1989. Age structure analysis of post-fire populations burned in 1972, 1976 and 1983, along with the rapid exhaustion of the seed bank from burned trees, suggest that the majority of seedlings were established within 3 to 10 yr after fire. Consequently, given the absence of nearby living seed bearers, little (if any) further recruitment can be expected in the even-aged, regener- ating populations. According to the tree density comparison (pre-fire vs post-fire), a shift from Picea- to Pinus-dominated communities occurred in most of the sites burned in 1981 or 1983, and in some of the sites burned in 1988 or 1989. The 1988 fire reduced the tree population density by 95 % in 10 of the 15 sites; total tree density decreased by at least 75 % in 28 out of 40 sites burned in 1989. This suggests that the areas burned in 1988 and 1989 will mainly regenerate as very open forests or lichen-heath communities that are more commonly found in the forest-tundra zone, north of the study area. Fire intensity, short fire interval, and unfavorable climate during and after fires are three plausible mechanisms associated with these post-fire vegetation changes.

Reduced postfire tree regeneration along a boreal forest-forest-tundra transect in northern Quebec

The large 1950s fires that burned >5500 km2 of land across a south—to—north climatic gradient in northern Quebec provide an opportunity to evaluate the role of fire in forest—tundra development on a demographic basis. The tree population density before and ° 30 yr after fire was estimated by censusing trees in plots of 400 m2 located in upland and lowland within four representative ecoregions of northern Quebec. The analysis of tree recruitment before and after fire, in 410 randomly selected sites along a transect crossing the upper boreal forest and forest—tundra zones, indicated that wild fires induced substantial depletion of tree populations. Taken as a whole, fires have significantly reduced the density of black spruce populations in forest—tundra uplands, but not in the lowlands. A reduction in tree population density of °75% was observed in 22% of upper boreal forest sites, and 45% and 93% of sites located in the forest and shrub subzones, respectively, of the forest—tundra zone. Complete exclusion of tree populations by fire was observed in 43% of upland sites in the northern part of the transect, while complete removal was a rare event in the southern part. Sustained reduction of tree population density after several destructive fires appears as one of the main deforestation processes in the subarctic zone. This leads to the patchy distribution of forest stands and scattered tree populations typical of the forest—tundra biome. Comparisons with paleoecological data suggest that the impact of the 1950s fires contribution to the expansion of the forest tundra into the upper boreal forest. The ecological impact of these fires was probably similar to those fires responsible for development of the forest tundra during the Holocene. It is suggested that the fire—climate interaction should be considered in order to predict the ecological impact of warming climate on high—altitude forest ecosystems.

Air temperature, tree growth, and the northern and southern range limits to Picea mariana

. Many models that simulate the long-term response of forests to climatic change use the assumption that northern and southern range limits are caused by the deleterious effects of cold and hot air temperatures, respectively, on individual tree growth and that growth declines symmetrically with air temperatures above and below some optimal value in between these extremes. To test the validity of this assumption, we combined physiological data for black spruce, Picea mariana, growing near the treeline in subarctic Quebec with a model of the biophysical and biochemical effects of temperature on photosynthesis. The physiological conditions allow black spruce to grow over a wider range of air temperatures than is reflected in its geographic distribution. In particular, the physiological data suggest that the northern range limit of black spruce is not caused by the direct effects of cold growing-season air temperatures on tree growth and that growth is optimal, with respect to temperature, at the southern range limit. While pollen data indicate large geographic changes in spruce abundance with past climatic changes, the current analyses suggest that the direct effect of air temperature on individual tree growth has not caused these changes. Until we better understand the effects of air temperature on ecological processes, the efficacy of climatic change analyses must be evaluated in terms of model assumptions.

Postfire succession of saproxylic arthropods, with emphasis on coleoptera, in the north boreal forest of Quebec.

Abstract Saproxylic succession in fire-killed black spruce [Picea mariana (Mill.) B.S.P.] coarse woody debris (CWD) in northern Quebec is estimated in this study using a 29-yr postfire chronosequence. Sampling was performed using both trunk-window traps and rearing from snag and log sections. A total of 37,312 arthropods (>220 taxa) were collected from both sampling methods. Two distinct colonization waves were identified. The onset of initial colonization occurs the year of the fire, whereas the second colonization phase begins only once debris falls to the ground. The initial colonization step is influenced by fire-associated species including subcortical predators, xylophages, and ascomycetes feeders. Abundance of most early colonizer species decline with time since fire with the disappearance of subcortical habitat. No noticeable species turnover occurred in snags thereafter. Lack of succession in snags is related to very low decomposition rates for postfire CWD because this substrate is unsuitable for species associated with highly decayed wood. Snag falling triggers fungal growth and concomitant saproxylic succession toward micro- and saprophagous species and increases accessibility for soil-dwelling organisms. Because the position of woody debris greatly influences overall physical properties of dead wood, the fall of burned CWD plays a major role in saproxylic community shift after fire.

Distribution and dynamics of balsam fir (Abies balsamea [L.] Mill.) at its northern limit in the James Bay area

A regional assessment of balsam fir distribution along a transect between 49° 30’ N and 53° 30’ N suggests that balsam fir exists as isolated populations scattered across large expanses of Quebec’s...

CLIMATE AND PICEA MARIANA SEED MATURATION RELATIONSHIPS: A MULTI‐SCALE PERSPECTIVE

One of the most important ways by which northern forests will respond to anticipated climate change is through variations in seed maturation. In this study, the relationship between growing degree-days (DD) >5°C and seed maturity was evaluated at three spatial scales. At the continental scale, the development of female gametophytes and embryos was evaluated as a function of the heat sums obtained from 11 sites distributed across the Canadian range of black spruce. At the regional scale, cone size and the percentage of germinated seeds formed in 1998, 1999, and 2000 were analyzed from seven sites situated along a latitudinal gradient in northern Quebec. At the local scale, cones were collected along perilacustral and insular transects according to their exposure to large water bodies, and from 10 islands located within a 2835-km2 hydroelectric reservoir. Our results confirm the 800–940 DD thermal sum threshold necessary for the complete maturation of black spruce embryos at several populations distributed across the total range of the species. Along the regional south-to-north climatic gradient, the percentage of germination can be predicted by a sigmoid function of a thermal sum (y = 2.8 + 25.1/[1 + e(x−896.6)/84.6]; r2 = 0.85, P < 0.0001) that attains a plateau at around 800–940 DD. Once the 800–940 DD threshold is attained, variations in the percentage of seed germination are mainly associated with inter-tree differences and local site factors (thickness of organic matter, tree density, tree height, tree age, and fetch). In the springtime, cold enclaves are created by the presence of the hydroelectric reservoir. The seed germination percentages in these enclaves varied from 0.6% ± 0.7% to 14.9% ± 19.1% (mean ± SD) according to the site (compared to 22.7% ± 15.1% for a site not exposed to the reservoir), which was equivalent to the germination percentages for sites at latitudes 1–3° farther north. These data suggest that the potential for black spruce regeneration increases strongly beyond the 800 DD isotherm, which evokes the possibility that subarctic open forests may become more dense under the current anticipated climate changes.

Impact of fire on Picea mariana and Pinus banksiana seedlings in subarctic lichen woodlands

. The demography of Picea mariana (black spruce) and Pinus banksiana (jack pine) seedlings was monitored through five censuses over 13 months in four different seed bed types after fire of four severity levels in lichen woodland. Most seeds germinated just before early frost in late summer 1990 or immediately after snow thaw in early spring 1991; the germination rate subsequently decreased. For both species, germination rate decreased along a gradient of fire severity. The proportion of Pinus seeds that produced a seedling surviving 13 months after sowing was 4.3 % and 0.4 % respectively in the intact lichen mat and in the mineral soil seed bed type. For Picea these values are 3.2 % and 0.2 % respectively. The low germination rate in a severely burned seed bed type appeared to be associated with the formation of a water-repellent crust at the soil surface following the fire. Seedlings were contagiously distributed and were more frequent in flat and hollow microsites, where there is probably more water available than on bumps or among pebbles. Properties of experimentally burned seed bed types may differ from those under natural fires where regeneration by seed generally occurs following dispersal. However, the high germination rate observed in the intact lichen mat suggests that scattered lichen woodland patches may respond to increased seed input by a higher frequency of seedling establishment.

Effects of altitude and tree height on the distribution and biomass of fruticose arboreal lichens in an old growth balsam fir forest

Abstract:Biomass and distribution of arboreal fruticose lichens were studied along two environmental gradients, height along vertical tree axis and altitude, in an old growth balsam fir (Abies balsamea [L.] Mill.) forest in the McGerrigle Range, Gaspe Provincial Park, Quebec. Biomass was estimated by subsampling 53 balsam firs selected from five plots, systematically distributed along an elevational transect. Three vegetation belts (mountain, subalpine, and alpine) located between 720 and 1068 m (above sea level) were sampled. Alectoria sarmentosa, Bryoria spp. and Usnea spp. represented the bulk of epiphytic lichen vegetation. Both gradients influenced lichen biomass and species diversity on balsam fir. The bulk of lichen biomass was found at intermediate heights in trees. In the mountain belt, the number of lichen species was positively correlated with tree height and diameter. Total lichen biomass on trees was predicted by tree diameter and vegetation belt (ln[biomass + 1] = 2.91x + k; R2 = 0.87; k = -...