Serge Payette

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.

Light rings in subarctic conifers as a dendrochronological tool

Abstract Light rings are characterized by one or a very few latewood-cell layers, an indication of shortened growing seasons, and are particularly frequent in black spruce (Picea mariana (Mill.) BSP) at the treeline in Quebec. The construction of a light-ring chronology spanning the period AD 1398–1982 showed that the highest frequency (>25%) of light rings among 160 trees and krummholz occurred in 1593, 1620, 1634, 1784, 1816, 1817, 1853, 1969, and 1972. These diagnostic rings be a useful cross-dating tool for dendroecologists working with living and dead krummholz with a low-growth variability. About two-thirds of the 65 light-ring years coincide with years (or triads) of major volcanic eruptions. The climatic conditions (low temperature) occurring at the end of the growing season, in part induced by the climatic effect of volcanism, seem to initiate light rings.

A matter of divergence: Tracking recent warming at hemispheric scales using tree ring data

[1] No current tree ring (TR) based reconstruction of extratropical Northern Hemisphere (ENH) temperatures that extends into the 1990s captures the full range of late 20th century warming observed in the instrumental record. Over recent decades, a divergence between cooler reconstructed and warmer instrumental large-scale temperatures is observed. We hypothesize that this problem is partly related to the fact that some of the constituent chronologies used for previous reconstructions show divergence against local temperatures in the recent period. In this study, we compiled TR data and published local/regional reconstructions that show no divergence against local temperatures. These data have not been included in other large-scale temperature reconstructions. Utilizing this data set, we developed a new, completely independent reconstruction of ENH annual temperatures (1750–2000). This record is not meant to replace existing reconstructions but allows some degree of independent validation of these earlier studies as well as demonstrating that TR data can better model recent warming at large scales when careful selection of constituent chronologies is made at the local scale. Although the new series tracks the increase in ENH annual temperatures over the last few decades better than any existing reconstruction, it still slightly under predicts values in the post-1988 period. We finally discuss possible reasons why it is so difficult to model post-mid-1980s warming, provide some possible alternative approaches with regards to the instrumental target and detail several recommendations that should be followed in future large-scale reconstruction attempts that may result in more robust temperature estimates.

CONTRASTED DYNAMICS OF NORTHERN LABRADOR TREE LINES CAUSED BY CLIMATE CHANGE AND MIGRATIONAL LAG

The northern Québec-Labrador tree lines are the most climatically stressed tree ecosystems of eastern North America. In particular, white spruce (Picea glauca) tree line populations distributed between 56 degrees N and 58 degrees N and 61 degrees W and 66 degrees W show contrasted responses to recent changes in climate according to their geographic position relative to the Labrador Sea. Along the coast, the northernmost latitudinal and altitudinal tree lines responded positively to warming over the last 50 years with invading spruce several tens of meters above the current tree line. In contrast, white spruce tree lines across the wind-exposed Labrador plateau are located much higher in altitude and have receded a few tens of meters beginning around AD 1740-1750 and have not yet recovered. Whereas no field evidence of recent fire and insect damage was found, all inland tree line stands were progressively damaged likely due to mechanical defoliation of wind-exposed trees. Massive tree death in the 19th century caused a reduction in the number of seed-bearing trees, and declining tree lines were not replenished by seedlings. Recent warming reported for northern latitudes has not been strong enough to change the regressive tree line trajectory in interior Labrador. However, white spruce expansion above coastal tree line in the northernmost forest site in Labrador is in line with current climatic trends. It is hypothesized that the species is still advancing toward its potential tree line higher in altitude due to delayed postglacial migration. The slow advance of white spruce in northernmost coastal Labrador is likely caused by the rugged topography of the Torngat-Kaumajet-Kiglapait mountains.

THE CREATION OF ALTERNATIVE STABLE STATES IN THE SOUTHERN BOREAL FOREST, QUEBEC, CANADA

The southernmost spruce-lichen woodlands in the Parc des Grands-Jardins, Quebec, Canada, are situated 500 km south of their usual range in the northern lichen woodland zone. Their co-occurrence within a spruce-moss forest matrix suggests the ex- istence of alternative stable states. We investigate the possibility of these spruce-lichen woodlands as an alternative stable state along with the factors contributing to their origin and spatiotemporal distribution. Analysis of plant macrofossils, charcoal, head capsules of defoliating insects, and pollen were used along with vegetation surveys to reconstruct the past and present disturbance dynamics along an east-west transect, corresponding to a precipitation and fire frequency gradient. At each site, spruce budworm head capsules were found preceding the charcoal layer delineating the shift to spruce-lichen woodland, dem- onstrating the compound disturbance (insect-fire) origin of the lichen woodlands. Moss forests previously occupied all lichen woodland sites, with the oldest record starting ca. 8300-9400 yr BP. A change to a higher fire frequency around 2500 yr BP was evident in the lichen woodland zone of the park. A lower fire frequency west of the lichen woodland zone likely is a result of orographic precipitation. While the spruce budworm affects the entire region, lichen woodlands are found exclusively within an increased fire frequency zone. Thus, it is the superimposition of these two disturbance factors that is responsible both for their creation and spatial distribution. Lichen woodland inception dates range between 580 and 1440 yr BP, demonstrating that these lichen woodlands have maintained their open structure with time and have not transformed into closed forests. Their persis- tence, along with their previous moss forest histories and current occurrence adjacent to closed moss forests, indicate that they are an alternative stable state to the spruce-moss forests and not a successional stage. In contrast to other examples of reported alternative stable states, this one is a result of natural disturbances inherent to the system and not anthropogenic impacts.

Recent Advance of the Arctic Treeline Along the Eastern Coast of Hudson Bay

1 The very cold, wind-exposed coastal environment along the eastern Hudson Bay coast (northern Quebec, Canada) has resulted in a treeline which runs parallel to the coast (i.e. north-south). We have measured changes in the longitudinal position of the black spruce (Picea mariana (Mill.) B.S.P.) treeline during the last centuries, using tree ring analysis of spruce stem development. 2 Treeline trees () 2.5 m high) were sampled along transects in valley sites of four different rivers draining towards Hudson Bay, from the present treeline to the forest limit. The year of initiation of supranival stem growth (i.e. when stems first developed above the snowpack) was determined for trees, whorled growth forms and dead shrubs. Supranival tree stems developed synchronously in the valleys, from the late 1800s to the mid 1900s, in the area presently occupied by treeline trees, indicating that only krummholz (shrubby spruces) were present at these sites during the Little Ice Age. 3 A corresponding displacement of the treeline of about 12 km towards Hudson Bay has occurred in the area since the late 1 800s, most likely as a result of recent warming. The recent treeline shift also resulted from the development of vertical tree stems from pre-established krummholz, suggesting that stem growth was a direct response to favourable summer growth conditions and reduced wind-driven snow abrasion and supercold wind-chill. The absence of treeline trees originating from seeds indicates that the recent warming has not been of sufficient magnitude or duration to promote sexual reproduction.

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.

Recent Fluctuations of the Lichen-Spruce Forest Limit in Subarctic Quebec

1 Black spruce (Picea mariana) experienced increased radial growth and stem height over the last 100 years at treeline in subarctic Quebec, suggesting warmer and snowier conditions. We tested if recent climate change also induced a shift from forest to krummholz during the Little Ice Age and stand reversion from krummholz to forest during the 1900s. Whether the shifts caused measurable displacements of the forest limit were also examined. 2 Growth forms of living and dead spruces were compared in five lichen-spruce stands located 0, 2, 4, 6 and 8 km south of the krummholz limit. The age structure of supranival shoots (stems standing above the snow cover) at each site was also determined. 3 A reversion from lichen-spruce forest to krummholz probably occurred during the mid-1800s. Since the late 1800s, the forest limit moved 4 km northward most likely in response to milder winter conditions. Krummholz changed progressively to forest as spruce height and frequency of the tree growth form increased. Thus the northward advance of the forest limit resulted from structural changes of pre-established spruces, whereas there was no evidence for a recent spruce establishment in the tundra.

The Range Limit of Boreal Tree Species in Quebec Labrador: an Ecological and Palaeoecological Interpretation

Most boreal tree species in northern Quebec-Labrador spread towards their northern range limit from different source areas during the Holocene. Two major pathways were utilized by the dispersing species, one on either side of the slowly decaying Laurentide ice. The Labradorean pathway was deglaciated early during the Holocene, thus favoring the northward expansion of boreal trees adapted to the humid environment. The Hudsonian pathway was formed later and it included a less diversified flora with black spruce [Picea mariana (Miller) Britton, Sterns and Poggenburg] as the only tree species to have reached the northern tree-line. In eastern Quebec-Labrador, tree species were able to occupy their potential sites throughout the region. In western Quebec-Labrador, many tree species have not yet colonized all the potential sites. The northern range limit of black spruce throughout northern Quebec-Labrador appears out-of-phase with present climate. White spruce [Picea glauca (Moench) Voss] is in equilibrium with present climate in the eastern part of the region, whereas it is in the process of expansion along the Hudson Bay coast. Balsam fir [Abies balsamea (L.) Miller] is slowly expanding in the boreal James Bay area. The spread of jack pine (Pinus banksiana Lambert) followed a SW-NE direction towards central Quebec-Labrador, a pattern probably determined by climate-mediated fire regimes. This species is at equilibrium with present climate and it has not gone beyond the southern edge of the forest-tundra because of late arrival and possibly unfavorable conditions. Populus probably arrived early after deglaciation; it is not known to what extent both species (P. tremuloides Michaux and P. balsamifera L.) are at equilibrium with present climatic conditions at their northern range limit. The geographical pattern of final deglaciation most likely controlled the spread of tamarack [Larix laricina (Du Roi) K. Koch]. This species showed contrasting abundance east and west of 74°W, i.e. along a line where the residual ice lasted until ca. 5500-5000 BP. Tamarack is still spreading towards the tree-line along the Hudson Bay coast. Instead of important latitudinal movements, boreal tree species reacted to postglacial environmental change by changes in their regional abundance.

Biomass offsets little or none of permafrost carbon release from soils, streams, and wildfire: an expert assessment

As the permafrost region warms, its large organic carbon pool will be increasingly vulnerable to decomposition, combustion, and hydrologic export. Models predict that some portion of this release w ...