ABSTRACT: Origin and dynamics of wooded palsas in the Rivière Boniface area, northern Québec.

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Abstract

Permafrost is defined as earth materials below 0°C for two or more years. In subarctic environments, the critical factor for permafrost formation is snow depth. In eastern Canada, maximum permafrost expansion occurred duringthe Little Ice Age. In the discontinuous permafrost zone, permafrost occurs most commonly in boreal and subarctic peatlands in the form of palsa mounds of variable size and height. The incidence of permafrost in peatlands increasesalong a south-north gradient. Further north, permafrost development also occurs in mineral soils in wet depressions. Low air temperature, reduced precipitation and a thin snow cover are the most influential factors inducing palsa formation. Organic palsas refer to peaty permafrost mounds that are distributed above the peatland surface, ranging from 0.5 to 10 m in height and exceeding 2 m in diameter. Mineral palsas correspond to permafrost moundsdeveloped in mineral sediments and controlled by the same physical processes as in organic palsas. Permafrost growth modifies profoundly the characteristics of a peatland by elevating its surface above the water table. The up-thrusting of the water-saturated peat layer due to permafrost growth causes a shift towards dry to mesic lichen-shrub vegetation. Because the surface of the peatland becomes dryer, tree establishment is possible. Three types of wooded palsas were observed in the study area: palsas covered by a dense mature forest, palsas covered by a dense young forest and palsas covered by black spruce shrubs. Wooded palsas are permafrost mounds with a cover of tree species greater than 10%. Depending on site exposition to snow-drift conditions, they are generally colonized by shrubby and arborescent black spruce (Picea mariana (mill.) BSP). The stratigraphy of a palsa consists of a sequence of various peat materials associated with the environment at the time of deposition. In organic wooded palsas, fen peat is generally buried by forest peat, the black arrow shows the forest peat layer and the white arrow the fen peat layer). The up-thrusting of mineral palsas causes the burial of the vegetation and soil developed nearby. The main objectives of this study are to determine the period of formation of wooded palsas and to evaluate their dynamics. We hypothesized that palsas with a dense mature forest were formed 1500-1000 years BP and that the two other types of palsas were formed during the Little Ice Age. The study area is located in the Rivière Boniface basin (57º45’N; 76º20’W), 130 km southeast of Inukjuaq and 30km inland from the Hudson Bay coast. The area is in the zone of discontinuous but widespread permafrost, 10 km south of the arctic tree line. At the Inukjuaq weather station, the mean annual temperature is -7 °C, ranging from -26 °C (in February) to 9 °C (in July), with a mean frost free period of approximately 60 days. Annual precipitation totals 450-500 mm, 40% of which fall as snow. The growing season extends from mid-June to mid-August. Black spruce is the dominant species in valleys and wet depressions, whereas hilltops are covered by lichen-tundra vegetation. Nine sites (S1, S2, S3…S9) were selected to represent both mineral and organic palsas, and each of the three types of wooded palsas. A transect was layed out across each palsa, and the site characteristics were recorded. The vegetation of each palsa was described along the transect, using linear cover of dominant species within 20 cm long, contiguous segments. Plants were recorded using 10 cover classes (class 1: < 10%; class 2: 10-20%; class 3: 20-30%, etc.). The data were averaged within 2 m long, contiguous segments to show the mean linear cover of the species. Topography, thickness of snow cover and active layer, and height of the tallest black spruce were also measured at each 2 m segments along the transects.At each site dominated by organic palsas, the uppermost section of fen peat and the lowermost section of forest peat were sampled for radiocarbon dating and determination of maximum and minimum age of each palsa, respectively. Buried organic matter in disturbed soils associated with surface up-thrusting was sampled for dating of mineral palsas. Radiocarbon dates of buried soils correspond to maximum age of palsa formation. Both conventional and AMS methods were used to date the samples. The 14C dates were calibrated in calendar years using CALIB REV 5.0.1 in accordance with intcal04.14c. (alive and dead) sampled at each site. A 7.5 x 10 m quadrat was established on palsas of three sites (S1, S2 and S3), where the diameter and height of every black spruce was measured. In addition, a cross-section of each black spruce was taken at the base of the main stem for age determination. Annual tree rings in each cross-section were counted and cross-dated using long chronologies of diagnostic light rings. At the sites where palsas are covered by a dense mature forest, the approximative age of the organic palsas, basedon radiocarbon dating of forest and fen peat, was 1200-900 cal. years BP. The life span of the oldest trees was 1463-1544 AD and 1348-1512 AD for sites S1 and S2, respectively. The life span of all the sampled black spruce (dead and alive) in sites S1 and S2 plotted over time shows individuals of all ages, suggesting that the forest likely at equilibrium. Black spruce size structure at sites S1 and S2 shows a negative exponential trend, further suggesting that these forests are at equilibrium. At site S3 and S7, the approximative age of palsas, based on radiocarbon dating of buried organic matter, is 500-400 cal. years BP and 650-400 cal. years BP, respectively. The life span of the oldest tree was 1554-1644 AD at site S3 and 1567-1660 AD at site S7. These results suggest that these palsas were formed at the beginning of the Little Ice Age, and are colonized by younger trees. Palsas at sites S5, S6 and S8 are covered by black spruce shrubs and were formed around 550-450 cal. years BP. The life span of the oldest trees in site S5, S6 and S8 was 1466-1732 AD, 1546-1660 AD and 1527-1636 AD, respectively. This suggests that these palsas also formed at the beginning of the Little Ice Age. At site S9, thepalsa was covered by black spruce shrubs and formed around 1100-900 cal. years BP. The life span of the oldest tree at this site was 1322-1436 AD. The main results of this study are: (1) wooded palsas covered by dense mature forest at equilibrium formed at 1200-1000 cal. years BP, (2) wooded palsas covered by younger dense forest formed at the beginning of the Little Ice Age, and (3) wooded palsas covered by shrubby black spruces formed at 1200-1000 cal. years BP or at the beginning of the Little Ice Age.

Bibliography of Canadian Geomorphology