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The warming of the high-latitude regions is improving the growing conditions of many vegetal species and generates increases in the vegetal productivity at the circumpolar scale. This greening of the Arctic and subarctic tundra seems to be heterogenous at the continental scale. Indeed, Nunavik and Labrador underwent more greening than the rest of Canada and Alaska between 1984 and 2012. The greening is also heterogenous at the landscape scale, because 50 to 60% of the shrub stands underwent productivity increases since 1984 compared to only 15% pour the forest stands. Knowing the processes involved in the heterogenous responses of the different type of plant stands to climate change is essential to predict the future dynamics of northern ecosystems and their repercussions on the climate of these regions.
Our aim is to identify the processes involved in the heterogenous increases of productivity between forest and shrub stands at the Nunavik treeline. To do so, we are going to compare the climate sensitivity of the radial growth of the woody species found in spruce stands (Black spruce - Picea mariana (Miller) BSP, and Dwarf shrub - Betula glandulosa Michx.) and in shrub stands (Betula glandulosa only) in Nunavik. We will also compare the reproductive success (seed germination) of Betula glandulosa in these two types of stands.
Study sites are in Nunavik, the North American region that underwent the most important greening between 1984 and 2012. More precisely, the sampling occurred in two stations: the Boniface river research station (57°45’ N, 76°10’ O) and the lac à l’Eau-Claire research station (56°20’N ,74°27’O). The Boniface river research station is located approximately 10 km south of the artic treeline. Black spruce (Picea mariana (Miller) BSP) is the only tree species and covers about 30% of the region. The remaining is covered by shrub tundra, with Betula glandulosa as the dominant specie. There is also wetlands, exposed hilltops, and sandy terraces. The lac à l’Eau-Claire station is located about 175 km south of the Boniface river, where the same type of environment is found.
In the summers of 2018 and 2019, stem sections of Black spruce and Betula glandulosa were collected in 6 shrub stands and 3 spruce stands at the Boniface river, and in 5 shrub stands and 3 spruce stands at the lac à l’Eau-Claire. These samples will be used to evaluate the climate sensitivity of these woody species. In the summer of 2019, Betula glandulosa seeds were collected in 4 shrub stands and 2 spruce stands. Germination trials were conducted in the summer of 2020.
Satellite images suggest that the increase in productivity is more important in shrub stands than in forest stands. We then expect the climate sensitivity of the radial growth of Betula glandulosa to be higher in shrub stands than in spruce stands. In spruce stands, we expect that the competitive forces resulting of the presence of many plant functional types will generate lower climate sensitivity. We also expect that the climate sensitivity will be higher at the Boniface river station and lower at the lac à l’Eau-Claire. For the reproductive success, we expect that seeds coming from shrub stands will have a higher germination rate than those coming from the spruce stands.