1045 avenue de la Médecine
Alterations in temperature and precipitation regimes as a result of climate change generally result in improved growing conditions, leading to an increase in the primary productivity of Arctic and sub-Arctic ecosystems. This phenomenon is often referred to as the "greening of the North". Although this greening has been widely documented at the circumpolar scale, little information is available at the scale of the different plant communities found in the Arctic and sub-Arctic landscapes. The main objectives of this thesis are therefore to characterize the major vegetation formations in Nunavik along a latitudinal gradient extending from lichen woodlands bioclimatic zone to the prostrate shrub tundra, to assess the rate of vertical structure development and to evaluate the productivity of woody species.
My first objective is to quantify the variation of NDVI within the different dominant terrestrial plant communities present in Nunavik. This will allow us to identify the plant formations that contribute the most to the observed increase in productivity. My second objective is to quantify the temporal evolution of plant structure and primary productivity of plant communities along a latitudinal gradient in Nunavik. My last objective is to evaluate the climatic sensitivity and performance of woody species along a latitudinal gradient, which will allow the identification of buffer zones to the observed changes.
The analysis of the vertical structure and primary productivity of plant communities is conducted along a latitudinal gradient in Nunavik extending from forest tundra to prostrate shrub tundra (ca. 52nd to 62nd parallel). In order to capture the variability of the different vegetation formations, eight sampling stations were selected. The southern portion of the gradient includes the Clearwater Lake and Boniface River stations, located in forest tundra vegetation zone. The central portion of the gradient includes three stations located in shrub tundra: the Le Roy Lake station, ca. 90km north of the Boniface River station, the Payne Lake station and another station ca. 100km east of Puvirnituk. The northern portion of the latitudinal gradient comprised three stations located in the prostrate shrub tundra vegetation zone, one station near Chukotat River, one station at Deception Bay and one station near the Inuit community of Ivujivik.
In order to quantify NDVI variation, annual summer NDVI mosaics were generated from Landsat scenes for the period 1984 to 2020. The ecological mapping of the vegetation of Northern Quebec was then superimposed on the mosaics to calculate the average NDVI per vegetation polygon per year. The vertical structure of each sample site was obtained from vegetation surveys to determine the cover, size and biomass of the different functional groups. Shrub productivity and height growth development were determined from dendrochronological analyses. Climate sensitivity of shrubs was assessed using mixed effects models.
Since the increase in primary productivity in Arctic and sub-Arctic regions is mainly due to the expansion of shrubs, shrubby formations are expected to show the largest increases in NDVI. We should find evidence of a decrease in the complexity of plant community structure along the latitudinal gradient due to increased climatic and edaphic constraints. Vertical structure and ecosystem productivity of woody species should decrease with increasing latitude. We should observe a structural change in recent decades, particularly important in the central portion of the transect, due to a shift from an open tundra ecosystem to an upright shrub tundra ecosystem. Carbon balance is expected to decrease along the latitudinal gradient and also to accumulate more rapidly in shrubby formations due to increased productivity.