About the Project
The aim of this PhD project is to gain deeper insights in the possible drivers behind observed changes in boreal forest functioning focussing on the following three drivers:
1. Expansion of boreal forest cover. Temperature increases should cause a shift of the tree line, but very few studies have demonstrated this empirically and there are very few estimates of the rate of forest expansion in this region. Contributions of forest expansion to the net carbon sink in this region are still poorly quantified.
2. Changes in forest disturbance regimes. Fire, droughts and insect attacks are important disturbances affecting boreal vegetation functioning, and are all likely increasing with global change. More frequent fires may lead to shifts towards more deciduous trees (Johnstone et al. 2010), while increases in any disturbance types will lead to decreases in mean forest age. Productivity of re-growing young forests is much higher than older forests, and shifts in forest age thus affect carbon uptake and loss.
3. Changes in primary productivity or respiration of existing vegetation. Increases in season length and mean growing season temperature will increase both photosynthetic productivity and respiration, nonetheless neither the magnitude of these increases nor the balance between productivity and respiration are well understood.
Methodology- Remote sensing data will be complemented with on-the-ground forest plot data and tree ring data to assess the contributions of the described drivers to boreal vegetation functioning. Long-term Landsat data will be analysed to quantify expansion of forests northwards (driver 1). For assessing changes in forest disturbance regimes (driver 2), suitable optical bands, and indices could include Normalized Difference Vegetation index (NDVI), Normalised Difference Water Index (NDWI), Normalised Burn Index (NBI) and Tassled Cap Wetness. Algorithm selection will be explored with methods such as Landsat-based Detection of Trends in Disturbance and Recovery (LandTrendr), and Breaks For Additive Season and Trend Monitor (BFAST), both of which have been developed for forest disturbance detection from Landsat data. These various products will be compared against highly detailed information on forest age, composition, biomass and productivity for over 30 thousand sites distributed across Quebec from forestry inventory data, and allow selection of most suitable metrics for reconstructing changes in forest age, composition, biomass stocks and productivity. To study effects of temperature on productivity and respiration (driver 3), we will further combine satellite products with tree ring data allowing assessing long-term changes. This project may explore implications of changes in vegetation composition on atmospheric CO2 using vegetation model scenarios of atmospheric carbon uptake and release in atmospheric transport models.
Keenan and Riley (2018) Greening of the land surface in world’s cold regions consistent with recent warming, Nature climate change, 8(9); 825-828;
Johnstone et al. (2010) Changes in fire regime break the legacy lock on successional trajectories in Alaskan boreal forests, Global Change Biology, 16(4);
Myneni et al. (1997) Increased plant growth in the northern high latitudes from 1981 to 1991, Nature 386, 698–702 (1997);
Piao, et al. (2017) Weakening temperature control on the interannual variations of spring carbon uptake across northern lands, Nature climate change 7, 359-363;
Wang et al. (2013) Accelerating carbon uptake in the Northern Hemisphere: evidence from the interhemispheric difference of atmospheric CO2 concentrations, Tellus B: Chem. and Phys. Met., 65(1), 20334;
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