Some of the fastest warming rates on earth are found in the northern hemisphere high-latitudes. As a result, large changes in ecosystem functioning are expected in these regions. Satellite observations reveal indeed that Northern hemi-sphere boreal forests have experienced a long-term greening trend since 1982 (Fig. 1, Myneni et al. 1997). Based on atmospheric CO2 records northern hemisphere land regions have also been identified as strong carbon sinks (Wang et al. 2013). Finally, northern hemisphere mid and high latitude atmospheric CO2 records show a strong increase in the seasonal CO2 amplitude over the past 50 years (Fig. 2, Graven et al. 2013). The amplitude increase reflects a substantial increase in seasonal vegetation carbon uptake and release. It is, however, not clear, whether this increase is caused just by changes of boreal forest functioning or whether other biomes and processes may also play a role (e.g. Zeng et al. 2014). It is also not clear whether the increased amplitude is an indication of a net carbon sink or rather e.g. of forest demographic changes (changes in turnover times). Nevertheless the most likely overall driver for these observed changes is the strong temperature increase in northern hemisphere high latitudes (Keenan and Riley, 2018). This is supported by an analysis of the atmospheric CO2 drawdown in spring observed at Barrow, a high latitude observation site, which correlates with spring temperature anomalies (Piao et al. 2017). Intriguingly this correlation seems to have weakened over the past one to two decades (Piao et al. 2017). This would suggest a change in vegetation functioning. On the other hand recent analysis of variation in spring vegetation greenness, which is closely related to productivity, has not broken down over time (Joyce et al., in prep.). Thus altogether we have still not fully understood the numerous pathways and feedbacks through which temperature, and associated changes in fires, droughts, insect outbreaks, and permafrost melting, affect boreal vegetation functioning.
The aim of this PhD project is to gain deeper insights in the possible drivers behind observed changes in boreal forest functioning. The project will focus on the role of the following three drivers:
- Expansion of boreal forest cover. Theory predicts that 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.
- 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 regrowing young forests is much higher than older forests, and shifts in forest age thus affect carbon uptake and loss.
- 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. Pixel-based variables and spatial-based features such as textures and segmentation will also be explored for land-cover change detection. 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 Quebec 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 from Quebec and the global tree ring database (ITRDB) 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.
References Graven H. D. et al. (2013) Enhanced Seasonal Exchange of CO2 by Northern Ecosystems Since 1960, Science, 341, 1085;
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;
Zeng, N. et al. (2014), Agricultural Green Revolution as a driver of increasing atmospheric CO2 seasonal amplitude, Nature, 515, 394–397.