RECOVERY

The overall objective of the projectl is to better understand the response of northern vegetation to climate change and its implication on carbon cycling. Through combining more established satellite-based coarse and novel high-resolution vegetation cover and vegetation cover change datasets as well as climate data, we will analyze pattern and causes of greening trends for both regions with stable and dynamic land cover. In a second step, we will constrain a state-of-the-art DGVM, i.e. the LPJmL, to reproduce the observed vegetation responses and will improve model parameterizations if necessary. The LPJmL simulates vegetation dynamics, including wildfire and permafrost, and their related carbon, water and nitrogen cycling, in a common modelling framework. In a final step, we will couple the improved DGVM carbon fluxes to an atmospheric transport model (TM3) to reproduce the observed CO2 amplitude trends and apply factorial simulations to attribute the drivers, processes and mechanisms that are responsible for the increasing CO2 amplitude. With our project we would like to test the following key hypotheses in regards to emerging northern ecosystem responses including the northern greening trend and the enhanced northern seasonal CO2 exchange:H1: Vegetation responses to warming trends: Increased plant productivity caused by (i) plant physiological responses (temperature-, moisture-, permafrost-, nutrient-driven) (ii) shifts in vegetation phenology (increased growing season length)H2: Gradual vegetation cover changes: Increased plant productivity caused by (i) expansion of boreal forest biome into tundra (tree line advance; temperature-driven) (ii) shrub expansion within tundra and boreal ecosystems (temperature-driven)H3: Abrupt vegetation cover changes: Shifts in disturbance regimes manifested by (i) compositional shifts towards increased fraction of deciduous versus evergreen vegetation within boreal systems (triggered by changing fire regime) contributing to the increasing CO2 amplitude (ii) increases in fire and drought-related tree mortality causing carbon loss that counterbalances the increasing CO2 amplitudeAlternative explanations for changes in northern ecosystem dynamics and corresponding links to CO2 amplitude trends that are related to changes in light-use efficiency, CO2 fertilization and land use change in more temperate regions will also be assessed to yield a thorough understanding. Our general focus on carbon uptake processes, as opposed to carbon loss processes (e.g. ecosystem respiration), is because atmospheric studies show that the largest contribution to the enhanced northern CO2 seasonal cycle was during the growing season when uptake processes dominate.