Nynne Marie Rand Ravn:
Low temperatures in the Arctic have through geological times caused a build-up of carbon (C) in the soil while carbon dioxide (CO2) has been removed from the atmosphere, because ecosystem respiration (ER) has been exceeded by the photosynthesis (GEP). Increasing air temperature, as occurring in current years, is likely to increase ER due to increased decomposition and increased substrate input e.g. due to permafrost thaw. If GEP is left unchanged and decomposition of old C from the soil is occurring this might result in an increase of the emission of CO2 to the atmosphere probably resulting in further warming.
This PhD thesis addresses different aspects of climate change effects on C dynamics in the Arctic. The focus has been on i) changes in ER, age of the C sources, GEP and the net ecosystem exchange (NEE) in response to long- and short-term climate manipulations and ii) comparisons of CO2 fluxes and organic nutrient utilization between ecosystems occurring from different latitudes and dominated by different vegetation types. These aspects are important to understand the effects of climate change on the CO2 balance in the Arctic and its potential positive feedback on global climate change. Furthermore, comparisons between arctic ecosystems across different latitudes or dominated by different vegetation can validate predictions based on data from one ecosystem to other arctic settings.
To improve our understanding of climate change effects on CO2 fluxes during the snow free season, field measurements were carried out at two subarctic shrub heaths in Abisko (North Sweden), and at a low-arctic shrub heath at Disko (West Greenland), exposed to climate change manipulations. Increased summer temperature is the recurrent manipulation, in the short-term at the low arctic heath and in long- and short-term in the subarctic. Other treatments were long-term addition of fertilizer or litter and shading, and short-term increased winter snow depth and shrub removal. In the attempt to compare arctic ecosystems, a laboratory study was conducted and the possibility of plant and microbial utilization of an organic compound at thaw was investigated for high arctic (Zackenberg, East Greenland) and subarctic (Abisko), mesocosms dominated by Cassiope tetragona or Salix hastata/Salix arctica. Furthermore, the ambient CO2 flux was measured at a low arctic heath (Disko) with patches dominated by different vegetation.
Enhanced temperature increased ER in the short-term at the low arctic heath and the δ13C value derived from ER were increased indicating emission of old soil C in response to summer warming. A long lasting response of ER to warming was also evident at the subarctic heaths and after as long as 22 years of summer warming indications of emission of old C were observed, though most pronounced in the spring. I found no short-term response of summer warming on GEP at the low arctic heath and the measurements of NEE showed an increased emission of CO2 to the atmosphere during two snow free seasons. Increased winter snow depth and shrub removal did also cause increase in the emission of CO2 to the atmosphere, as GEP was limited.
Based on the observations presented in this PhD thesis I conclude that increased summer temperature in the short-term perspective increases the emission of CO2 to the atmosphere and that contribution of old soil C might also be increased. The consequence can be a short-term positive feedback on climate change. In the long-term, higher temperatures will still stimulate ER, but the loss of old soil C is probably less pronounced. I speculate that enhanced plant biomass in response to long-term warming can cause GEP to increase and leave NEE unaltered. Hence, the risk of warming induced long-term positive feedback on climate change might be reduced. The new balance in the C cycling might though be sensitive to limitations of GEP due to for instance late snowmelt or herbivory.
Ecosystems at higher latitude might respond most rapid to increased temperature and nutrient addition compered to ecosystems from lower arctic latitude but the pattern in the responses seem similar between different shrub heaths.