| Summary: | Predicting the intensity of climate change and its consequences on our livelihood requires an understanding of the factors that drive changes in biosphere - atmosphere exchange of greenhouse gases and how these feedback onto the global climate system. To achieve this understanding, it is imperative that monitoring, modelling and manipulation experiments of drivers of greenhouse gas fluxes be intensified across the globe. The Arctic tundra represents an important biome in the context of global climate change. This is because of the highly sensitive nature of the Arctic tundra to climatic perturbations and the potential for widespread feedbacks with global consequences. In this thesis, I present and discuss the findings of an investigation of comparable drivers of the seasonality in carbon dioxide (CO 2 ) fluxes across heterogeneous Arctic tundra ecosystems. Due to the remoteness and the harsh climatic conditions in the Arctic tundra, inter-comparison studies of Arctic ecosystems are still rare. A standardized post-processing procedure ensures site inter-comparability and allows putting the results from the individual sites into a broader circumarctic context. The results of this study have identified and quantified important drivers that need to be considered when future pathways of CO 2 exchanges between ecosystems and the atmosphere are assessed in the light of climate change in the Arctic. Using light response curve parameters as site specific descriptors which illustrate comparable ecosystem CO 2 flux characteristics, we have identified vegetation indices (like LAI and NDVI) and air temperature as important drivers of spatial variations in CO 2 fluxes across the heterogeneous Arctic tundra landscape during the peak of the growing season. Moreover, while interannual variations in peak season CO 2 net uptake were driven mainly by radiation-related and temperature-related variables in a low Arctic mixed tundra in Daring Lake and a high Arctic heath tundra in Zackenberg, LAI and radiation were the main drivers of observed trends in photosynthetic uptake in a subarctic fen in Stordalen. During the growing season, while there were no discernable drivers of CO 2 fluxes in Stordalen, growing season length showed significant controls on net ecosystem exchange (NEE) in Zackenberg and with gross primary production (GPP) and ecosystem respiration (R e ) in Daring Lake. This is important considering the recent observations and predictions of a lengthening of the growing season across the Arctic tundra. In an experimental manipulation of snow cover, we observed that predicted increases in winter precipitation would shorten the growing season. Yet, an enhancement of the photosynthetic capacity of the changing vegetation and landscape compensates for the shorter growing season due to increase snow cover and duration. Other drivers of growing season CO 2 fluxes were mainly air temperature, growing degree days and photosynthetic active radiation in a high and a low Arctic tundra ecosystem. Upscaling Arctic tundra NEE based on an acquired understanding of the drivers of NEE during this research venture, shows an estimation of reasonable fluxes at three independent sites in low Arctic Alaska. However, this later project is still ongoing and its findings are only preliminary.
|
|---|