Temperature and Microtopography Interact to Control Carbon Cycling in a High Arctic Fen Patrick F. Sullivan, 1,2, * Seth J. T. Arens, 1,2 Rodney A. Chimner, 3 and Jeffrey M. Welker 1,2 1 Environment and Natural Resources Institute, University of Alaska, Anchorage, AK 99501, USA; 2 Department of Biological Sciences, University of Alaska, Anchorage, AK 99508, USA; 3 Ecosystem Science Center, School of Forest Resources and Environmental Science, Michigan Technological University, Houghton, MI 49931, USA ABSTRACT High arctic wetlands hold large stores of soil carbon (C). The fate of these C stores in a changing climate is uncertain, as rising air temperatures may differ- entially affect photosynthesis and ecosystem respi- ration (ER). In this study, open-top warming chambers were used to increase air and soil tem- peratures in contrasting microtopographic positions of a high arctic fen in NW Greenland. CO 2 ex- change between the ecosystem and the atmosphere was measured on 28 dates over a 3-year period. Measurements of the normalized difference vege- tation index, leaf and stem growth, leaf-level gas exchange, leaf nitrogen, leaf d 13 C, and fine root production were made to investigate the mecha- nisms and consequences of observed changes in CO 2 exchange. Gross ecosystem photosynthesis (GEP) increased with chamber warming in hol- lows, which are characterized by standing water, and in hummocks, which extend above the water table. ER, however, increased only in hummocks, such that net ecosystem exchange (NEE) increased in hollows, but did not change in hummocks with chamber warming. Complementary measurements of plant growth revealed that increases in GEP corresponded with increases in C allocation to aboveground biomass in hummocks and below- ground biomass in hollows. Our results and those of several recent studies clearly demonstrate that effects of climate change on the C balance of northern wetlands will depend upon microtopog- raphy which, in turn, may be sensitive to climate change. Key words: climate change; CO 2 ; fine root; Greenland; ingrowth core; minirhizotron; peat- land; photosynthesis; respiration; wetland. INTRODUCTION Northern wetlands cover large areas of the Earth surface (346–500 Mha) and play a fundamental role in the global climate system, as reservoirs for carbon (C) (Gorham 1991; Zoltai and Martikainen 1996; Wieder 2001). Recent studies suggest that inter-annual climate variability and long-term directional change may turn northern wetlands that have been sinks into net sources of CO 2 to the atmosphere (Oechel and others 1995; Waddington and Roulet 1996; Alm and others 1999; Joiner and others 1999; Griffis and others 2000; Aurela and others 2002; Lafleur and others 2003). The cold, water-logged soils of arctic wetlands have long held rates of decomposition below rates of plant pro- duction, creating large stocks of soil C (for example, Received 19 April 2007; accepted 24 October 2007; published online 20 November 2007. *Corresponding author; e-mail: paddy@uaa.alaska.edu Ecosystems (2008) 11: 61–76 DOI: 10.1007/s10021-007-9107-y 61