Light controls on growing season Net Ecosystem Exchange across Arctic tundra

This paper presents a comparison study of carbon dioxide (CO₂) response to light on 11 tundra sites from the Subarctic to high Arctic. Flux data has been collected using the micrometeorological eddy covariance technique. The sites range from dry Arctic heaths, wet mires with elevated palsa mounts, and wet fens. Growing season length ranges from 51 days at a northeast Greenland heath site (Zackenberg) to 118 days at a northern Swedish palsa mire (Abisko). Earliest start of growing season was in Abisko and Kaamanen (Finnish permafrost-free aapa mire) on 29th of May while the end of growing season ranged from 23rd August in Zackenberg to 25th September in a Russian tundra site (Seida). We have selected the most productive two-week period in summer and comparison has been done based on common light response curve (LRC) parameterization. The functional response of CO₂ exchange to light has been interpreted from the LRCs and their parameters used to investigate similarities and differences in aspects of ecosystem CO₂ exchange. This study shows that there is a lot of variability between Arctic tundra sites. Seida was the most productive site with 7.4, 3.97 μmol m^-2 s¹ of CO₂ and 0.06 μmol m^-2 s¹ of CO₂ per μmol m^-2 s¹ of PPFD (photsythetic photon flux density); for the LRC parameters fcsat, Rd and alpha, respectively. Abisko, northeast Siberian tundra (Chokurdakh) and Kaamanen were the most comparable sites with similar LRC parameter values ranging from 4.48 to 6.34, 1.5 to 2.0 μmol m^-2 s¹of CO2 and 0.015 to 0.02 μmol m^-2 s¹ of CO₂ perμmol m^-2 s¹ of PPFD; for fcsat, Rd and alpha respectively and goodness of fit ranging between 0.95 and 0.96. Atqasuk (Alaskan tundra) and Zackenberg were the least productive sites. The LRC's shows fastest vegetation response to light in Seida and the slowest in Samoylov Island (Russian wet polygonal tundra) and Zackenberg. This could be explained by ecosystem differences with contrasting vegetation and hydrology. Parameter differences between sites could be explained by differences in growing season length, hydrology, LAI and vegetation composition. Climate change effect will vary considerably in the Arctic tundra due to heterogeneity and differences in vegetation composition, hydrology and phenology.