Increased turnover but little change in the carbon balance of High-Arctic tundra exposed to whole growing season warming

Tundra ecosystems constitute large stocks of carbon and might therefore, if climate warming releases CO2, induce positive feedback and amplify temperature increase. We studied the effect of a 2.5degreesC temperature increment, induced by controlled infrared irradiation, on various components of the carbon balance of a High Arctic tundra ecosystem at Zackenberg in Northeast Greenland (74degreesN, 21degreesW) over the 1999 growing season. Gross photosynthesis (P-gross), belowground respiration (R-soil), and canopy respiration (R-canopy) were regularly determined with closed dynamic CO2 exchange systems, and the whole-growing season C-balance was reconstructed by relating these components to potentially controlling factors (green cover, soil moisture, radiation, soil and canopy temperature, and thawing depth). Thawing depth and green cover increased in heated plots, while soil moisture was not significantly affected. P-gross increased 24.2 owing to both a green cover and a physiological influence of warming. Belowground respiration was enhanced 33.3 mainly through direct warming impact and in spite of lower Q(10) in the heated plots; the factors controlling R-soil, were day of the year and soil moisture. did not differ significantly between treatments, although green cover was higher in the heated plots. This tundra ecosystem acted as a relatively small net sink both under current (0.86 mol CO2 m(-2)) and heated (1.24 mol CO2 m(-2)) conditions. Nevertheless, turnover increased, which was best explained by a combination of direct and indirect temperature effects, and delayed senescence.