The response of tundra plant biomass, aboveground production, nitrogen, and CO₂ flux to experimental warming

We manipulated air temperature in tussock tundra near Toolik Lake, Alaska, and determined the consequences for total plant biomass, aboveground net primary production (ANPP), ecosystem nitrogen (N) pools and N uptake, and ecosystem CO₂ flux. After 3.5 growing seasons, in situ plastic greenhouses that raised air temperature during the growing season had little effect on total biomass, N content, or growing-season N uptake of the major plant and soil pools. Similarly, vascular ANPP and net ecosystem CO₂ exchange did not change with warming, although net primary production of mosses decreased with warming. Such general lack of response supports the hypothesis that productivity in tundra is constrained by the indirect effects of cold temperatures (e.g., low nutrient availability or short growing-season length) rather than by cold growing-season temperatures per se. Despite no effect on net ecosystem CO₂ flux, air warming stimulated early-season gross photosynthesis (GP) and ecosystem respiration (ER) throughout the growing season. This increased carbon turnover was probably associated with species-level responses to increased air temperature. Warming increased the aboveground biomass of the overstory shrub, dwarf birch (Betula nana), and caused a significant net redistribution of N from the understory evergreen shrub, Vaccinium vitis-idaea, to B. nana, despite no effects on soil temperature, total plant N, or N availability. Thus, although air warming had no effect on total ecosystem N pools, it did mediate N distribution within the plant community. The early-season stimulation of GP is consistent with warming effects on leaf expansion of dominant shrubs (including B. nana) observed in other studies in tussock tundra. The stimulation of ER probably resulted from maintenance and growth respiration associated with higher aboveground B. nana biomass production and higher root N uptake and/or turnover in the greenhouses. The species responses to warming are consistent with changes in plant species assemblages associated with past warming and suggest that future warming may increase the abundance of B. nana in tussock tundra. We conclude that a 4°C rise in air temperature alone will significantly stimulate carbon turnover in tundra but will have little effect on net carbon balance.