Climate change is altering the terrestrial carbon cycle in Artic tundra through the direct and indirect effects of warming, including the lengthening of the growing season due to earlier snowmelt. To predict future responses of this vast carbon reservoir to environmental change, we must understand how the underlying biological components of the carbon cycle – photosynthesis and respiration – vary through the short growing season, characterized by highly variable precipitation and temperature, and a near 24-hour photoperiod. To quantify seasonal trends in photosynthesis and respiration, measurements of leaf-level carbon dioxide fluxes were collected in five day intervals in two dominant tundra species, Eriophorum vaginatum, an evergreen tussock, and Betula nana, a deciduous woody shrub, grown under ambient and warmed conditions at the Arctic LTER in Toolik Lake, Alaska.
Photosynthetic rates increased over the season when expressed on nitrogen basis, with Betula exhibiting higher rates than Eriophorum throughout the season. Respiration (in darkness and light) decreased through the season in both species (P < 0.05), with Betula producing higher rates. The inhibition of respiration by light, measured via the Kok-method, increased through the growing season (P < 0.001), and ranged from 0-60% inhibition for both species. The temperature sensitivity of respiration (Q10) differed between species (P < 0.001), was not influenced by growth temperature, and exhibited variation during the early growing season (P < 0.001). For both species, the warming treatment did not significantly elevate or depress rates of gas exchange. The increases in photosynthesis and decreases in respiration suggest plant growth and energy demand drive gas exchange over the growing season, though species differences highlight the potential influence of growth form; Betula, as a deciduous shrub, must rapidly assimilate and allocate carbon into leaf and woody biomass during the short growing season, compared to the evergreen Eriophorum. The light inhibition of respiration proved to be highly influential in decreasing carbon efflux rates at mid-season. Neglecting this inhibitory effect at the ecosystem scale could lead to overestimations of gross primary productivity, especially in the night-less tundra. Further, the temperature acclimation of respiration and photosynthesis may explain the lack of a stimulatory warming effect on either species. The seasonal controls on photosynthesis and respiration collected in this study can be used to describe the current carbon balance of this region and more accurately predict its future status.