Adaptational lag has been reported for Eriophorum vaginatum populations in Alaska's moist tussock tundra, suggesting that this dominant tussock-forming sedge may be vulnerable to replacement by woody shrubs as Arctic temperatures continue to rise. The role of physiological factors, including temperature optima for photosynthesis, that underlie this adaptational lag are unknown. Physiological performance of E. vaginatum was investigated in transplant gardens containing tussocks from three ecotypes, representing populations from Coldfoot (67.3°N), Toolik (68.6°N), and Sagwon (69.4°N). Identical gardens were located at Sagwon and Toolik, and half of the transplanted tussocks were passively warmed with open-top-chambers (OTC). Photosynthetic CO2 response curves (A/Ci curves) were measured on five tussocks per experimental treatment combination (i.e., ecotype and OTC vs. ambient) at both transplant gardens. Curves were made at measurement temperatures of 15, 20, and 25°C. Each A/Ci curve was analyzed to determine maximum Rubisco activity (Vcmax) and maximum electron transport rate (Jmax). These data, along with measurements of dark respiration (Rd) were expressed relative to measurement temperature, and a slope or Q10, in the case of Rd, was computed for each relationship. Slopes and Q10 values were analyzed separately for each garden with two-way ANOVA, where ecotype and treatment were the main effects.
Slopes of Vcmax and Jmax, as well as the Q10 of Rd, did not differ significantly among ecotypes at either garden (p > 0.05). There was a tendency toward greater temperature responsiveness of photosynthetic capacity under the warming treatment at the Toolik garden, though this pattern varied with ecotype such that no significant difference between treatments was observed at either garden (p > 0.05). The Q10 of Rd was significantly higher for tussocks exposed to the warming treatment at the Sagwon garden (p = 0.026), though no difference existed at Toolik (p > 0.05). These analyses indicate that anticipated ecotypic differences in photosynthetic capacity and respiration rates do not exist, as the southernmost ecotype from Coldfoot had no physiological advantage, despite adaptation to a warmer growing environment. Therefore, these physiological measures do not underlie adaptational lag in E. vaginatum. However, greater temperature responsiveness in photosynthetic capacity within a subset of ecotypes at the Toolik garden suggest variation in phenotypic plasticity among ecotypes. At the Sagwon garden, the significantly greater temperature responsiveness in Q10 values across ecotypes has the potential to reduce E. vaginatum carbon gain in the northernmost reaches of Alaska's moist tussock tundra as Arctic temperatures continue to rise.