How will climate warming affect photosynthetic assimilation rates in sub-Antarctic vegetation?  We studied photosynthetic responses to temperature in two dominant species on Macquarie Island, the umbelliferan megaherb Stilbocarpa polaris (Macquarie Island Cabbage) and the tussock grass Poa foliosa. A Free Air Temperature Increase (FATI) system of infra-red ceramic lamps was used to increase leaf temperatures by +1 to +3 °C under field conditions on Macquarie Island.  Adult plants were transplanted to a common garden where they were assigned to either control or warming treatments  arranged in five randomised blocks. Air and leaf temperatures were monitored with thermocouples, and infra-red video thermography was used to determine spatial variation in effects of heating treatments on distribution of leaf temperatures within plant canopies. Gas exchange measurements were made on fully expanded leaves 6 weeks after imposition of treatments, and compared with values measured in plants growing naturally along an altitudinal gradient with similar variation in temperature.  Assimilation rates were measured with variation in  leaf temperature above and below ambient conditions to determine whether thermal acclimation had occurred in response to FATI treatments.

Results/Conclusions

Macquarie Island is a very windy location. Consequently, leaf temperatures were often similar to air temperatures except when wind speeds were low. Nevertheless, FATI treatments effectively raised average leaf temperatures above ambient. However, average daily leaf temperatures were increased  differentially by 1.4°C for Stilbocarpa and 0.3°C for Poa,   reflecting the much greater area and more horizontal  display of leaves in the megaherb. Under ambient conditions, maximum assimilation rates of  7.7 µmol CO₂ m^-2 s^-1 were measured at 12-16°C in Poa, with this apparent plateau due to increasing stomatal limitations to photosynthesis. In contrast, assimilation rates increased from 7.3 to 10.2 µmol CO₂ m^-2 s^-1 with increase in leaf temperature from 8 to 18°C in Stilbocarpa, consistent with apparent increase in photosynthetic capacity. FATI warming increased photosynthetic capacity and associated assimilation rates in Poa at temperatures from 12-16°C, but induced a systematic reduction in both photosynthetic capacity and assimilation rates in Stilbocarpa. These results were consistent with a parallel set of measurements made along an  altitudinal gradient at Macquarie Island, and  suggest that Poa may be better able to increase carbon gain in response to climate warming than Stilbocarpa.