How will global warming affect sub-Antarctic vegetation? To answer this question, it is necessary to understand the thermal environment of dominant plant species and how variation in resources affects photosynthetic responses to temperature. Our study focussed on vegetation on Macquarie Island in the Southern Ocean where the seasonal and daily range in temperature is much less than in alpine or tundra environments at equivalent northern latitudes. Soils on Macquarie Island are rich in nitrogen, which decreases with distance from seal and penguin colonies. Water is abundant under prevailing cool conditions, but under brief periods of sunlight, as might become more frequent with climate warming, leaves adapted to high humidity may experience water deficit. We studied two dominant species: the megaherb, Stilbocarpa polaris and tussock grass, Poa foliosa. Measurements were made along altitudinal transects to determine patterns in leaf resource investment, carbon gain and water use in relation to growth temperature and nitrogen availability. Leaf temperature was monitored on species pairs at sea level, mid-slope and at the upper distributional limits of 165m. Gas exchange characteristics were measured in response to variation in temperature and leaves were harvested for determination of nitrogen content, δ¹³C and specific leaf area.

Results/Conclusions

Average daytime temperature decreased by 0.7ºC per 100m altitude. However, the daily range in leaf temperatures was greater in the large-leaved megaherb, with lower minima and higher maxima. Warmer leaf temperatures and higher photosynthetic temperature optima gave Stilbocarpa similar assimilation rates to Poa when modelled for their average daytime leaf temperatures. In both species, photosynthetic temperature optima were lower at the cooler, higher altitudes. Leaf nitrogen content decreased with increasing altitude with no concurrent changes in specific leaf area. Photosynthetic capacity followed gradients in leaf nitrogen such that assimilation rates were highest near the coast and declined with altitude. The tussock grass had similar leaf nitrogen content to the megaherb, but invested more nitrogen in photosynthetic capacity. Higher photosynthetic capacity enabled Poa to function with a higher assimilation rate at lower stomatal conductance, and hence also greater water use efficiency and higher δ¹³C than Stilbocarpa. Stomatal closure increasingly limited assimilation rates when leaf temperatures increased above 12ºC, particularly in Stilbocarpa, implying that water deficit may limit response to warming or to increased exposure to sunny periods when leaf temperatures can exceed 12ºC. These results suggest that Poa may benefit more from warming than Stilbocarpa.