Comparative Responses to Temperature of the Major Canopy Species of Tasmanian Cool Temperate Rain-Forest and Their Ecological Significance .II. Net Photosynthesis and Climate Analysis

Abstract

Net photosynthesis was measured in foliage of Tasmanian rainforest canopy species grown at 20°C and acclimated sequentially to a range of temperatures. Some trends in photosynthetic response (both instantaneous and acclimatory) correlated with aspects of species' distributions with respect to altitude, latitude and climate, and with their frost resistance. This was particularly evident in Athrotaxis selaginoides D.Don and Nothofagus cunninghamii (Hook.) Oerst. which are common at high altitudes. High-altitude provenances of these species showed a low optimum acclimation temperature (16-17°C) and maintained a high rate of photosynthesis (as a proportion of their maximum rate) at 8°C (84% and 76% respectively). However, the co-occurring winter-deciduous Nothofagus gunnii (Hook. f.) Oerst. showed a higher optimum acclimation temperature for photosynthesis (23°C) and a lower rate of photosynthesis at 8°C (60% of its maximum rate) and is apparently adapted photosynthetically to summer temperature conditions. Provenances of N. cunninghamii showed trends in photosynthetic responses (maxima and responses to extremes) and specific leaf area which correlated with the climate of the collection site and with frost resistance. This population variation may permit the very wide geographic and climatic range of this species, allowing tolerance of extreme temperatures as well as a relatively high competitive ability under more equable climates. Atherosperma moschatum Labill. showed a low photosynthetic tolerance of high and low temperatures compared with the other species. This is consistent with the general restriction of this species to microhabitats with an ameliorated climate and indicates that its wide latitudinal range is not due to a broader photosynthetic tolerance of temperature than co-occurring species. The determinants of the narrow latitudinal range of the Tasmanian endemic species, particularly those which are common at low altitudes (Phyllocladus aspleniifolius (Labill.) Hook, f., Eucryphia lucida (Labill.) Baill. and Lagarostrobos franklinii (Hook. f.) C.J. Quinn are less clear. Limited acclimation to high temperatures in E. lucida (and P. aspleniifolius under some conditions), and in the high altitude species, N. gunnii and A. selaginoides, suggests that the sensitivity of these species to high summer temperatures may directly limit their distribution. However, interpretation of the sensitivity of these species to high summer temperatures and low precipitation shown by the climate analysis is complicated by the interactions of these climatic features with the incidence of fire.