Comparative response to temperature of the major canopy species of Tasmanian cool temperate rainforest and their ecological significance. II. Net photosynthesis and climate analysis.
Net photosynthesis was measured in foliage of seedlings of Nothofagus cunninghamii, N. gunnii, Atherosperma moschatum, Eucryphia lucida, Athrotaxis selaginoides, Phyllocladus aspleniifolius and Lagarostrobus franklinii grown at 20°C and acclimatized to temp. between 8° and 35°C. Some trends in photosynthetic response (both instantaneous and after acclimatization) correlated with aspects of the species' distributions with respect to alt., lat. and climate, and with their frost resistance. This was particularly evident in A. selaginoides and N. cunninghamii, which are common at high alt. High-alt. provenances of these species showed a low opt. acclimatization temp. (16-17°C) and maintained a high rate of photosynthesis (as a proportion of their max. rate) at 8°C (84% and 76% respectively). However, the co-occurring winter-deciduous N. gunnii showed a higher opt. acclimatization temp. for photosynthesis (23°C) and a lower rate of photosynthesis at 8°C (60% of its max. rate) and is apparently adapted photosynthetically to summer temp. 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 temp. as well as a relatively high competitive ability under more equable climates. A. moschatum showed a low photosynthetic tolerance of high and low temp. 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 temp. than co-occurring species. The determinants of the narrow latitudinal range of the Tasmanian endemic species, particularly those which are common at low alt. (P. aspleniifolius, E. lucida and L. franklinii) are less clear. Limited acclimatization to high temp. in E. lucida (and P. aspleniifolius under some conditions), and in the high alt. species N. gunnii and A. selaginoides, suggests that the sensitivity of these species to high summer temp. may directly limit their distribution. However, interpretation of the sensitivity of these species to high summer temp. and low precipitation shown by the climate analysis is complicated by the interactions of these climatic features with the incidence of fire.