Foliar temperature tolerance of temperate and tropical evergreen rain forest trees of Australia

Australian rain forests extend from tropical climates in the north to temperate climates in the south, providing an opportunity to investigate physiological responses to temperature of both temperate and tropical species within the same forest type. Eight, rain forest canopy tree species were selected to cover the 33 degrees latitudinal range of rain forests in eastern Australia. Temperature tolerance was measured in 6-year-old plants grown in a common environment, by exposing leaves to a series of high temperatures during late summer and a series of freezing temperatures during midwinter. Damage was evaluated based on chlorophyll fluorescence measurements made 2 h after exposure and by visual assessment of leaf damage made a week after exposure. Leaves of the tropical species were more heat tolerant and less frost tolerant than leaves of the temperate species, which is consistent with their climate distributions. In contrast, the temperature tolerance of the photosynthetic apparatus was unrelated to climate in a species' native habitat. However, the tropical species underwent significant photoinhibition during winter. All species maintained the integrity of the photosynthetic apparatus and avoided tissue damage over a similar span of temperatures (about 60 degrees C), reflecting the similar annual temperature ranges in Australia's temperate and tropical rain forests. Chlorophyll fluorescence measurements and visual assessment of leaf damage provided different estimates of the absolute and relative temperature tolerances of the species, thus emphasizing the importance of a direct assessment of tissue damage for determining a species' temperature tolerance.