Tropical rainforests, which are already restricted and degraded by human activities like many ecosystems, are likely to be further threatened by the rising temperatures anticipated with climate change. Prediction of changes to plant distributions under rising temperatures has involved models based on temperature tolerances estimated from either native distributions or leaf-level physiology. Australian rainforests provide a unique opportunity to investigate the responses of plants to temperature within the same forest type, as they cover a wide range of climates from cool temperate to tropical. We sought to determine if there were consistent differences in the physiology of temperate and tropical rainforest trees that could explain their contrasting climatic distributions. We studied eight evergreen rainforest tree species, from different families, across a 33o latitudinal gradient, with an associated range in mean annual temperature of 9 to 23oC. Physiological responses to temperature were measured in seedlings grown in controlled environments under a range of contrasting temperatures.
Results/Conclusions
Leaves of tropical species were found to reach maximum rates of photosynthesis when developed under higher temperatures than temperate species. More importantly, tropical species maintained high photosynthetic rates over a narrower range of developmental temperatures. Similarly, when fully-expanded leaves were exposed to new growth temperatures, tropical species maintained high photosynthetic rates over a narrower range of temperatures than temperate species. Although temperate species showed maximum growth at lower temperatures than tropical species, these temperatures were 8oC higher than would be predicted from their maxima for photosynthesis and native climates. This discrepancy between photosynthesis and whole plant growth in the temperate species was not explained by changes in biomass allocation. 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. In conclusion, rainforest tree species of Australia showed several physiological differences that were consistent with their native distributions. The narrower temperature tolerance of photosynthesis in the tropical species suggests they will be more susceptible to predicted future temperature increases than temperate species. However, photosynthetic responses did not scale up to whole-plant growth, suggesting that photosynthetic responses cannot be used in isolation to predict present and future distribution of rainforest species.