By analysing the ecosystem fluxes of carbon dioxide and radiation in seven tropical forests over two years, researchers found that the optimum temperature for photosynthesis rises with average growing season temperatures. The rise could be due to changes in the pores of plants, and the way in which they respire, the scientists say.

"We hold the view that tropical forests will not be so vulnerable to warming as indicated by [the current] optimum temperature for photosynthesis," explained Zheng-Hong Tan of Hainan University in China.

Until now, there has been no consensus on what will happen to tropical forests during continued climate change. A warming of 3–5°C at the end of the Palaeocene epoch 56 million years ago seemed to spare neo-tropical vegetation, but that warming was slower and shorter-lived than that expected to occur under today’s climate change.

The main concern is that tropical forests could already be growing at the optimum temperature for photosynthesis. If this optimum temperature is exceeded, photosynthesis will decline, and one of the world’s biggest carbon sinks will begin to disappear – ushering in yet more climate change.

Tan and colleagues – who are based at various institutions in China, the US, Japan, Panama, Brazil and Australia – have attempted to find out exactly what will happen to the photosynthesis of tropical forests as temperatures rise. To do so, they collected data on the flux of carbon dioxide, heat and radiation with a time resolution of 30 to 60 minutes, spanning two years or more, for seven forests in Southeast Asia and the Amazon. For the same period, they also collected various meteorological data.

The analysis of the data allowed them to estimate the optimum temperatures for photosynthesis in the forests, and how photosynthesis will likely change in response to global warming. They found that the optimum temperatures were already close to the average growing temperatures, but tended to be higher in forests with higher growing season temperatures.

This rise in optimum photosynthesis temperature with growing temperature did not seem to be due to any change in biochemistry. Instead, the researchers believe the rise is due to changes in the pores, or stomata, of the leaves, and the way in which they respire—all helped along by the greater concentration of carbon dioxide in the air.

"This can be considered as a positive signal for tropical forests," they write in Environmental Research Letters (ERL) .

Still, Tan and colleagues believe further work is necessary to fully establish the link between optimum temperature for photosynthesis and climate change, and are now examining drought as well as warming and carbon dioxide.

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