Effects of rising temperatures and CO 2 on the physiology of tropical forest trees

Using a mixture of observations and climate model outputs and a simple parametrization of leaf-level photosynthesis incorporating known temperature sensitivities, we find no evidence for tropical forests currently existing ‘dangerously close’ to their optimum temperature range. Our model suggests that although reductions in photosynthetic rate at leaf temperatures ( T L ) above 30°C may occur, these are almost entirely accountable for in terms of reductions in stomatal conductance in response to higher leaf-to-air vapour pressure deficits D . This is as opposed to direct effects of T L on photosynthetic metabolism. We also find that increases in photosynthetic rates associated with increases in ambient CO 2 over forthcoming decades should more than offset any decline in photosynthetic productivity due to higher D or T L or increased autotrophic respiration rates as a consequence of higher tissue temperatures. We also find little direct evidence that tropical forests should not be able to respond to increases in CO 2 and argue that the magnitude and pattern of increases in forest dynamics across Amazonia observed over the last few decades are consistent with a CO 2 -induced stimulation of tree growth.