LAUSR

Photosynthetic carbon uptake by tropical forests is of critical importance in regulating the earth's climate, but rising temperatures threaten this stabilizing influence of tropical forests. Most research on how temperature affects photosynthesis focuses on fully sun-exposed leaves and little is known about shade leaves, even though shade leaves greatly outnumber sun leaves in lowland tropical forests. We measured temperature responses of light-saturated photosynthesis, stomatal conductance and the biochemical parameters VCMax (maximum rate of RuBP carboxylation) and JMax (maximum rate of RuBP regeneration, or electron transport) on sun and shade leaves of mature tropical trees of three species in Panama. As expected, biochemical capacities and stomatal conductance were much lower in shade than in sun leaves, leading to lower net photosynthesis rates. However, the key temperature response traits of these parameters-the optimum temperature (TOpt), and the activation energy-did not differ systematically between sun and shade leaves. Consistency in the JMax to VCMax ratio further suggested that shade leaves are not acclimated to lower temperatures. For both sun and shade leaves stomatal conductance had the lowest temperature optimum (~25°C), followed by net photosynthesis (~30°C), JMax (~34°C), and VCMax (~38°C). Stomatal conductance of sun leaves decreased more strongly with increasing vapor pressure deficit than that of shade leaves. Consistent with this, modeled stomatal limitation of photosynthesis increased with increasing temperature in sun but not shade leaves. Collectively, these results suggest that modeling photosynthetic carbon uptake in multi-layered canopies does not require independent parameterization of the temperature responses of the biochemical controls over photosynthesis of sun and shade leaves. Nonetheless, to improve the representation of the shade fraction of carbon uptake dynamics in tropical forests, better understanding of stomatal sensitivity of shade leaves to temperature and vapor pressure deficit will be required.