Relative enhancement of photosynthesis and growth at elevated CO₂ is greater under sunflecks than uniform irradiance in a tropical rain forest tree seedling

The survivorship of dipterocarp seedlings in the deeply shaded understorey of South‐east Asian rain forests is limited by their ability to maintain a positive carbon balance. Photosynthesis during sunflecks is an important component of carbon gain. To investigate the effect of elevated CO 2 upon photosynthesis and growth under sunflecks, seedlings of Shorea leprosula were grown in controlled environment conditions at ambient or elevated CO 2 . Equal total daily photon flux density (PFD) (∼7·7 mol m −2 d −1 ) was supplied as either uniform irradiance (∼170 µ mol m −2 s −1 ) or shade/fleck sequences (∼30 µ mol m −2 s −1 /∼525 µ mol m −2 s −1 ). Photosynthesis and growth were enhanced by elevated CO 2 treatments but lower under flecked irradiance treatments. Acclimation of photosynthetic capacity occurred in response to elevated CO 2 but not flecked irradiance. Importantly, the relative enhancement effects of elevated CO 2 were greater under sunflecks (growth 60%, carbon gain 89%) compared with uniform irradiance (growth 25%, carbon gain 59%). This was driven by two factors: (1) greater efficiency of dynamic photosynthesis (photosynthetic induction gain and loss, post‐irradiance gas exchange); and (2) photosynthetic enhancement being greatest at very low PFD. This allowed improved carbon gain during both clusters of lightflecks (73%) and intervening periods of deep shade (99%). The relatively greater enhancement of growth and photosynthesis at elevated CO 2 under sunflecks has important potential consequences for seedling regeneration processes and hence forest structure and composition.