Paper Reviewed
Tomimatsu, H., Iio, A., Adachi, M., Saw, L.-G., Fletcher, C. and Tang, Y. 2014. High CO2 concentration increases relative leaf carbon gain under dynamic light in Dipterocarpus sublamellatus seedlings in a tropical rain forest, Malaysia. Tree Physiology 34: 944-954.

The authors of this intriguing paper - Tomimatsu et al. (2014) - first note that "within a closed tropical rainforest, CO2 concentration is often higher on the forest floor than above the canopy during late night and early morning because of soil respiration," as has been found to be the case for the Pasoh tropical rain forest of Malaysia, as reported by Aoki et al. (1978) and Ohkubo et al. (2008), as well as when "compared with the CO2 concentration of a gap area," as documented by Tang et al. (2003). And, therefore, the six scientists decided to explore the subject in more detail, hypothesizing that the higher atmospheric CO2 concentration near the ground may well "compensate for leaf carbon loss caused by the low light."

Working in the understory of the Pasoh rain forest - determined to either confirm or refute their thinking on the subject - Tomimatsu et al. measured the photosynthetic rates of fully-expanded younger leaves of Dipterocarpus sublamellatus seedlings along a gap edge in the forest with an intermittent light regime and under CO2 concentrations of 350 and 700 ppm. This work revealed that the steady-state photosynthetic rate was more than 50% higher at the higher of the two CO2 concentrations at all photon flux densities. In addition, they discovered that when gas exchange measurements were conducted during the presence of a single lightfleck of 30 minutes duration in the same leaves exposed to 350 and 700 ppm air, the total carbon gain of the leaves "increased by about two fold."

As the take-home message of their work, the six scientists write their study suggests that "high CO2 increases photosynthetic light-use efficiency under both steady-state and fluctuating light conditions, which should be considered in assessing the leaf carbon gain of understory plants in low-light environments." And this benefit will likely increase in magnitude as the CO2 content of the atmosphere continues its upward climb.

References
Aoki, M., Yabuki, K. and Koyama, H. 1978. Micrometerology of Pasoh Forest. Malayan Nature Journal 30: 149-159.

Onkubo, S., Kosugi, Y., Takanashi, S., Matsuo, N., Tani, M. and Nik, A.R. 2008. Vertical profiles and storage fluxes of CO2, heat and water in a tropical rainforest at Pasoh, Peninsular Malaysia. Tellus 60B: 569-582.

Tang, Y., Okuda, T., Awang, M., Nik, A.R. and Tani, M. 2003. Sunfleck contribution to leaf carbon gain in tree seedlings from gap and the understory. In: Okuda, T. (Ed.) Pasoh - ecology of a lowland rain forest in Southwest Asia. Springer, Tokyo, Japan, pp. 251-260.