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Elevated CO2 Overpowers the Growth-Retarding Impacts of Ozone on Chinese Pine Growth

Paper Reviewed
Liu, Z., Chen, W., Fu, W., He, X., Fu, S. and Lu, T. 2015. Effects of elevated CO2 and O3 on leaf area, gas exchange and starch contents in Chinese pine (Pinus tabuliformis Carr.) in northern China. Bangladesh Journal of Botany 44: 917-923.

It has long been established that enhanced fossil fuel combustion is increasing the amount of carbon dioxide (CO2) in the atmosphere and that that increase is expected to benefit Earth's plants, as literally thousands of field and laboratory experiments have demonstrated that the more CO2 there is in the air, the better plants grow. However, the concentration of atmospheric ozone (O3) is also expected to increase, as higher levels of nitrogen oxides, carbon monoxide and volatile organic compounds react in the presence of sunlight within the atmosphere to produce this plant-damaging air pollutant. The question remains, therefore, which of the two substances -- the fertilizer (CO2) or the pollutant (O3) -- will win out and control the destiny of Earth's future plant life?

A new study conducted by six ecologists from the Chinese Academy of Sciences (Liu et al., 2015) provides an answer with respect to Chinese pine (Pinus tabuliformis Carr.), an ecologically and monetarily important indigenous coniferous tree species inhabiting a range of approximately 3 million square kilometers in northern China. Working at the Shenyang Botanical Garden (42.4°N, 128.1°E), Liu et al. designed an open-top chamber field study in which they grew four-year-old Chinese pine seedlings for a period of 90 days in either control (350 ppm) or elevated (700 ppm) CO2 at either control (40 nmol/mol) or elevated (80 nmol/mol) O3.

So what did their study reveal?

Working alone, elevated CO2 increased net photosynthesis and leaf area of Chinese pine by 43 and 47 percent, respectively, whereas elevated O3 decreased these two parameters by 43 and 22 percent, respectively. When in combination, however, the growth-enhancing benefits of atmospheric CO2 enrichment won out over the growth-retarding influence of elevated O3, resulting in a 22 percent increase in leaf area and 7 percent increase in net photosynthesis compared to plants growing under controlled or ambient CO2 and O3 conditions. Thus, the positive effect of CO2 was able to fully compensate for, and thereby counterbalance, the negative impact of O3 on these parameters.

Additionally, Liu et al. measured the starch content of the trees growing under the different treatments, since "starch is an important energy storage material (Hoekstra and Buitink, 2001), which is directly generated by photosynthesis in most plants and plays an important part in improving the resistance of plants to stress (Donnelly et al., 2001)." And as they report, after 90 days exposure to elevated CO2, Chinese pine starch contents were significantly increased by 27 percent compared to control. Elevated O3, in contrast, significantly reduced the starch content. But when combined together, Liu et al. report that "the positive responses of starch [to elevated CO2] were enough to balance the negative effect by O3 exposure."

In the future, therefore, it is clear that the destiny of Chinese pine trees will be far more influenced by the positive impacts of atmospheric CO2 enrichment than by the negative impacts of ozone pollution. And that being the case, why aren't more scientific organizations and media outlets telling this story?

References
Donnelly, A., Craigon, J., Black, C.R., Colls, J.J. and Landon, G. 2001. Does elevated CO2 ameliorate the impact of O3 on chlorophyll content and photosynthesis in potato (Solanum tuberosum)? Physiologia Plantarum 111: 501-511.

Hoekstra, F.A. and Buitink, J. 2001. Mechanisms of plant desiccation tolerance. Trends in Plant Science 8: 431-438.

Posted 21 March 2016