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Feeding China's Giant Pandas: Is There a Role for Rising CO2?
Li, Y., Zhang, Y., Zhang, X., Korpelainen, H., Berninger, F. and Li, C. 2013. Effects of elevated CO2 and temperature on photosynthesis and leaf traits of an understory dwarf bamboo in subalpine forest zone, China. Physiologia Plantarum 148: 261-272.

In providing some background for their study, the authors note that dwarf bamboo (Fargesia rufa Yi), which grows in the understory of subalpine coniferous forests in China, is one of the main foods of the giant pandas that live there. So it was only natural for them to wonder how the ongoing rise in the atmosphere's CO2 concentration might impact the small bamboo plants and, thereby, the pandas source of food. More specifically, they were curious about the phenomenon of photosynthetic acclimation (or down-regulation, which is a decrease in originally-stimulated photosynthetic rates) that is sometimes observed in CO2-enriched air, where the initial CO2-induced stimulation of photosynthesis gradually erodes away over time.

What was done
Seeds of the dwarf bamboo plants were collected and sown in seedbeds where the resultant seedlings were allowed to grow for four years, after which healthy seedlings of a uniform size were transplanted into 120 20-L pots filled with surface soil taken from the natural forest floor. Then, starting in May of 2010, the planted pots were placed within eight enclosed-top growth chambers constructed from materials having a transparency of 85% (walls) and 82% (tops), where they grew until the end of the study in September 2010 in air of either ambient or double-ambient CO2 and either ambient or ambient +2.2°C temperature, while numerous measurements were made on the young trees over the course of the 150-day experiment.

What was learned
The six scientists report that "the light-saturated net photosynthetic rates of the dwarf bamboo increased by 57.6% under elevated CO2," while in the case of both elevated CO2 and elevated temperature together, the increase in net photosynthesis was 36.9%. They also report that they "found no evidence of photosynthetic down-regulation in the dwarf bamboo." In addition, they say "there were no significant reductions in the nitrogen concentration based on mass in the dwarf bamboo," and that "there were even increases in the N concentration based on [leaf] area when exposed to elevated CO2." And they speculate that "the lack of observed photosynthetic down-regulation may be related to this result."

What it means
In the concluding paragraph of their paper, Li et al. write that the dwarf bamboo plants could adjust their "physiology and morphology to enable the capture of more light, to increase water use efficiency and improve nutritional conditions." However, they also indicate that elevated temperature had just the opposite effects on water use efficiency and nutritional traits of leaves. But in the end, they report that "the combination of elevated CO2 and elevated temperature showed no significant interaction effect on the nutritional traits of leaves." And, therefore, their ultimate conclusion was that if and when "the dwarf bamboo confronts warmer climate for a long term, elevated CO2 will be beneficial," as it will lead to the production of more equally-nutritious dwarf bamboo tissue.

Reviewed 11 September 2013