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Growth Responses of Populus euphratica Trees to Concomitant Changes in Groundwater Depth and Atmospheric CO2 Content
Chen, Y., Chen, Y., Xu, C. and Li, W. 2011. Photosynthesis and water use efficiency of Populus euphratica in response to changing groundwater depth and CO2 concentration. Environmental Earth Sciences 62: 119-125.

The authors write that "Idso (1998) suggested that elevated CO2 affects plant growth dependent upon plant water status: it has less effect on plants in the well-watered optimal growth phase, but exerts more effect under non-lethal dry conditions, and is most beneficial to plants under severe drought conditions."

What was done
In a further assessment of this phenomenon, Chen et al. measured leaf transpiration rate (E) and net photosynthetic rate (PN) in Populus euphratica trees growing just within the northern edge of the Taklimakan Desert in Xinjiang, northwestern China, where the riparian trees dominate the indigenous vegetation because of their tolerance of severe drought and the high salinity and alkalinity of the region's soils. This they did in four different locations, where mean soil water contents at groundwater depths of 4.12, 4.74, 5.54 and 7.74 meters were 10.9, 9.5, 3.5 and 1.3%, respectively, making their measurements at atmospheric CO2 concentrations of either 360 or 720 ppm, after which they calculated the trees' water use efficiencies (WUE = PN/E) when measured under the two atmospheric CO2 concentrations.

What was learned
In the case of each CO2 concentration, there was no statistical difference between the leaf water use efficiencies of the first three groundwater depths; but the mean WUE at the higher of the two CO2 concentrations was 44% greater than the mean measured at the lower CO2 concentration. The WUE of the lowest and driest of the four groundwater depths, however, was statistically different from the WUEs of the other three groundwater depths; and the mean WUE of the trees growing under this most stressful condition when measured at the higher of the two CO2 concentrations was 86% greater than the mean measured at the lower CO2 concentration.

What it means
In light of their several findings, Chen et al. concluded that with respect to the plant water use efficiency of Populus euphratica trees, those growing "under a mild water stress show a weak responsiveness, and those under a moderate drought stress display a strong responsiveness to CO2 enrichment."

Idso, S.B. 1998. Three phases of plant response to atmospheric CO2 enrichment. Plant Physiology 87: 5-7.

Reviewed 6 April 2011