Shang, H., Yu, S.,
Liu, W. and Zhang, R.
2020
"Impacts of global warming
on the radial growth and
long-term intrinsic water-use
efficiency (iWUE) of Schrenk
spruce (Picea schrenkiana
Fisch. Et Mey) in the
Sayram Lake basin,
northwest China."
Forests 11: 380,
doi:10.3390/f11040380
Full summary of study here:
http://www.co2science.org/articles/V23/aug/a7.php
MY SHORT SUMMARY:
Rising atmospheric CO2 improves plant growth and plant water use efficiency (WUE). The WUE of spruce trees in the Sayram Lake basin of northwest China has risen by 50% since the mid-1800s, due to rising atmospheric CO2.
In nearly all of the thousands of elevated CO2 studies, and greenhouse owners actual experience, rising CO2 improves plant growth. WUE also increases because of reduced plant stomatal apertures open time.
Stomata are pores that suck in CO2 from the air. When they are open, they also lose water. With a higher atmospheric CO2 level, stomata stay open for a shorter period of time to get the needed amount of CO2. The shorter open time means losing less water by "transpiration", the plant equivalent of perspiration.
Plants tend to need less water at higher CO2 levels to produce the same (or more) growth (aka "biomass"). This study looked at historic trends in the stable carbon isotope ratio (δ13C) of tree rings, to infer long-term changes in intrinsic WUE (iWUE).
Results in Figure 1:
