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Elevated CO2 Aids Drought Recovery in Desert Species
Hamerlynck, E.P., Huxman, T.E., Loik, M.E. and Smith, S.D.  2000.  Effects of extreme high temperature, drought and elevated CO2 on photosynthesis of the Mojave Desert evergreen shrub, Larrea tridentataPlant Ecology 148: 183-193.

What was done
The authors grew seedlings of the evergreen perennial shrub Larrea tridentata in glasshouses receiving atmospheric CO2 concentrations of 360, 550, and 700 ppm for one year.  In addition, half of the seedlings had water withheld from them for three months prior to a nine-day high temperature treatment.  Thus, the authors studied the interactive effects of elevated CO2, soil moisture and heat stress on gas exchange in this woody species common to the Mojave Desert.

What was learned
Elevated CO2 largely offset the detrimental effects of drought and high temperature on water relations and photosynthesis in this species.  Midday leaf water potentials, for example, were similar in all well-watered plants, regardless of atmospheric CO2 concentration.  In droughted plants, however, leaf water potentials indicated that internal plant water status became increasingly more favorable with atmospheric CO2 enrichment (-5.9, -4.6 and -3.4 MPa for plants grown at 360, 550 and 700 ppm CO2, respectively).

Measurements of chlorophyll fluorescence used to ascertain photosynthetic efficiency indicated that seedlings grown at 700 ppm CO2 recovered to pre-heat treatment values, while those grown at 360 and 500 ppm exhibited reductions in photosynthetic efficiency of up to 20%.  Averaged across the entire experiment, rates of net photosynthesis increased with atmospheric CO2 enrichment.  Photosynthetic rates of seedlings grown at 550 and 700 ppm CO2 were 31 and 90% greater, respectively, than rates displayed by ambiently-grown control plants.

What it means
As the CO2 content of the air continues to rise, it is likely that seedlings of Larrea tridentata will be able to better deal with episodes of drought and high temperature stress that currently limit seedling establishment and survivorship in their arid home range.  Indeed, the current paper demonstrates that elevated CO2 concentrations reduce the deleterious effects of drought and high temperature on both plant water relations and photosynthesis in this species.  Thus, it is almost guaranteed that the Mojave Desert will become a greener and more vegetated biome in the years to come.