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The Photosynthetic Responses of Three Desert Shrubs to Elevated CO2
Naumburg, E., Housman, D.C., Huxman, T.E., Charlet, T.N., Loik, M.E. and Smith, S.D.  2003.  Photosynthetic responses of Mojave Desert shrubs to free air CO2 enrichment are greatest during wet years.  Global Change Biology 9: 276-285.

What was done
A FACE experiment was begun in a Mojave Desert ecosystem in Nevada, USA, in the spring of 1997 to determine the responses of native desert vegetation to atmospheric CO2 enrichment.  The dominant species within the 25-m-diameter circular FACE plots was an evergreen perennial shrub known commonly as creosote (Larrea tridentata).  Also present was the drought deciduous shrub Ambrosia dumosa and the winter deciduous shrub Krameria erecta.  In this paper, the authors report on photosynthetic and other gas exchange data collected from these shrubs during five years of fumigation with air of either 360 or 550 ppm CO2.

What was learned
On average, elevated CO2 increased rates of net photosynthesis in Larrea, Ambrosia and Krameria by 31, 32 and 63%, respectively.  Moreover, the photosynthetic enhancements persisted over the entire duration of the five-year study.  The highest absolute rates of photosynthesis were observed in CO2-enriched shrubs during times of comparatively high soil moisture following spring and sporadic summer rains.  Surprisingly, atmospheric CO2 enrichment did not consistently affect stomatal conductances in any species, although it did reduce those in Larrea by 25 to 50% during the summer dry season.

What it means
As the air's CO2 content continues to rise, the three shrubs monitored in this study will likely respond by exhibiting enhanced rates of photosynthesis, which should lead to greater biomass production, particularly when soil moisture is high.  Indeed, as indicated by the authors, "future elevated CO2 effects in desert ecosystems will strongly depend on concurrent climate changes, with above average precipitation years resulting in the greatest seasonal increase in carbon uptake."

Reviewed 9 April 2003