PARENT SESSION
Oral Session #58: Elevated CO₂. Presiding: A. Finzi.
Thursday, August 9, 2001. 8:00 AM to 12:00 PM. Madison Ballroom C.

Effect of Free Air CO₂ Enrichment on photosynthesis in Mojave Desert shrubs: Results from three years with differing precipitation regimes.

Naumburg, Elke^1,2, Housman, David¹, Huxman, Travis³, Charlet, Terri¹, Stortz-Lintz, Diane⁴, Smith, Stan¹, ^{1 2 3 4}

ABSTRACT- It has been suggested that desert species will show the strongest responses to rising atmospheric carbon dioxide due to the strong water limitations in these systems. We report here the effect of 55 Pa atmospheric CO₂ on photosynthesis of three Mojave Desert shrubs over 3 years. Daily integrated photosynthesis (A_day) was significantly enhanced by elevated CO₂ for two of the three species. For the drought-deciduous Ambrosia dumosa no significant A_day enhancement by elevated CO₂ was observed. For the evergreen Larrea tridentata, the overall enhancement of A_day was 9% and for the winter deciduous Krameria erecta the enhancement effect was 17%. Two dates during early onset of the summer dry season showed the largest enhancements of A_day in Larrea, suggesting that elevated CO₂ may extend the photosynthetically active season for this species. In contrast, A_day enhancements of Krameria were greatest early in the growing season under higher soil moisture conditions. Neither species experienced significant reductions in stomatal conductance under elevated CO₂ during the diurnals. This suggests that elevated CO₂ will not extend the growing season into summer drought via water conservation and improved soil moisture conditions, a result that is confirmed by soil moisture measurements. Overall, our results suggest that different functional types differ in their photosynthetic responses to elevated CO₂, which may lead to differences in growth and ultimately competitive interactions.

KEY WORDS: daily photosynthesis, elevated co2, stomatal conductance, desert