PARENT SESSION
Oral Session #53: Elevated CO₂: Communities, ecosystems, soils.
Presiding: G. Lin
Wednesday, August 7. 8:00 AM to 11:30 AM. Coconino Meeting Room, TCC.

Responses of the annual plant component of a Mojave Desert ecosystem to elevated CO₂.

Huxman, Travis^*,1, Nagel, Jennifer², Zitzer, Stephen³, Charlet, Terese⁴, DeFalco, Leslie⁴, Housman, David⁴, Griffin, Kevin², Smith, Stanley⁴, ¹ Univeristy of Arizona, Tucson, AZ² Columbia University³ Desert Research Institute⁴ University of Nevada, Las Vegas

ABSTRACT- Annual plants represent a major source of plant diversity and contribute up to 50% of the primary production in the Mojave Desert. The annual plant component is also likely to be the most responsive to global change because changes in climate directly affect factors important to the evolution and ecology of this life form. Understanding their response to global change is important for an overall understanding of deserts, and may provide a nice model system in which to develop generalities of global change impact to terrestrial ecosystems. We have evaluated the dynamics of annual plants at the Nevada Desert FACE Facility since 1997 across a number of different scales of biological organization. While individual plant size has consistently increased across all species at elevated CO₂, the mechanistic determinant has been species-specific. Some species, such as Bromus madritensis, show increases in photosynthetic rate, while other herbaceous species show reductions in stomatal conductance, leading to greater water-use efficiency. In addition to enhanced photosynthetic rates, reduction in tissue construction costs may also drive increases in growth rates at elevated CO₂ and appears to explain species-specific differences in growth rate. Thus, accelerations in growth rate have shifted species composition, through alterations of competitive hierarchies, and for a number of species, have had life history consequences. Larger plants at elevated CO₂ are constrained to produce proportionally similar numbers of seeds, but also have reduced resource reserves to allocate toward this seed crop. The result is a consistent decrease in seed nitrogen that occasionally results in generational effects, thus reducing the performance characteristic of future offspring. These patterns of CO₂ effect seem to have been important in the general observation that invasive species increase in abundance and relative contribution to production at elevated CO₂ as compared to native species.

KEY WORDS: Mojave Desert, Elevated CO₂, Annual Plants, Seed Production