PARENT SESSION
Organized Oral Session 41: Ecological responses to precipitation: Scaling patterns and processes from the genome to the ecosystem
Organizer(s): ME Loik and SD Smith
Thursday, August 11, 8:00 AM - 11:30 AM, Meeting Room 510b, Level 5, Palais des congrès de Montréal

Interactions of precipitation and elevated CO₂: El Niño and annual plants in the Mojave Desert.

Smith, Stanley^*,1, Huxman, Travis², Charlet, Therese¹, Zitzer, Stephen³, Babcock, Derek⁴, DeSoyza, Amrita⁴, ¹ University of Nevada, Las Vegas, Las Vegas, NV, USA² University of Arizona, Tucson, AZ, USA³ Desert Research Institute, Las Vegas, NV, USA⁴ University of Nevada, Reno, Reno, NV, USA

ABSTRACT- The annual plant driven pulse-reserve paradigm in desert ecosystems may be particularly responsive to elevated CO₂, given the rapid growth rates that characterize desert annuals. In the El Nino year of 1998, we observed substantially increased biomass of desert annuals at elevated CO₂, particularly the invasive grass Bromus madritensis. We hypothesized that the pronounced differential increase in seed rain in Bromus would result in a community shift toward the invasive grass (Nature 408:79-82). However, the subsequent six growing seasons (1999-2004) had below-average rainfall in either the winter or spring, including an apparent seed-bank-depleting high-density winter germination event followed by a spring drought in 2000, which resulted in 70% annual plant mortality prior to seed set and extremely low seed rain. We thus observed very low Bromus populations in 2001-2004 and a reestablishment of a native dicot dominated annual community. Over the 1998 to 2004 period, aboveground biomass was enhanced by 57% at elevated CO₂ in Bromus versus 39% in native annuals, whereas reproductive allocation was not different between ambient and elevated CO₂ in both wet and dry years in all species. This year (2005) is again an extremely wet El Nino cycle that is resulting in a reemergence of Bromus and a second invasive annual grass, Schismus arabicus. We will present comparative density, productivity, and carbon allocation results for Bromus and sympatric native annuals in the 1998 and 2005 El Nino cycles, one and eight years after the start of continuous CO₂ fumigation in this FACE experiment. Although we see a clear stimulation in growth and enhanced allocation to seeds at elevated CO₂ in all species, our results show that the long-term response of desert annual communities to elevated CO₂ will be highly variable in this episodic, pulse-driven ecosystem.

Key words: Precipitation, Elevated CO₂, Bromus, Mojave Desert