PARENT SESSION
Poster Session #45: Elevated CO₂ I.
Wednesday, August 7. Presentation from 5:00 PM to 6:30 PM. Exhibit Hall B & C, TCC

Influence of microsite and elevated CO₂ (FACE) on recruitment, survival and growth of desert shrubs .

Babcock, Derek^*,1, Zitzer, Stephen², Housman, Dave³, Fenstermaker, Lynn¹, Nowak, Robert², Smith, Stan³, Coleman, James¹, ¹ Dept. of Earth and Ecosystems Science; Desert Research Institute, Las Vegas, NV² Dept. Environmental Resource Sciences, University of Nevada, Reno, NV³ Dept. Biological Sciences, University of Nevada, Las Vegas, Las Vegas, NV

ABSTRACT- Seedling recruitment plays a major role in determining plant community structure. Positive growth responses to increasing atmospheric CO₂ are predicted for desert plants, however, no studies have documented the response of naturally recruited desert shrubs to elevated CO₂ . Using Free-Air Carbon Dioxide Enrichment (FACE), we began continuous fumigation of an undisturbed Mojave Desert ecosystem with 550 mol mol^-1 CO₂ in April 1997. During Fall 1997, a major recruitment event occurred for the two dominant shrub species at our site, the evergreen Larrea tridentata and drought-deciduous Ambrosia dumosa. We measured seedling recruitment, survival and growth beneath shrub canopies and in intercanopy microsites. Recruitment of Larrea (0.05 seedlings m^-2) was not influenced by CO₂ , but Ambrosia recruitment doubled under elevated CO₂ (0.87 vs. 0.40). Survival after 4 years was significantly greater for Ambrosia (28%) than for Larrea (15%). Overall survival after 2 years tended to be greater under elevated than ambient CO₂ (50% vs. 32% respectively), but after 4 years was similar (22% vs. 20%). Microsite alone had no influence on survival. Heights after 4 years were similar (6.7 vs. 5.9 cm, respectively, for Ambrosia and Larrea), but Ambrosia stem diameter (3.53 mm) and canopy volume (82 cm³) tended to be 2-3 times larger than Larrea. Elevated CO₂ had no effect on Ambrosia height and stem diameter, but tended to produce larger canopies (+11%). The reverse occurred for Larrea with increases of 10% and 16% for height and stem diameter, respectively, and a decrease in canopy volume of 7%. Species-specific microsite effects were similar, producing taller seedlings with smaller stems and canopies in understory microsites. Our results suggest the need to continue monitoring survival and growth responses of these slow-growing and long-lived shrubs to elevated CO₂ in order to better understand the impact of rising CO₂ on community structure in this desert ecosystem.

KEY WORDS: FACE, elevated, recruitment