PARENT SESSION
Poster Session #11: Elevated CO₂.
Monday, August 6, 2001. Presentation from 3:30 PM to 5:00 PM. Exhibition Hall

Canopy and biomass responses of native undisturbed Mojave Desert shrubs to multiple years of exposure to elevated CO₂ using FACE technology.

Babcock, Derek¹, Zitzer, Stephen², Coleman, James³, Seemann, Jeffery ², Smith, Stanley⁴, ^{1 2 3 4}

ABSTRACT- Desert ecosystems are expected to have the largest relative increases in productivity due to rising atmospheric CO₂. Our experimental design consisted of 9 circular plots (25 m diameter), of which 3 were continuously fumigated with 550 ppm CO₂ beginning 28 April 1997 (using Free Air CO₂ Enrichment technology), 3 with ambient CO₂ and 3 with no fumigation. All plots were located in an undisturbed Mojave Desert site. Canopy dimensions of all shrubs were measured and biomass estimated using allometric equations from off-plot harvests. Initial shrub densities, canopy areas and biomass were not different for ambient versus elevated plots. Shrub species per plot varied from 10-14, but four species (Ambrosia dumosa, Larrea tridentata, Lycium andersonii and L. pallidum) comprised 71-79% of total shrub density. During a wet year, drought deciduous Ambrosia had the largest relative increase in canopy and biomass, 33% and 94% for ambient and elevated CO₂ respectively. During a dry year, Ambrosia grew little under ambient CO₂ but had a 10% increase under elevated CO₂. Conversely under ambient CO₂, drought deciduous Lycium canopies and biomass had slight decreases during a wet year and slight increases during a dry year, while under elevated CO₂ there were small increases regardless of rainfall. Evergreen Larrea canopy and biomass had their greatest relative increases, 30%, during a wet year under elevated CO₂ and none under ambient CO₂. During a dry year, Larrea canopies and biomass had 10% increases regardless of CO₂ level. Growth responses of desert shrubs are CO₂, rainfall and species dependent, and our current results indicate elevated CO₂ will increase the rate of carbon accumulation in this desert ecosystem.

KEY WORDS: desert shrubs, elevated carbon dioxide, face, allometry