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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