Prickly pear (Opuntia engelmannii) carbon pools in a desert grassland. Vogl, Adrian^*,1, McMurtry, Chad¹, Archer, Steve¹, ¹ School of Natural Resources, Tucson, AZ, United States

ABSTRACT- Woody plant proliferation in global dry lands may have significant consequences for the global carbon cycle. However, estimates of its effect on carbon pools are highly uncertain. Remote sensing is an important tool for assessing changes in aboveground carbon pools accompanying woody plant expansion, but such approaches do not currently account for the contribution of cacti that have increased substantially in many arid and semi-arid rangelands. In addition, the carbon mass contained in Opuntia patches is not well documented. The objectives of this study conducted at the Santa Rita Experimental Range, Arizona were to (a) develop algorithms to predict Opuntia engelmannii biomass from non-destructive dimensional measurements of patch geometry; (b) apply these algorithms in field surveys to estimate biomass of O. engelmannii on landscapes with contrasting land use histories; and (c) quantify O. engelmannii carbon density and estimate patch and landscape-scale carbon mass. Circumference of Opuntia patches was a good predictor of biomass (R² = 0.92), however it increasingly underestimated biomass in patches greater than 5.4 m circumference. An oblate ellipsoid volume algorithm based on patch height and diameter best predicted biomass of all patch sizes (R² = 0.97). Field surveys of management units with contrasting land use histories revealed large variation in O. engelmannii biomass (ranging from 0 in some pastures to 5,094 ± 1,663 kg/ha in others). Data indicate prickly pear can contribute significantly to aboveground carbon pools in areas where little or no woody vegetation is detected by remote sensing.

KEY WORDS: carbon sequestration, carbon density, remote sensing, land cover change