No abstract

Although desert ecosystems are predicted to be the most responsive to elevated CO 2 , low nutrient availability may limit increases in productivity and cause plants in deserts to allocate more resources to root biomass or activity for increased nutrient acquisition. We measured root respiration of two Mojave Desert shrubs, Ambrosia dumosa and Larrea tridentata, grown under ambient ($ 375 ppm) and elevated ($ 517 ppm) CO 2 concentrations at the Nevada Desert FACE Facility (NDFF) over five growing seasons. In addition, we grew L. tridentata seedlings in a greenhouse with similar CO 2 treatments to determine responses of primary and lateral roots to an increase in CO 2 . In both field and greenhouse studies, root respiration was not significantly affected by elevated CO 2 . However, respiration of A. dumosa roots o1 month old was significantly greater than respiration of A. dumosa roots between 1 and 4 months old. For both shrub species, respiration rates of very fine (o1.0 mm diameter) roots were significantly greater than those of fine (1-2 mm diameter) roots, and root respiration decreased as soil water decreased. Because specific root length was not significantly affected by CO 2 and because field minirhizotron measurements of root production were not significantly different, we infer that root growth at the NDFF has not increased with elevated CO 2 . Furthermore, other studies at the NDFF have shown increased nutrient availability under elevated CO 2 , which reduces the need for roots to increase scavenging for nutrients. Thus, we conclude that A. dumosa and L. tridentata root systems have not increased in size or activity, and increased shoot production observed under elevated CO 2 for these species does not appear to be constrained by the plant's root growth or activity.