Differences in hydraulic integration within canopies of two desert shrub species, Encelia farinosa and Ambrosia dumosa

            Many desert shrub species split morphologically into independent hydraulic units (IHUs) as they mature. IHUs are hypothesized to be an adaptation to living in dry and heterogeneous soil-water environments, because they introduce redundancy into a plant’s hydraulic system, may prevent spread of embolisms, and may decrease water loss through hydraulic redistribution. Division into IHUs occurs in many shrubs in the Sonoran and Mojave Deserts, including Ambrosia dumosa. However, some species, such as Encelia farinosa, which commonly co-occurs with A. dumosa, never physically split and appear to maintain an integrated hydraulic system. In order to better understand the development of different degrees of hydraulic integration in desert shrubs, we compared the hydraulic architectures of young A. dumosa and young E. farinosa shrubs. We hypothesized that heterogeneous soil-water supply would cause significantly more spatial variation in water use and water status within A. dumosa canopies than within canopies of E. farinosa. Young shrubs of the two species were planted in containers designed to allow heterogeneous and homogeneous watering treatments. Spatial variation of leaf water potentials and stomatal conductances within the canopies of the two species were measured in response to soil water heterogeneity. Results show that young A. dumosa shrubs possess IHUs long before they physically split, and that young E. farinosa shrubs possess integrated hydraulic systems. The difference may have important implications for the water relations of these species and their longevity in environments with high degrees of temporal and spatial heterogeneity in water availability.