The significance of soil water redistribution facilitated by roots (an extension of "hydraulic lift", here termed hydraulic redistribution) was assessed for a stand of Artemisia tridentata using measurements and a simulation model. The model incorporated water movement within the soil via unsaturated flow and hydraulic redistribution and soil water loss from transpiration. The model used Buckingham-Darcy's law for unsaturated flow while hydraulic redistribution was developed as a function of the distribution of active roots, root conductance for water, and relative soil-root (rhizosphere) conductance for water. Simulations were conducted to compare model predictions with time courses of soil water potential at several depths, and to evaluate the importance of root distribution, soil hydraulic conductance and root xylem conductance on transpiration rates and the dynamics of soil water. The model was able to effectively predict soil water potential during a summer drying cycle, and the rapid redistribution of water down to 1.5 m into the soil column after rainfall events. Results of simulations indicated that hydraulic redistribution could increase whole canopy transpiration over a 100-day drying cycle. While the increase was only 3.5% over the entire 100-day period, hydraulic redistribution increased transpiration up to 20.5% for some days. The presence of high soil water content within the lower rooting zone appears to be necessary for sizeable increases in transpiration due to hydraulic redistribution. Simulation results also indicated that root distributions with roots concentrated in shallow soil layers experienced the greatest increase in transpiration due to hydraulic redistribution. This redistribution had much less effect on transpiration with more uniform root distributions, higher soil hydraulic conductivity and lower root conductivity. Simulation results indicated that redistribution of water by roots can be an important component in soil water dynamics, and the model presented here provides a useful approach to incorporating hydraulic redistribution into larger models of soil processes.

The temporal patterns of soil water potential in a stand of Artemisia tridentata in central Utah, USA, were monitored during the summer, which included small periodic rainfall events, and over the winter, when most of the soil recharge occurs in this environment. The pattern of recharge, when compared to an area cleared of aboveground vegetation, strongly indicated that the downward movement of water to 1.5 m was primarily conducted via roots by the process known as hydraulic redistribution. Rainwater was moved rapidly downward shortly after the rain event and continued over a period of a few days. For rainwater reaching a 0.3–1.5 m depth, the portion redistributed by roots was estimated to range from 100% for small rainfall events (<8 mm) to 74% for a 36‐mm event. Simulations with a model of soil water movement that compared situations with and without hydraulic redistribution by roots, indicated that during the fall–spring recharge period, 67% of all water moved downward below 0.1 m was via roots, while 87% of the water moved below 0.3 m was via roots. These results indicate that rapid downward movement of rainwater by roots can be a significant mechanism of soil water recharge to depth in arid and semiarid ecosystems. Corresponding Editor: F. C. Meinzer

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Root growth and physiological responses to elevated CO # were investigated for three important Mojave Desert grasses : the C $ perennial Achnatherum hymenoides, the C % perennial Pleuraphis rigida and the C $ annual Bromus madritensis ssp. rubens. Seeds of each species were grown at ambient (360 µl l −" ) or elevated (1000 µl l −" ) CO # in a glasshouse and harvested at three phenological stages : vegetative, anthesis and seed fill. Because P. rigida did not flower during the course of this study, harvests for this species represent three vegetative stages. Primary productivity was increased in both C $ grasses in response to elevated CO # (40 and 19% for A. hymenoides and B. rubens, respectively), but root biomass increased only in the C $ perennial grass. Neither above-ground nor belowground biomass of the C % perennial grass was significantly affected by the CO # treatment. Elevated CO # did not significantly affect root surface area for any species. Total plant nitrogen was also not statistically different between CO # treatments for any species, indicating no enhanced uptake of N under elevated CO # . Physiological uptake capacities for NO $ and NH % were not affected by the CO # treatment during the second harvest ; measurements were not made for the first harvest. However, at the third harvest uptake capacity was significantly decreased in response to elevated CO # for at least one N form in each species. NO $ uptake rates were lower in A. hymenoides and P. rigida, and NH % uptake rates were lower in B. rubens at elevated CO # . Nitrogen uptake on a whole rootsystem basis (NO $ jNH % uptake capacityiroot biomass) was influenced positively by elevated CO # only for A. hymenoides after anthesis. These results suggest that elevated CO # may result in a competitive advantage for A. hymenoides relative to species that do not increase root-system N uptake capacity. Root respiration measurements normalized to 20mC were not significantly affected by the CO # treatment. However, specific root respiration was significantly correlated with either root C : N ratio or root water content when all data per species were included within a simple regression model. The results of this study provide little evidence for up-regulation of root physiology in response to elevated CO # and indicate that root biomass responses to CO # are species-specific.

The use of water in the upper 1 m of the soil profile by 3 common herbaceous species of the southern Great Plains was examined by labeling soil water with 'Hz0 and Hzl*O. Uptake of