Environmental and physiological controls on water source use by semi-arid riparian tree species

A general paradigm in semi-arid and arid systems is that woody plants with dimorphic root systems will exhibit preferential use of deeper soil water because it represents a more stable source of water than short duration pulses of shallow soil moisture derived from summer rainfall. However, whether this holds across all woody species and whether use of deeper soil water interacts with use of shallow soil water is not determined for many species in different ecosystems. Understanding the amount of water plants derive from groundwater and shallow soil water is critically important to accurate calculations of local and regional water balance. The focus of this research was to determine if dominant woody species in semi-arid riparian ecosystems used shallow soil water and how depth to groundwater and defoliation might affect root proliferation and water uptake. This research found that the functional grouping “phreatophytes” encompasses a variety of responses to environmental variation. Stable isotopic analyses determined that Prosopis velutina Woot. (Velvet mesquite) and Populus fremontii Wats. (Frémont cottonwood) used shallow soil water derived from summer rainfall, and the proportion of shallow soil water was higher at sites with greater depth to groundwater. In contrast Salix gooddingii Ball (Goodding willow) did not use shallow soil water at any location regardless of depth to groundwater. Field experiments using defoliation treatments, to limit carbon assimilation and reduce plant photosynthate pools, confirmed that Prosopis velutina exhibited flexible response in water uptake patterns in response to defoliation. Defoliation, which presumably reduced available photosynthate, increased the reliance of this species on shallow soil water; contrary to predictions that woody species should maintain extensive deep root systems to buffer themselves from seasonal drought. Greenhouse experiments with Populus fremontii and Prosopis velutina also indicated changes in belowground biomass of fine roots, which were associated with changes in water-source use for Populus fremontii, but not for Prosopis velutina. These results imply that in terms of predicting plant response to changes in future climates, or modeling fluxes of water from the soil to the atmosphere that are largely controlled by plant transpiration, intra- and interspecific variability will need to be considered.