Submitted to: Functional Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/1/2003
Publication Date: 1/1/2003
Citation: SNYDER, K.A., WILLIAMS, D.G. DEFOLIATION ALTERS WATER UPTAKE BY DEEP AND SHALLOW ROOTS OF PROSOPIS VELUTINA (VELVET MESQUITE). FUNCTIONAL ECOLOGY. 2003. V. 17(3). P. 363-374. Interpretive Summary: The woody species mesquite (Prosopis velutina) is a widespread colonizer of areas that were formerly grass-dominated. Along the San Pedro River in Arizona, this species is quite common in areas underlain by shallow groundwater that were formerly dominated by Sacaton grasses. The objective of this research was to understand how two populations of mesquite at sites with differing depth to groundwater used groundwater and shallow soil moisture from monsoon rain. It is predicted that global climate change may alter the seasonal weather patterns in this region, so we were also interested in assessing if this species is able to use both groundwater derived from winter precipitation and shallow soil water derived from summer precipitation. Because plants have a finite amount of carbon to allocate to root growth, we were interested in assessing if carbon-limited trees (carbon limitations were imposed by removing leaves) would interact with groundwater availability to favor use of one water source over another water source. Mesquite at the site with greater depth to groundwater derived a greater percentage of its water from shallow soil water. Carbon-limited trees appeared to become disconnected from groundwater at both sites and, therefore, had to use a greater proportion of shallow soil water to satisfy transpiration demands. These data indicate both environmental conditions and internal plant constraints on carbon allocation interactively determine what sources of water will be used to satisfy plant water demand. These data also indicate mesquite is very flexible in its water use strategies; this flexibility is one of the attributes of mesquite that contributes to its rapid encroachment in many semiarid systems.
Technical Abstract: 1. Prosopis velutina Woot. (Velvet Mesquite) at a site with limited groundwater availability derived a greater percentage of water from shallow soil at the onset of the summer rainy season than did trees at a site with greater availability of groundwater. Predawn leaf water potentials were not a strong indicator of shallow water use for this species with roots in multiple soil layers. 2. We experimentally defoliated P. velutina plants to determine if reduced-canopy photosynthesis would alter vertical patterns of root activity. After natural rain events, hydrogen isotope ratios of xylem sap indicated that defoliated P. velutina took up a greater percentage of its water from shallow soils than did undefoliated plants. 3. Irrigation with deuterium-labelled water further demonstrated that undefoliated plants were able to use shallow soil water. Defoliation appeared to affect the ability of trees to use deep-water sources. 4. Reduced carbon assimilation limited water uptake from deep soil layers. These data highlight that there are internal physiological controls on carbon allocation that may limit water uptake from different soil layers. During periods of high vapour pressure deficit or soil drought, when leaf gas exchange and carbon assimilation decline, this may create positive feedbacks where plants are unable to forage for deep water due to carbon limitations.