HR: 1330h
AN: B52A-1032    [PDF]
TI: Seasonal Water Usage by $\it Juniperus Ashei$: Assessment With Stable Isotopes of Hydrogen and Oxygen
AU: * McCole, A A
EM: amy_mccole@mail.utexas.edu
AF: University of Texas at Austin, Geological Sciences Department, 1 University Station C1100, Austin, TX 78712-0254 United States
AB: The recent expansion of {\it Juniperus ashei} (Ashe juniper) on the Edwards Plateau of Central Texas has important implications for the ecosystem structure, productivity and hydrology of the region. Ashe juniper expansion may negatively impact the ecology and hydrology of the Edwards Plateau. The Ashe juniper's morphology, rooting habit, and ability to photosynthesize throughout the year suggests greater water loss will occur in areas where Ashe juniper is prevalent compared to areas dominated by grasses. However, past studies have reached conflicting conclusions regarding Ashe juniper's effect on the water budget. A better understanding of the patterns of Ashe juniper's water use will aid in the understanding of how the Ashe juniper affects groundwater recharge, herbaceous productivity and evapotranspirational water loss. Stable isotopes of hydrogen and oxygen from precipitation, soil water, plant xylem water, and groundwater reveal the current ecosystem hydrology. A comparison of the isotopic compositions of potential water sources and juniper stem water indicates the source water utilized by the Ashe juniper. At the Honey Creek State Natural Area, Comal County, Texas the plant, soil and spring water as representative groundwater were sampled at approximately two month intervals over an annual cycle from two adjacent watersheds and analyzed for hydrogen and oxygen isotope composition. Soils were sampled at depth intervals of 5 to 10 cm and soil water content measured. Mass balance calculations using oxygen isotope data from the dry periods of the year, late summer and winter, indicate the Ashe juniper derives between 72% and 100% of it water from groundwater. In contrast, during the wet periods of the year, spring and fall, mass balance calculations indicate that between 45% and 100% of Ashe juniper's water is derived from soil water. Hydrogen isotope data from a subset of samples are consistent with these results. Bowens ratio measurements of evapotranspiration were compared with isotope data to partition evaporative and transpirative water loss from Ashe juniper stands and grass areas. The isotope data from the dry and wet periods of the year show a seasonal shift from nearly exclusive groundwater use by the Ashe juniper during the dry season to significant soil water use by the juniper during the wet season. This seasonal shift indicates the presence of Ashe juniper can appreciably reduce groundwater resources both by lateral roots intercepting potential recharge during the wet season and direct uptake of groundwater by deep roots during the dry season. Ashe juniper will directly compete with grasses for soil water during the wet season, limiting herbaceous productivity.
DE: 4870 Stable isotopes
SC: Biogeosciences [B]
MN: 2003 Fall Meeting