Document: ALE-3-32-41

Trees on the edge: Water relations and leaf carbon isotope discrimination in a Chihuahuan Desert wash.

TRAFTON, A.*, K.R.HULTINE and D.G.WILLIAMS

University of Arizona, Tucson ,AZ 85721 USA ¹

Abstract:
Ephemeral channels that drain broad alluvial fans in desert regions provide an extremely heterogeneous resource environment for plants. Saturated conditions in the channel sub-surface emerge following episodic flow events and disappear as water seeps into the deeper aquifer or is lost by evapotranspiration. Local precipitation that wets upper soil layers can be an additional source of water for plants in these habitats during the growing season. The success of tree species that occupy these ephemeral channels in the American Southwest, depends primarily on their ability to access these pulses that are largely unavailable during most of the growing season. To address plant responses to pulses of water, we measured the plant water relations and carbon isotope discrimination of three co-occurring tree species (Juglans major, Prosopis velutina, and Celtis reticulata) at two sites along an ephemeral drainage in southeastern Arizona. The channel aquifer is perched at one site by the presence of an impervious granite sill. The highly fractured limestone and conglomerate at the second site allows water to drain rapidly into the very deep regional aquifer. Predawn water potentials in June before the precipitation and runoff events was highest in J. major at both sites and was higher among all the species at the perched-aquifer site than at the fractured-bedrock site. P. velutina and C. reticulata expressed a greater degree of seasonal variation in predawn water potential than did J. major. Deuterium and oxygen isotope ratios of xylem water indicate that P. velutina uses water strictly from unsaturated layers in the alluvial soils while C. reticulata and J. major predominantly use the groundwater that seeps through fissures or that ponds above bedrock layers. These results suggest that plants employ multiple strategies to exploit limited resources. Leaf carbon isotope ratios will be analyzed to examine intra- and interspecific differences in gas exchange characteristics that arise in these spatially and temporally heterogeneous environments.

Keywords: leaf carbon isotope, predawn water potential, riparian, phreatophyte

This abstract is being presented at: 3:30 PM in session:
WATER RELATIONS