PARENT SESSION
Oral Session #17: Plant Ecology: Water Relations.
Presiding: W. Pockman
Monday, August 5. 1:00 PM to 3:45 PM. Coconino Meeting Room, TCC.
Desert shrub water relations with respect to soil characteristics and plant functional type.
HACKE, UWE*,1, SPERRY, JOHN1, 1 University of Utah, Salt Lake City, Utah
ABSTRACT- Soil characteristics influence plant communities in part through water relations. By hypothesis, finer textured soils in arid climates should be associated with more negative plant and soil water potentials during drought, greater resistance of xylem to cavitation, and shallower root systems than coarse soils. These hypotheses were tested by comparing the water relations of Great Basin shrubs growing in sand vs. loam soils. The eight study species varied in typical rooting depth and vegetative phenology. Xylem pressures for a species averaged 1.1 MPa more negative in the loam vs. sand site, despite higher precipitation at the loam site. Root xylem at the loam site were 0.9 MPa more resistant to cavitation on average than at the sand site for the same species. There was a strong trend for shallower rooting depths at the loam vs. sand site. Within a species, roots were consistently more vulnerable to cavitation than stems, and experienced more cavitation during the growing season. Greater than 80% loss of xylem conductivity (PLC) was estimated in shallow roots of three species at the loam site by the end of July, with two of the three showing extensive leaf drop and branch mortality. Transpiration rate was negatively correlated with PLC, with a tendency for lower gas exchange rates in loam vs. sand. At the sand site, cavitation resistance was negatively correlated with estimated rooting depth. Drought deciduous species had the shallowest root systems and greatest resistance to cavitation. In contrast, two species with phreatophytic tendencies were summer active and were the most vulnerable to cavitation. The cavitation resistance of roots in desert plants may help explain species differences in limits to soil water extraction, magnitude of hydraulic lift, and response to late summer rain pulses.
KEY WORDS: desert shrub communities, plant-soil interactions, root water uptake, xylem cavitation