homepageReturn to News/Meetings and Events/Professional Society Information
Ecological Society of America, 83rd Annual Meeting in Baltimore, 2 - 6 August 1998
Theme: "Ecological Exchanges Between Major Ecosystems"
ORAL PRESENTATION
SNYDER, KEIRITH A.* and DAVID G. WILLIAMS.
University of Arizona, Tucson, AZ 85721 USA.
Environmental and physiological controls on water source utilization patterns.
Several studies have shown that obligate phreatophytes rarely utilize surface soil moisture, but under some circumstances these species can switch from groundwater to surface soil moisture use. The physiological explanation for why some populations and species use surface soil moisture and others do not is unresolved. We hypothesized that species with access to a stable water source, in this case groundwater or perennial streamwater, would be less likely to grow lateral surface roots to acquire soil moisture from precipitation. We identified water sources utilized by cottonwood (Populus fremontii) and mesquite (Prosopis velutina) to determine if spatial and temporal water availability influenced water uptake behavior. Cottonwood and mesquite trees along a perennial river reach did not utilize monsoon-derived soil moisture and primarily used groundwater. However, at an ephemeral reach both species utilized monsoon-derived soil moisture, and mesquite used a greater proportion of surface soil moisture than cottonwood. The trade-off between producing a deep root system or maintaining small lateral surface roots is also not known, but is likely influenced by whole plant carbon balance. Therefore, greenhouse and field experiments were initiated to determine if changes in root allocation patterns and root function are in response to whole plant carbon limitations that may arise from differences in below ground water availability.
POSTER PRESENTATIONS
KOEPKE, DAN F.* and DAVID G. WILLIAMS.
University of Arizona, Tucson, AZ 85721 USA.Comparative gas exchange and water relations in young and old shoots of a desert riparian phreatophyte.
Water relations and gas exchange characteristics of year-old and basally resprouting stems were examined in the deciduous phreatophytic shrub seep-willow (Baccharis glutinosa Pers.) in a semi-arid riparian ecosystem in southern Arizona, USA. We hypothesized that root-to-leaf path hydraulic conductance would be lower in year-old stems than in resprouts and differences would be found between stem types on plants occupying relatively xeric ephemeral wash sites compared to those of plants at mesic perennial stream locations. Predawn and midday leaf water potential did not differ between stem types at either site. However, net photosynthetic rate (A), stomatal conductance (gs) and leaf-specific hydraulic conductance (LSC) were consistently higher in new than in old stems, except early in the growing season at the perennial stream site. Differences in A, gs, and LSC between new and old stems at the xeric site in late summer, for example, were 11.4 and 10.5 micromol*m-2*s-1, 0.52 and 0.38 mol*m-2*s-1, and 7.87 and 6.95 mmol*m-2*s-1*MPa-1, respectfully. Constrictions at shoot junctures may account for some of the differences found in gas exchange and hydraulic characteristics, since leaves on year-old stems are produced from expanding axillary shoots and from terminal growth in the resprouting stems. The complex architectural and physiological dynamics in this important desert phreatophyte have implications for the water balance and productivity of riparian ecosystems in the desert southwest.
SCHAEFFER, SEAN M.,* and DAVID G. WILLIAMS.
University of Arizona, Tucson, AZ 85721 USA.
Controls on transpiration in desert riparian forest canopies.
Micrometeorological, physiological, and stem sap flow measurements were made to investigate controls on cottonwood/willow canopy transpiration at perennial and ephemeral stream sites on the upper San Pedro River watershed in southeastern Arizona, USA. The stand density was greater for cottonwood/willow (609 stems/ha) at the perennial stream site compared to the ephemeral site (189 stems/ha) and may have contributed to differences in the sensitivity of transpiration to humidity between the two sites. Trees at the ephemeral stream site reached maximum transpiration rates at lower upwind air evaporative demand when compared to trees at the perennial site (1.75 kPa and 2.5 kPa for cottonwood respectively, 2 kPa and 2.5 kPa for willow, respectively). Apparently, stand structural characteristics that develop under different hydrological regimes in desert riparian forests play a large role in determining the sensitivity of transpiration to environmental driving variables and hence daily and seasonal patterns of canopy transpiration. Differences in stomatal conductance, coupled with environmental driving variables, of trees between perennial and ephemeral streamsites may help determine if boundary layer processes are important in determining patterns of transpiration. Variation at the stand level should be considered when parameterizing models that predict responses of these ecosystems to future climate or hydrologic conditions, or when aggregating transpiration fluxes across the entire river system.
WILLIAMS, D.G.1*, D. GOODRICH2, A. CHEHBOUNI3, B. MACNISH1, T. MADDOCK1, S. MORAN2, J. SHUTTLEWORTH1, J. TOTH2, C. WATTS4, L. HIPPS5, D. COOPER6, J. SCHIELDGE7, Y. KERR8, H. ARIAS4, M. KIRKLAND6, R. CARLOS6, W. KEPNER9, and B. JONES9.
1University of Arizona, 2USDA Agricultural Research Service, 3ORSTOM France, 4IMADES Mexico, 5Utah State University, 6Los Alamos National Laboratory, 7Jet Propulsion Laboratory, 8CNES/CESBIO FRANCE, 9US Environmental Protection Agency.
The Semi-Arid Land-Surface-Atmosphere (SALSA) Program - an international collaboration to study global change.
The primary objective of the Semi-Arid Land-Surface-Atmosphere (SALSA) Program is to investigate the consequences of natural and human-induced change on the basin-wide water, energy, and carbon balance of semi-arid regions at event, seasonal, interannual, and decadal time scales. SALSA is a long-term program whose current research and integrated measurement efforts are focused on the San Pedro River basin which originates in northern Sonora, Mexico and flows north into southeastern Arizona, USA. The 1997 objectives were as follows: 1) to improve the diagnosis of surface fluxes used in atmospheric models and compare remote and in situ observations with real-time model runs; 2) to initiate the development and validation of a coupled soil-vegetation-atmosphere transfer (SVAT) and vegetation growth model; 3) to conduct in situ and remote measurements to quantify and develop models for groundwater, surface water, and evapotranspiration interactions, identify plant water sources, and investigate controls on riparian canopy transpiration; 4) to develop and validate aggregation schemes with data over highly heterogeneous surfaces; and 5) to develop a multi-scale system of landscape pattern indicators of changing ecological condition using remotely sensed data.
Return to News/Meetings and Events/Professional Society Information
homepage