HR: 1340h
AN: H33C-0474    [Abstracts]
TI: Spatially Explicit Observations Elucidate Simple Scalars of Forest Canopy Transpiration Along Moisture Gradients in Semi-Arid and Humid Climates
AU: * Mackay, D S
EM: dsmackay@buffalo.edu
AF: Department of Geography, State University of New York at Buffalo, 105 Wilkeson Quadrangle, Buffalo, NY 14261 United States
AU: Loranty, M M
EM: mloranty@buffalo.edu
AF: Department of Geography, State University of New York at Buffalo, 105 Wilkeson Quadrangle, Buffalo, NY 14261 United States
AU: Adelman, J D
EM: jadelman@uwyo.edu
AF: Department of Botany, University of Wyoming, 1000 E. University Avenue, Laramie, WY 82071 United States
AU: Ewers, B E
EM: beewers@uwyo.edu
AF: Department of Botany, University of Wyoming, 1000 E. University Avenue, Laramie, WY 82071 United States
AU: Kruger, E L
EM: kruger@calshp.cals.wisc.edu
AF: Department of Forest Ecology and Management, University of Wisconsin - Madison, 1630 Linden Drive, Madison, WI 53706 United States
AB: The ability to scale from point measurements to watersheds is critical for making predictions in hydrology. Assumptions are often made that averaging point measurements and scaling them up using a cookie-cutter or paint-by-numbers approach will capture relevant spatial gradients. Two issues are whether such assumptions are valid and how these assumptions hold across environmental conditions. We made spatially explicit measurements and modeling at two field sites representing semi-arid and humid climates. For our semi-arid site we identified a topography-soil moisture gradient in an alpine watershed near Laramie, Wyoming. For our humid climate we chose a wetland-to-upland gradient in the Chequamegon National Forest near Park Falls, Wisconsin. At both sites we used cyclic sampling designs to efficiently quantify spatial trends using geostatistics. Spatial data was collected for sap flux using Granier type sensors. In addition, we collected spatial soil moisture, vapor pressure deficit, and leaf area index with the same level of spatial detail, at both study sites. At the Wyoming site our dominant species was Lodgepole pine, as it spans most of the topography-soil gradients from the edges of riparian areas to upslope positions. At the Wisconsin site we selected aspen as our focus species, as it is a dominant species in terms of transpiration in the region and it grows over a wide variation in topographic positions from wetland to upland. We found that the semivariagrams of soil moisture at both sites showed ranges of about 110 meters on low soil moisture days and 80 meters on high soil moisture days. Neither site showed differences in sap flux per unit xylem along soil moisture gradients. However, once we scaled the sap flux measurements to the whole tree using basal area, we found that the semi-arid site had much higher fluxes near the stream compared to upslope, but at the humid site the uplands had much higher scaled sap fluxes than the wetlands. It appears that both sites allow simple scalars to spatially predict transpiration, but we hypothesize that the scaling behavior differs because the semi-arid site is water-limited because of too little water, and the humid site is water-limited because of too much water. To test this hypothesis we used a forest canopy model to interpret the opposing responses, and a stochastic parameter restriction and selection scheme to assess predictive uncertainty associated with the model-supported interpretations.
DE: 1818 Evapotranspiration
DE: 1851 Plant ecology
DE: 1866 Soil moisture
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting