PARENT SESSION
Oral Session # 56: Plant Ecology IV: Plant - Water Relations II.
Presiding: D Rosenthal
Wednesday, August 6. 1:30 PM to 5:00 PM, SITCC Meeting Room 200.
Simulated and measured tree water uptake using tritium as a tracer.
Rebel, Karin*,1, Riha, Susan1, Fahey, Timothy1, Seaman, John2, 1 Cornell University, Ithaca, NY2 Savannah River Ecology Lab, Aiken, SC
ABSTRACT- Methods for determining in the field if tree species differ in where they take up water in the soil profile are limited, as are methods to determine root distribution for modeling purposes. In this study, tritium transpired by trees periodically irrigated with tritium enriched water at a southern US coastal plain site was measured and simulated. Tritium in transpired water was measured by liquid scintillation analysis following collection in bags. Water and tritium uptake were simulated using a multi-layer one-dimensional water balance model. Soil water movement and storage were simulated using a capacitance approach, soil water and tritium uptake were simulated using an absorption approach. The potential demand for water and tritium uptake was partitioned among soil layers based on relative root density distribution, estimated using inverse modeling techniques. When soil water content decreased in surface layers, the demand was transferred between layers. Modeled and measured tritium activity in the transpiration water were compared, as well as the measured differences between tree species. The measured tritium activity in the transpiration water followed the simulated tritium activity well. When tritium activity in the upper part of the soil profile was high relative to the lower part of the profile, a large range in the values of measured tritium activity in the transpiration water was observed. This measured increase in range of the tritium activity co-occurred with an increase in simulated transpired tritium activity. Differences in measured tritium activity in the transpiration water was most apparent between understory and overstory trees, comparing the same species as well as among species. These results imply that roots of individual trees and tree species were taking up water in different parts of the soil profile. On a stand basis the root distribution estimated using inverse modeling techniques appeared to well represent water and tritium uptake.
Key words: modeling, tree water uptake, soil-plant-atmosphere interactions