PARENT SESSION
Thursday, August 10, 8:00-11:30 am
COS 79 - Stoichiometry, allometry, and scaling
Mississippi, Mezzanine Level, Cook Convention Center
Presiders: D Jenerette

A comparison of sap flux scaled evapotranspiration and catchment water balance.

Ford, Chelcy^*,1, Hubbard, Robert², Kloeppel, Brian³, Vose, James¹, ¹ USDA Forest Service Coweeta Hydrologic Lab, Otto, NC² USDA FS RMRS, Fort Collins, CO³ Institute of Ecology, University of Georgia, Athens, GA

ABSTRACT- While many studies have validated sap flux measurements on short time scales, the performance of these probes over long time scales, and the feasibility of scaling sap flux based transpiration (E) measurements to the landscape is unknown. Important questions such as– 1) how well can we scale from the tree to the watershed?, and 2) what are the main sources of variation at each scaling step?– remain largely unanswered. To address these questions, we made continuous precipitation (P) and stream discharge (R_o) measurements, and sap flux measurements on 40 trees in three plots for two years in a 13.5-ha watershed (WS17) within the Coweeta Basin in western North Carolina. The vegetation layer in WS17 is a 50-yr old Pinus strobus L. plantation. Interception was estimated using equations developed on a nearby stand. Allometric equations coupled with vegetation surveys conducted on 17 permanent plots were used to estimate sapwood area to ground area ratios of WS17. Sap flux scaled E for 2004 and 2005 was 769 and 659 mm. Scaled evapotranspiration (E_t) estimates were 9% and 11% lower than P-R_o for 2004 and 2005, respectively. Consistent underestimates may have resulted from omitting ∼3% of the basal area attributable to deciduous competitors. Calculating catchment E_t using only one sap flux measurement plot (instead of three) resulted in underestimates of 27% and 22% for 2004 and 2005, respectively. Additionally, omitting key scaling steps produced compensating errors, highlighting the importance of incorporating these sources of variability when scaling from the tree to the landscape.

Key words: Allometry, Scaling, Pinus strobus