PARENT SESSION
Poster Session # 15: Plant Ecology.

Wednesday, August 6 Presentation from 5:00 PM to 6:30 PM. SITCC Exhibit Hall B.

Canopy water use, groundwater fluctuations and hydraulic gradients beneath adjacent plantation and grassland areas.

Engel, Victor ^*,1, Jobbagy, Esteban², Jackson, Robert¹, ¹ Biology Department, Durham, NC, USA² Catedra de Forrajes, Buenos Aires, BA, Argentina

ABSTRACT- Heat dissipation techniques were used to measure sap flow in E. camaldulensis (n=18) in a 50 yr. old plantation in the Pampas region of central Argentina. Piezometer readings of water table depths along a transect from the plantation interior to the surrounding grassland were also recorded. Transpiration rates derived from sap flow measurements ranged from 2 to 4 mm d^-1 during the summer of 2002-3 and correlate well (r²=0.65) with values derived from water table fluctuations beneath the plantation. Residual analysis shows the variance in transpiration estimates between these two techniques is most closely related to moisture content in the vadose zone. These results suggest root uptake of soil water by E. camaldulensis alternates from deep groundwater to more shallow sources depending on availability and evaporative demand. During dry periods between precipitation events, piezometer readings showed a sharp decline (>1 m) in water table depth at the plantation—grassland boundary. Mass balance calculations suggest this local hydraulic gradient caused a net flux of groundwater (∼2 to 3 mm d^-1) from the topographically low grassland areas to the more upland plantation. To test our assumptions regarding plantation root activity, source water uptake, and groundwater contributions from the surrounding grassland, we employed a simulation model of the soil-plant atmosphere continuum that includes hydraulic constraints on transpiration and groundwater transport. Model results compare favorably with sap flow and water table observations, while on-going sensitivity analyses allow investigations of the influence of stand structure (e.g. root depths, leaf area, tree height) on annual water balance. The results show the importance of changes in vegetation structure on local hydrologic budgets.

Key words: transpiration, Eucalpytus camaldulensis, groundwater