Direct measurement of groundwater uptake through tree roots in a cave



Recent intensive research investigating groundwater extraction of Pinus radiata D. Don plantations in south east South Australia has focused on measurement of sapflow rates in P. radiata stems above ground. An extended dry period in summer 2001 provided an opportunity to install sapflow sensors in a P. radiata root clump, which extended through a limestone cave to an unconfined aquifer approximately 14 m below the surface. Over 5 days in March 2001, sapflow velocities were monitored in three individual roots extending from a 58-year-old P. radiata tree. A second sensor set installed in the tree stem enabled direct comparison of water flow through the tree stem and roots. Sap-flux density averaged 0·840 m3 m−2 day−1 and 0·240 m3 m−2 day−1, in the roots and stem, respectively. Sapwood cross-sectional area of the root clump was 0·012 m2 compared to 0·193 m2 for the main tree stem. Total water flow through the stem over 5 days, averaged 0·046 m3 day−1 compared to 0·010 m3 day−1 through the root clump. Thus, at least 22% of tree water use came directly from the unconfined aquifer via these roots. The tree was possibly obtaining more than 22% of its water from this source, if additional tree roots had aquifer access. These results present direct evidence that P. radiata trees can extract water from an unconfined aquifer at depth, if their roots are able to reach the watertable, providing direct support to intensive water balance studies in the region that infer groundwater extraction. Copyright © 2010 John Wiley & Sons, Ltd.