In situ water absorption by tree fine roots measured in real time using miniature sap-flow gauges

1. Root water uptake is a key process in the circulation of water in forest ecosystems. Until recently, water absorption by tree fine roots could not be measured in situ in undisturbed soil. 2. We present a new technique that allows continuous recording of the water absorption of fine root endings in mature stands without altering soil structure, hydrology or mycorrhizal infection. 3. The approach combines miniature sap-flow gauges mounted on small-diameter tree roots (3-4 mm) with a complete extraction and visual surface analysis of the adjacent absorbing fine root endings. This technique yields continuous data on water absorption per fine root surface area, and allows analysis of the spatial heterogeneity of root water uptake in the rhizosphere of forests. 4. We present the results of laboratory and field calibration experiments with Fagus sylvatica L. roots (3-4 mm), which show a good agreement between gauge flow data and synchronous gravimetric flow measurements for flows between 2 and >50 g h(-1) . Gauge readings were unreliable during low flows (<2 g h(-1) ) at night. In these periods, which cover approximate to10% of daily flow, we used an empirically derived linear relationship between root temperature difference and flow. 5. Measurements on F. sylvatica root endings during 10 summer days showed daily water absorption maxima ranging between 0.20 (rainy days) and 0.58 mmol m(-2) root surface area s(-1) (bright or overcast days). The corresponding daily maxima of leaf transpiration rate were approximate to10 times higher (2-4 mmol m(-2) leaf area s(-1) ). 6. The combination of miniature sap-flow gauges and determination of fine root surface area provides a promising tool for analysing water absorption by tree root systems in situ .