Localization of tree water uptake in Scots pine and Norway spruce with hydroiogical tracers

Abstract

This study explores the potential of hydroiogical tracers for determining the proportion of water uptake by Scots pine (Pinussylvestris L.) and Norway spruce (Piceaabies (L.) Karst.) from different layers in the upper 3 dm of podzolized forest soil. To improve the resolution of the tracer technique, a pair of tracers was employed. One tracer was the naturally occurring vertical gradient in the oxygen isotope ratio,δ18O, of soil solution. The second tracer was a 3H solution placed in the mor layer. A three-component mixing model of water uptake was used to simulate the content of these two tracers in the xylem sap. Each component in the mixing model represented a horizontal layer of soil with a characteristic composition of δ18O and 3H. Two stands were investigated. In both stands, the volume-weighted water uptake by Scots pine occurred at a depth of 8–17 cm in the upper B horizon. This was below the concentration of fine roots in the mor layer and the upper few centimeters of mineral soil. In one of the two study stands, Norway spruce was present. It had mean uptake depths ranging from within the mor layer to 5 cm below the mor–mineral soil contact. An uncertainty in the water uptake depth for each tree of ± 1–2 cm was estimated from a Monte Carlo analysis of uncertainties in the model inputs. In addition to these specific results, the study demonstrated that isotopic tracers provide a simple and effective method for determining the vertical distribution of water uptake.