A hydrological tracer study of water uptake depth in a Scots pine forest under two different water regimes

Little is known about the vertical distribution of water uptake by trees un der different water supply regimes, the subject of this study, conducted in a Scots pine stand on sandy loam in northern Sweden. The objective was to determine the water uptake distribution in pines under two different water regimes desiccation (no precipitation) and irrigation (2 mm day(-1) in July and 1 mm day(-1) in August), and to relate the uptake to water content, ro ot and soil texture distributions. The natural O-18 gradient in soil water was exploited, in combination with two added tracers,H-2 at 10 cm and H-3 a t 20 cm depth. Extraction of xylem sap and water from the soil profile then enabled evaluation of relative water uptake from four different soil depth s (humus layer, 0-10, 10-25 and 25-55 cm) in each of two 50-m(2) plots per treatment. In addition, water content, root biomass and soil texture were d etermined. There were differences in vertical water uptake distribution bet ween treatments. In July, the pines at the irrigated and desiccated plots t ook up 50% and 30%, respectively, of their water from the upper layers, dow n to 25 cm depth. In August, the pines on the irrigated plots took up a gre ater proportion of their water from layers below 25 cm deep than they did i n July. In a linear regression, the mean hydraulic conductivity for each mi neral soil horizon explained a large part of the variation in relative wate r uptake. No systematic variation in the residual water uptake correlated t o the root distribution. It was therefore concluded that the distribution o f water uptake by the pines at Aheden was not a function of root density in the mineral soil, but was largely determined by the unsaturated hydraulic conductivity.