Seasonal charges in water source of four families of loblolly pine (Pinus taeda L.)

Source water utilized by four families of loblolly pine (Pinus taeda L.) wa s assessed by comparing the H isotope composition (deltaD) of xylem sap and of soil water from four depths (0-20 cm, 20-40 cm, 1.2 m, and 2.1 m) acros s 1 year. Soil water deltaD values varied with soil moisture content in the well-drained, sandy site and at each of the four soil depths. In September and November 1997 and May through November 1998, xylem sap deltaD values c losely matched the soil water deltaD values of the upper soil horizons (0-2 0 and 0-40 cm) and, therefore, reflected significant water uptake from uppe r regions of the soil profile. In March 1998, xylem sap deltaD values close ly matched the soil water FD values of the 1.2 m soil depth and, therefore, indicated that trees during this portion of the growing season were obtain ing their water from deep in the soil profile. Analysis of source water use with a two-ended mixing model in the 3 months of collection that exhibited a range of soil water deltaD values across the soil profile confirmed that trees utilized different sources of water depending upon season of the yea r. In September 1997 and November 1998, source water uptake was primarily f rom the upper soil profile while in March 1998, source water uptake was fro m deep in the soil profile. With few exceptions, we did not fmd striking di fferences in source water use between drought-hardy families and those that were locally adapted.