Water table salinity, rainfall and water use by umbrella pine trees (Pinus pinea L.)

Abstract

The interactions between environmental conditions, particularly precipitation and water table salinity and tree water use were studied at the pinewood of Alberese, a stand of umbrella pine (Pinus pinea L.) trees growing along the Southern coastline of Tuscany and characterised by a sandy soil and a high water table level (ranging between 1 and 2 m depth). Data on sap flow, measured by heat pulse, or compensation technique, were compared between two contrasting sites (referred to as sites A and B), characterised by clear differences in the salinity levels of the water table. Site A, located near the karstic Uccellina hills was characterised by more favourable hydrologic conditions as it was likely receiving lateral rainfall drainage from the hills. Water electrical conductivity (EC) values at the upper surface of the water table of this site were lower than 12 dS/m. By contrast, the more typical site B, located further away from the hills, did not benefit from lateral water movement in the soil and showed values of EC of about 17–20 dS/m, half the value of seawater. This amounted to a difference in soil osmotic potential of about 0.4–0.5 MPa across sites. Despite this difference in salinity, measurements of needle water potential during September 2000 did not differ across sites (average of about −1.5 and −2.4 for pre-dawn and midday water potentials, respectively). In contrast to water potentials, the dynamics of sap flow clearly differed across sites. Larger seasonal reductions in maximum daily sapwood-related sap flux density were recorded at site B both during summer (0.005–0.015 10−3 m3 m−2 s−1) and spring-autumn (0.030–0.045 10−3 m3 m−2 s−1) than at site A (0.010–0.018 and 0.025–0.035 10−3 m3 m−2 s−1, for summer and spring-autumn, respectively). The different behaviour of water potentials and transpiration rates across sites could be explained by higher values of soil-to-leaf hydraulic resistance at site B during the dry season. Rainfall accumulated in the soil during winter formed a top layer of fresh water, which was then used by plants during the following spring/summer. When fresh water supplies were depleted, the pines drew from the underlying salty water with clear seasonal differences between the two sites.