Saugspannungsmessungen in der Krone eines Mammutbaumes (Sequoiadendron giganteum)

Abstract

Summary Scholander’s pressure chamber technique was used for determining suction tensions (= xylem water potentials) in the crown of a giant sequoia, Sequoiadendron giganteum (Lindl.) Buchholz. The influence of repeated cutting on readings was found to be considerable, when 20 minutes had elapsed between detachment of a branch and removal of a piece of stem. Equilibrium pressures became smaller in lateral shoots removed after some time from cut branches, indicating a withdrawal of the water column from the first cutting, probably through water uptake by leaf cells. Excessive trimming of branches and gradual detachment of small twigs from cut branches are not recommended, therefore, unless immediately after the first cutting. Daily fluctuations of suction tension at 7 m above the ground show a good agreement between vapour pressure deficits calculated from meteorological data and tensions in shaded twigs. Xylem tensions in twigs exposed to full sunlight gradually rise over the shadow curve from morning to noon, reaching a maximum difference of ∼ 7 atm. Minimum tensions are observed shortly before sunrise. Tension gradients from the lowest branches (7 m above ground) to the top (24–28 m) show only a slow rise when transpiration is reduced, especially immediately before sunrise. During the day, they are steeper in the shade (e.g. under cloudy sky) than in full sunshine. Measurements of potential osmotic pressures indicate that twigs in the top of the crown are unable to heighten their tensions as much as lower twigs because of osmotic difficulties. These are overcome in winter and early spring, when potential osmotic pressures are high and permit very steep gradients in full sun. A formula originally developed by Huber is further improved and serves as the basis for a discussion of static and dynamic components of xylem tensions.