Variation of the 18O/16O ratio in water samples from branches

The studies of the water turnover of plants may use the labelling of water by its natural variation of the 18O/16O ratio. The basic value of such a study is the isotope ratio in soil water, which is represented by the 18O/16O ratio in water samples from stem and branches, too. During the water transport from the soil water reservoir to the leaves of trees, no fractionation of the oxygen isotopes occurs. The oxygen isotope ratio within a single twig varies about +- 0/00 (variation given as standard deviation of the delta-values), within the stem of a large tree about +- 20/00. The results of water from stems of different trees at the site of the Nuclear Research Center Juelich scatter about +- 10/00. The delta-values from a larger area (Rur valley-Eifel hills-Mosel valley), which were collected in October 1978 during the end of the vegetation period, showed a standard deviation between +- 2.2 (Rur valley) and +- 3.60/00 (Eifel hills). The 18O/16O-delta-values of a beech wood from Juelich site are in the range of - 7.3 and - 10.10/00 (mean local precipitation 1974 - 1977: - 7.40/00). At the hill site near Cologne (Bergisches Land, late September 1978) we observed an oxygen isotope ratio of - 9.10/00 (groundwater at the neighbourhood between - 7.6 and 8.70/00). In October 1978 at an area from the Netherlands to the Mosel valley we found delta-values of branch water between - 13.9 (lower Ruhr valley) and - 13.1 (Eifel hills to Mosel valley) in comparison to groundwater samples from the same region: - 7.55 and - 8.39. There was no significant difference between delta-values from various species or locations within this area. Groundwater samples should normally represent the 18O/16O ratio of local precipitation. The low delta-values of branch water could be due to the rapid uptake of precipitation water of low 18O content in autumn to the water transport system of plants. (orig.)