Does salinity influence the distribution of exotic willows (Salix spp.) along the Lower River Murray?

Abstract

The weeping willow Salix babylonica and crack willow S. fragilis dominate the riparian vegetation of the River Murray between Mannum and Wellington, South Australia, Australia. The presence of S. fragilis is confirmed, over-riding earlier, informal references to S. × rubens. In this region S. babylonica is represented by females and S. fragilis is represented by males. The flowering times of the two species overlap but the growing season begins earlier for the former. No evidence of hybridization was found, although this is not conclusive. Salix babylonica is most common upstream of Murray Bridge whereas S. fragilis occurs mainly downstream. A field survey in 1999 showed that estimated soil water conductivity varied between sites (median 6.0 mS cm–1) along the river, but this was not reflected in the distributions of the species. In a glasshouse experiment, cuttings of both species proved intolerant of conductivities exceeding 7 mS cm–1. In the field, foliar salt levels indicated that neither species was salt-stressed, despite growing in moderately saline soil. Also, it appears that the trees may access less saline river water in preference to that in the soil. The distributions of the two species in this region could reflect the history of planting rather than salinity tolerance.