WILLOWS SPREADING BY SEED IMPLICATIONS FOR AUSTRALIAN RIVER MANAGEMENT

摘要

This is an initial assessment of the spread of willows (Salix) by seed in Australia. Its main aims are to assist river managers and provide a basis for further research Extensive observations, mainly in the Canberra-Cooma region, are supported by a review of literature. All willow species in Australia have been introduced. Each plant is usually either wholly male or female. Until recently, it was thought that in most species only one gender had been introduced, so that production of seed would rarelyor never occur, and local spread would be only by the deliberate planting of cuttings or by the rooting of accidentally detached branches. This assessment has shown that there are at least a dozen willow species and varieties spreading beyond parks andgardens. There are females of ten 'species', and all but one of these (S. babylonica) have now been proved to produce viable seed and resulting seedlings in rivers. There are also males of ten 'species', and "these hybridise with species of either the tree willow group or the shrub willow group. The seedling populations found varied from a few dozen individuals to half a million; they ranged from Tasmania through Victoria and eastern New South Wales. Seedlings of most species became established only inexceptional years, and the oldest seedlings found were about 15 years old. Regeneration by seed depends mainly on a favourable seedbed, a fertile mother within seeding distance (about 300 m) and a fertile, compatible father within pollinating distance (about 300 m). Except for S. cinerea (which has spread not only along rivers, but also into swamps, and even into moist forest), and S. nigra (which has spread along rivers as well as road ditches), willows have regenerated only on the continuously wet bare margins of rivers. Their seeds live only a few days, and need favourable conditions as soon as they fall, around November, and for several weeks or months thereafter. Willows can be valuable for river bank protection, where erosion is extreme, but their use for this purpose should be restricted to 'species' that do not spread readily into the river channels either by seed or by the rooting of detached branches. It is therefore recommended that, where willows are clearly preferable to native plants, only permanently male or sterile clones be used at river margins, provided that their branches are not fragile. Selective removal of fragile males (especially S. fragilis) growing in streams and any currently or potentially fertile females growing within 300 m of streams should be carried out. Females should not be planted anywhere.