Publications: Research reports and publications

Effects of willow removal on Australian and New Zealand stream ecosystems - A literature review of the potential risks and benefits

1 April, 2013
Cawthron Report 2315. Prepared for Ministry of Business, Innovation and Employment project C01X1002: Maintenance and rehabilitation of aquatic ecosystems. Effects of Willow Removal on Australian and New Zealand Stream Ecosystems - A literature review of the potential risks and benefits.

EXECUTIVE SUMMARY

Willows (Salix spp.) are exotic to New Zealand and Australia, but dominate many riparian habitats because they have been widely planted to control streambank erosion in degraded agricultural catchments, and because some species have become invasive.

Negative effects of willows on stream ecosystems include:

1) increased flooding and reduced land drainage due to the willow roots and branches reducing channel volume and increasing hydraulic roughness

2) consequential erosion and channel migration, and

3) further spread and loss of biodiversity through replacement of the native vegetation.

These issues have raised concern amongst resource managers and the community and in response, some resource management authorities throughout New Zealand and in south eastern Australia have started to implement reach-scale willow control and removal operations.

The destructive removal process and the associated potential risks to stream ecosystems have sometimes caused a public outcry; prompting the need for managers to consider whether the benefits outweigh the risks. This literature review presents the potential risks and benefits to inform resource managers whether reach-scale willow removal and subsequent re-establishment of native riparian vegetation may be an effective rehabilitation measure to increase stream health and the biodiversity of instream and riparian communities.

Very few scientific studies on willow removal effects have been conducted and documentation of such rehabilitation projects is equally scarce. Hence, potential benefits are inferred from studies on the mitigation of adverse effects of willows on stream and riparian ecosystems reported in the literature. Potential risks of willow removal are based on knowledge of the ability of willows to retain large amounts of fine sediment and organic matter, and to influence geomorphology and flow patterns. Further potential risks are associated with the loss of the functions that riparian vegetation fulfils.

A key finding of this review is that willow management is complex and context-dependent.The expected ecological benefits as well as potential risks are likely highly dependent onstream size, geomorphology, hydrology, catchment land use and associated stressors, and the extent of willow growth and the taxa involved. Setting management goals tempered by the spatial and temporal limitations to recovery will guide the cost-benefit analysis of intended operations and will be crucial to successful rehabilitation projects. Given the potential ecological risks and negative consequences that are involved with willow removal, this report provides management recommendations for when not to remove willows and for selecting streams where rehabilitation efforts are likely to be most efficient.