Weeds, including trees

Why weeds are a problem for braided rivers

Canterbury’s braided rivers are home to unique native plant communities especially adapted to growing in the challenging environment of shifting gravels, extreme temperatures and limited nutrients. This natural vegetation is often low-lying and sparse because of the dynamic morphology of braided rivers, providing the perfect habitats for braided river birds, reptiles, and invertebrates. 

Many invasive plants were brought into New Zealand by European settlers for agricultural or garden use. Others were planted along riverbanks to prevent flooding, with little to no understanding that braided rivers need to wander across a wide braidplain, not be confined to a few deep and narrow channels.

Several of these plants, including willow,  broom, and Russell lupin, have invaded braided rivers, changing their natural dynamic nature destroying their specialised ecology and vital ecosystem functions, including helping to clean water from the high level of nitrates generates by dairy farms.

These weeds are:

  • Easily established
  • Become dense stands, shading out and displacing threatened plants and whole native plant communities such as the cushion forming ‘forget-me-not’ (Myosotis uniflora) and the rare dwarf woodrush (Luzula celata)
  • Many fix nitrogen, which changes the chemistry of the soil, making it unsuitable for native plants
  • Some suck up huge quantities of water like a sponge, lowering the river level and water table
  • Entwined roots hold the gravel together, forming stable areas. The river erodes the edges, creating steep banks and restricting the water so that instead of braiding, it develops deep, fast-flowing channels unsuitable for wading birds to feed in
  • Dense stands take over the open spaces where braided river birds like to nest. This is deeply concerning for rare and endangered birds that nest exclusively along braided rivers, particularly the wrybill, black-fronted tern and kakī/black stilt, which normally feeds in shallow river braids
  • Provide cover for introduced predators to sneak up on nesting birds
Short 2-minute video showing the impact of weeds on braided river through modelling (NIWA research- see link to references below), and drone footage over the Orari River showing how closely modelling resembles reality.

Several programmes are underway to control or remove weeds in rivers. The responsibility for this control depends on who owns different sections of rivers and includes:

Weeds including willow, gorse, and broom have devastated the Orari River, completely altering the natural dynamic behaviour and sucking up huge volumes of water. There is virtually no room left for native plants or nesting sites for birds except in low lying patches prone to flooding.
Weeds including willow, gorse, and broom have devastated the Orari River, completely altering the natural dynamic behaviour and sucking up huge volumes of water. There is virtually no room left for native plants or nesting sites for birds except in low lying patches prone to flooding.

References and research papers

© BRaid 2020

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