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.
Russell lupin, broom, and gorse on braided river
Many invasive plants were brought into New Zealand by European settlers for agricultural or garden use. Some of these plants – willow, gorse, broom, and Russell lupin – have become invasive weeds along braided rivers, changing the natural conditions and specialised ecology.
Why are they a problem?
- 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, such as willows, 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 to predators such as stoats and cats to sneak up on birds
Gorse and broom infestation causing incised single channel in a previously braided river bed, completely destroying the ‘braided river’ morphology