River islands

Six Waikato River islands were returned to Waikato-Tainui through the 2010 Waikato River Settlement. Researcher Cheri van Schravendijk-Goodman (working for Waikato Raupatu River Trust) focuses on two of them in a restoration project.

Maurea Islands

These two islands are located immediately south of the Rangiriri Bridge. They are sometimes referred to as the Maurea Islands because they are near Maurea Marae.

One island is approximately 850 m in length and 180 m wide. The other island is smaller. The islands are dominated by exotic and pest plant species such as alder, crack willow, privet, yellow flag iris and gorse. There are few signs of native species.

Cheri explains that the islands were confiscated from Waikato-Tainui during the raupatu (confiscation of Māori land) in the 1860s. Although the tribe are pleased to have the islands returned, Cheri makes the point that the islands were not returned in the same condition that they were in when confiscated. The Waikato Raupatu River Trust is committed to restoring the islands as part of protecting and restoring the health and wellbeing of the Waikato River.

A baseline survey is being undertaken to identify what is on the islands and what needs to be done. A 2-year project will engage local marae knowledge and student support to help meet the restoration goals.

Cheri explains that these islands are important because they sit near a major stream – Te Onetea – so whatever is on the islands may travel not only down the river but also into the tributary stream and further afield.

Invasive plants

Pest plants of concern are those that are highly invasive – they can propagate quickly and easily:

• Yellow flag iris (Iris pseudacorus) takes over habitat that could be used by natives and has the potential to spread up Te Onetea Stream into Lake Waikare and ultimately into the Whangamarino Wetland, which is of international importance (under the Ramsar Convention). The plant has seeds built for flotation and are easily transported into any aquatic environment (saltwater or freshwater). They can also propagate from their root mass.
• Alligator weed (Alternanthera philoxeroides) can propagate when small parts of the plant break off and re-establish.
• Sweet reed grass (Glyceria maxima) sits in the water on the edge of the river. It has an extensive root system from which plants grow. It is propagated by seed and from stem fragments washed downstream. The plant has sharp leaves that can cut – like paper cuts – when walked through. Whitebait such as inanga tend to lay their eggs within riverside vegetation, but they avoid areas with sweet reed grass. Its increasing invasiveness is adding to the overall decline in available spawning habitat, making it more difficult for whitebait to find suitable places to spawn and lay their eggs.
• Wandering willy (Tradescantia fluminensis), also known as wandering jew, invades both shady and open areas. It establishes thick carpets under trees, which prevent native tree seeds (such as kahikatea) from getting into the soil (seed bank) ready for regeneration once the present trees fall down. (Kahikatea seeds can remain in the seed bank for many years until the conditions are right for growth.) Wandering willy prevents self-generation from taking place. The difficulty with eradicating wandering willy is that whitebait lay their eggs in amongst it in the absence of native grasses. Whilst the preference is to remove wandering willy to help with regeneration of native trees, there is some reluctance to remove what is potentially whitebait spawning habitat.

Managing pest plants

Managing pest plants will be a challenge for people participating in restoration. Cheri explains there are three ways it can be done:

• Biocontrol
• Chemicals
• Mechanical removal.

There are risks with all of these. For example, bringing in a new organism to control another (biocontrol) may have harmful consequences for native species – research for this can take 5–10 years. The chemicals may change native species, and mechanical removal may result in plant pieces breaking off and propagating elsewhere.

In the short term, the chemicals may be used to control some of the pest plants on the river islands. In the long term, scientists will investigate the use of native plants as competitors to the pest plants. This will involve physically damaging the pest plants’ root systems and then planting, for example, harakeke, which should be able to out-compete yellow flag iris. Once established, harakeke can get very dense, and it has adapted to cope with fluctuating river levels.

Invasive trees

The Waikato Raupatu River Trust would like to replace the islands’ willows and alders with natives such as kahikatea. In some stretches of the river, removal of exotic trees can destabilise the banks, contributing to erosion. Also, because trees like willows have been here since at least the 1850s, native fish species like tuna (eels) have adapted to them, for example, using their roots as habitat. On the river islands, the conundrum is around maintaining shaded areas for some natives and providing a frost and wind buffer to more sensitive native plant species, which these pest trees can also provide.

Cheri says deciding what to do will be case by case and will involve looking carefully at what the removal might mean for the surrounding area. A strategy will then be worked out. For example, treetops could be removed to let in light, allowing kahikatea to grow underneath. A toxin can be drilled into the willow base, which then supports the riverbank and remains a habitat for tuna while the natives are growing behind it.