Riparian planting around headwater streams — Fact sheet

This fact sheet is about planting around waterways and the benefits it can bring. It's based on research by Scion from the Planting eroding hill country in the Hawke’s Bay Region report.

When it rains, any water that doesn’t soak into the ground becomes runoff. This runoff picks up nutrients from things like fertiliser, other chemicals, animal waste, and loose soil sediment as it runs across the land and is carried to the closest stream, river or lake.

When trees and plants grow around waterways, runoff has to pass through their roots and the soil around them. This filters out many contaminants before they enter the water.

What affects water quality?

Agricultural activities can affect water quality through:

• fertilisers used to improve pasture growth. This adds to the nitrogen, phosphorus and potassium content of the soil (commonly known as NPK).
• animal waste (urine and faeces) adding nutrients to the soil
• defoliation (removal of grass) due to grazing can result in reduced soil cover and greater runoff during rain events.

Planted forests can affect water quality. Potential sources of contaminants from planted forests include: 

• leftover nutrients in soils and groundwater from past agricultural activities
• fertiliser or pesticides applied as part of forest management 
• disturbances during and after harvesting, particularly increased sediment (silt and clay) in waterways.

Runoff is much less common in established forested areas. This is because tree roots act like water filters. They soak up the water and absorb nutrients.

Headwater catchments catch rainwater high in the landscape, and headwater streams bring that water downhill.

Because they sit high in the landscape, headwater streams and the areas around them influence:

• the amount of water going downstream
• the quality of water going downstream
• the movement of organic matter (like leaves and branches) and inorganic matter (like silt, gravel, and clay) downstream
• the amount of sediment ending up downstream.

All these things influence the composition of land- and water-based life downstream. Because of this, it’s a good idea to plant riparian areas in the headwater catchments before trying to plant them downstream.

Moderating stream temperatures

Shade from plants helps moderate stream temperatures. These benefits are greatest in smaller headwater streams. Less light getting into streams means reduced solar radiation. This reduces algal growth and helps cool and moderate water temperatures.

Streambank stability

Plant roots reinforce the banks of streams, helping provide better bank stability. This means:

• reduced bank erosion, so less sediment getting into the water
• improved habitat for aquatic life, especially from undercut banks and tree roots.

Flood mitigation

Riparian vegetation (plants and trees) can help in heavy rain or floods when sediment, nutrients and other things like woody debris flow through waterways. The root systems and stems of streamside and riparian vegetation slow the speed of flood waters through the waterway. They also:

• decrease the ability of the fast-flowing water to erode the banks
• help retain floodwaters and their contaminants for processing in the riparian area
• trap woody debris transported downhill and downstream during floods. In return the trapped debris provides a food source and habitat diversity for plants, animals and aquatic life in and around the waterway.

Food sources

Sometimes shade from the forest limits the growth of algae as a food source in headwater streams.

When this happens, organic material from forested riparian areas provides a diverse food resource for both water and land ecosystems. This is from:

• large pieces of wood
• finer organic matter provided by branches, leaves and twigs
• organic matter from insects
• other waste or debris.

The organic material also contains higher quantities of plant biomass than non-forested areas.

Carbon sequestration

The scale of riparian forest planting is often small compared to other afforested areas. But trees in riparian areas, and large wood deposited in headwater streams, still capture and store carbon dioxide from the atmosphere. This is called carbon sequestration.

Biodiversity

Riparian areas are a source of high biodiversity, with plant, animal, and aquatic communities relying on them for food and shelter. They also provide ways for both land- and water-based animals to move around.

Research has shown that larger buffers provide bigger benefits. In general, riparian buffers less than 10m wide aren’t wide enough to prevent harvesting damage. The greatest benefits of buffers this size are:

• shade retention
• improving stability on channel banks
• limiting the transport of logging slash (woody debris from logging operations) into streams.

Widths of at least 30m or more on both sides of the stream limit the effects of harvesting, and maintain both riparian and stream ecosystem function and biodiversity.

Disclaimer

This work was completed in 2019. Since then, devastating weather events have affected the North Island in 2023. The Government is doing more work on species for erodible land in this region, so this material may no longer be accurate or up to date.