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Rivers and streams

Forestry streams

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Example of a forestry stream

The Mahurangi River (forest site) is an example of a forestry stream in the Auckland region. The ARC has been monitoring its water quality since 1986.

What is a forestry stream?

A forestry stream flows through a plantation of exotic (non-native) forest for most, or all, of its length. Most New Zealand exotic forests (almost 90%) contain pine trees (Pinus radiata) but there are some plantations of douglas-fir, cypress, eucalytus, and other softwood and hardware species.

In 2000, 7% of the land (about 33,000 hectares) in the Auckland region was covered by exotic forest. Planting of exotic forests has declined since 1998 and some plantations are being converted to pasture after the trees have been harvested.

The health of a forestry stream and the types of life (ecology) in and around it are influenced by the following factors:

Riparian vegetation

Mature exotic forest provides shade throughout the whole year and protects the forestry stream from direct sunlight. This ensures that the water stays at a fairly constant and cool temperature.

When native bush is allowed to grow along the stream banks, leaves and other organic matter from decaying forest vegetation wash into the stream and provide an important a food source for organisms living in the stream. Larger pieces of woody debris also provide a variety of habitats to suit different organisms.

Land use in the surrounding catchment

Exotic forest is often planted on land that is too steep and / or where the soil is too poor for other types of horticulture or agriculture. The ground in exotic forest plantations tends to be acidic, similar to the native bush environment.

There is little or no urban development in the catchment area so the water is almost free of contaminants such as heavy metals from domestic and industrial pollution. The levels of nutrient concentrations (nitrogen and phosphorus) in the stream may be slightly increased but there is little bacterial contamination.

Harvesting

When the forest is harvested, there are rapid and significant environmental changes. These are greatest when there is no riparian buffer and when the forest is harvested close to the stream margins. The water quality is often severely degraded by high levels of sediment and the water temperature increases because the shade has been removed.
High sediment levels can remain for several years after harvesting, particularly in the smaller forestry streams that have a lower water flow and cannot remove sediment as fast as the larger streams.

Typical water quality of a forestry stream

The lack of urban land cover and only occasional disturbances in the forestry catchment mean that forestry streams have high water quality, similar to native bush streams, and much higher than rural and urban streams. Forestry streams in mature exotic forest have high levels of dissolved oxygen, cool water temperatures , low levels of nitrogen and phosphorus.
However, after the forest has been harvested the water quality can be severely degraded.

Typical invertebrate ecology of a forestry stream

Forestry streams in mature, undisturbed exotic forest can have similar numbers and varieties of invertebrates (taxa), insect, , amphibian, fish, and bird life to those in native bush streams.

However, if after harvesting the water quality has been seriously degraded, only the more tolerant taxa can survive. This means the types of invertebrates change and become more similar to those found in rural streams.

As the environment starts to recover and the quality of the forestry stream habitat improves, the more sensitive taxa such as the mayflies, stoneflies, and dobsonflies start to return. This recovery process can take up to seven years in the smaller forestry streams

What is being monitored?

The Freshwater Ecology Programme is monitoring the biological health and the physical condition of some streams in the Auckland region.

The Rivers & Streams Water Quality Programme is monitoring the water quality.

When the results from both programmes are combined, they give a good indication of the overall health of a stream and its physical, chemical, and biological condition.

Technical Publications

The ARC has a number of technical publications relating to this subject. Follow this link to see the full list of ARC technical publications.

Continue to Native bush streams