TREAT Info-Notes | Replanting the Rainforest No. 5

Planting in a Riparian Location

Many of the plantings undertaken privately by TREAT members on their own property do not involve replanting streambanks. By contrast, most of the projects undertaken by TREAT as a group involve replanting creek or river bank areas for example, Massey Creek Project; Peterson Creek Project; Mazlin Creek Project; Upper Johnston River Project; Barron River Project. Replanting a streambank or riparian location often needs special attention to different details not present in non-riparian locations.

Riparian Environments

A riparian environment is one that involves water, either a creek, river or lake bank, or a swampy area. All these environments are subject to flooding, either for relatively short periods as on streambanks, or often for quite extended periods on lake banks and in swampy areas. These matters must be considered when choosing the species to replant. Streambanks have special problems associated with erosion. Here, the stability of the bank must be considered before any replanting can take place.

Streambank Erosion

It is important to solve any problems associated with stability of the streambank before starting any project to replant vegetation. Many factors affect the stability of the streambank. Some can be changed easily (eg. vegetation), or changed with some difficulty (eg. height and slope of the bank; channel width; stream velocity), while others can not be changed (eg. bank material or soil type).

There are two main types of bank erosion: (1) scouring where the bank material is removed particle by particle and the amount of sediment in the stream is increased to cause problems lower downstream; and (2) bank collapse where the bank fails and topples forward into the stream suddenly. Although collapse is the more dramatic and obvious form of bank erosion, scour is the more destructive. If collapsed material is not removed by scour, the bank will often remain stable and can be revegetated with success. It is scour that primes the bank for the next collapse.

The height and steepness of the bank may need to be modified to prevent further collapse. Also, revegetation can not take place on near vertical banks. In these instances, appropriate engineering work may be needed to batter back the banks to reduce their steepness. This process has the added advantage of widening the channel of the flooded stream, thereby reducing stream velocity and lowering the risks of erosion during periods of flooding.

Streambank erosion is strongly influenced by the density and type of riparian vegetation. Revegetating unstable streambanks and maintaining a healthy riparian vegetation is a relatively cheap way of providing a solution to streambank erosion and stability in the longer term.

Using Vegetation to Stabilise Streambanks

Vegetation improves bank stability by reducing scouring, by reinforcing the bank material and buttressing the bank above it, and by using excess water in the bank.

Vegetation growing on the bank or in the stream can reduce scouring by lowering the velocity of the stream, especially close to the toe of the bank. In this instance, grasses and reeds are better than widely spaced trees. However, if the grasses or reeds become too dense within the channel they may direct flow into the banks and increase scour. Also, channels choked by vegetation can lead to lower channel capacity and an increase in the frequency of nuisance flooding The roots of streambank vegetation reinforce soil in much the same way that steel rods are used to reinforce concrete. Root reinforcement of the bank material is usually the most important safeguard against bank collapse. Further, by stabilising the lower bank, the bank material above it is buttressed or supported against collapse. In this process, fine roots are better than thick roots. Trees that form a mat of fine roots within the surface layers of the soil and grasses are better at reinforcing the bank material.

Riparian vegetation can use excess water within the bank material. As most banks collapse when they are saturated with water, riparian vegetation, by using up the excess water and by reinforcing the bank material with their roots, can help stabilise the bank and reduce the risks of sudden collapse.

Replanting a Riparian Location

Plan before you start

In many respects, planning for replanting a riparian location proceeds much like that for a normal location (see Info-Note No. 2: Preparing the groundwork). You will need to prepare a site plan and consider the stability of the streambank and undertake any appropriate engineering works. Also, consult with other landholders, especially those upstream to ensure that there will be no long-term change in use of the stream system that may affect the stability of the streambanks in your area.

If stock are present, erect permanent exclusion fences. Limit stock watering points; these should be gravelled to reduce erosion and provide safe access for stock. Control feral animals that may cause damage.

Site preparation

Kill all unwanted vegetation using a suitable weedicide (eg. glyphosate) or by manual means, leaving the dead material on site as a mulch (see Info-Note No. 2 for more detail). Their roots continue to bind the soil for a short time even after the plants themselves have been killed. Avoid spraying weedicides into the watercourse. Retain the useful native vegetation (some pioneer species may already be present).

Planting

Planting-out is done similarly to that at a normal site (see Info-Note No. 3: Planting-out). Planting should take place as soon as the sprayed weeds are yellowing and collapsing. When manual weed control was used, planting should take place immediately. This takes advantage of the potential life of the mulch and the residual binding effects of the sprayed weeds' root systems. There are three distinct zones on a streambank (waters-edge, mid-bank, and top-bank), and two on a lake or swampy area (waters-edge or swamp, and top-bank). Water-edge species like lots of water, can tolerate flooding or swampy areas, and usually provide greater stability to the lake- or streambank. Mid-bank species can tolerate seasonal flooding. Seek advice on species suitable for these locations in your area (eg. uplands, foothills or coastal plains). All plantings should be on 1.5m spacings. This is to encourage quick canopy closure to intercept and reduce the size and velocity of individual rain droplets.

Site Maintenance

This is the most time consuming but important part of any tree planting project (see Info-Note No. 4: Maintenance & monitoring). Weeds must be controlled by mulching, hand weeding or selective weedicides. The fertiliser applied in the bottom of the planting hole will be sufficient for about 1-2 months. Additional fertiliser will be needed every 4 weeks during the main growth season for the first 2 years after planting. Recolonisation by ephemerals is an advantage because of the additional soil coverage and binding the new plants provide. However grasses, especially Guinea Grass (Panicum maximum), must be excluded because it will compete aggressively with the establishing trees. Flood damage caused by debris collecting on the young trees can be a problem for the first 2 to 3 years after planting. After the flood recedes, remove the debris and use it as mulch.


More Info-Notes

  1. Planning with a Purpose
  2. Preparing the Groundwork
  3. Planting Out
  4. Maintenance and Monitoring

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