John Grimmett
Forest Development Group, Gympie
Preamble
DPI Forestry currently manages 175,000 hectares of Hoop Pine (Araucaria cunninghamii) and Exotic pine (Pinus sp.) plantations situated principally in S.E. Queensland and within 200 kilometres of the coast.
Hoop Pine plantings comprise about 45,000 hectares and are generally situated on the fertile ex-scrub soils of the coastal ranges. Although soils are relatively stable, logging and certain re-forestation activities on some of the steeper slopes (up to 35 �can lead to unacceptable erosion during early summer storms and before the site has been properly stabilised.
Our major plantings of exotic pines comprising about 130,000 hectares, are situated on the depauperate 'Wallum' soils of the coastal strip. These soils are generally poorly structured and can be highly erosive despite the relatively gentle topography on which they occur.
Forestry Trials
Following a field day conducted by Paul Truong at Beerwah in October 1993 we decided to trial Vetiver grass (Vetiveria zizanioides) as a erosion control agent in two of our major plantation types in S.E. Queensland.
Three sacks of Vetiver vegetative material were obtained from the Ayr Research Station in October 1993. The large clumps were broken up into two to three leafed slips and planted into Vic Pots for growing-on prior to planting-approximately 500 plants resulted.
Material that was considered unsuitable for growing-on was placed in a bag and thrown in a corner of the nursery.
Much of this material, when inspected several weeks later was found to have initiated root and shoot growth, so we had obviously underestimated the extremely robust nature of Vetiver.
The Exotic Pine Trial
In November 1993, half the plants were deployed in an unstable waterway within a recently mounded second rotation exotic pine site at Toolara, east of Gympie. The waterway had been constructed with excessive fall and was draining too much area.
Active erosion was present, and previous attempts at stabilisation using cereal crops and couch grass had proved ineffective.
Approximately nine single and/or double rows, each five metres wide, were spaced fifteen metres apart along a section of the waterway. Plants in each row were planted at 20 cm centers into virtual clay subsoil forming the bottom of the waterway. Because the wet season was already in progress, there was little time available for the plants to establish themselves on the site before their presence was required. Consequently, several breakthroughs and washaways occurred due to excessive water velocity and volume. However, where the barriers did hold, a substantial volume of sand was trapped and built up against them.
Obviously, with more extensive and better design and deployment of planted barriers, stabilisation could have been achieved in that first year - at 20 cm spacing and growing in the clay, it took a full season for the plants to develop sufficiently to form an effective barrier.
The Hoop Pine Trial
Also in November 1993 the remaining Vetiver plants were established in an eroding unprotected gully in Hoop Pine plantation at Imbil, south west of Gympie. The site was steep (>200) and had recently been broadcast burnt, strip sown with kikuyu and millet, and replanted following harvest.
Again plants were spaced about 20 centimetres apart. Rows of Vetiver were planted for each metre of fall, which equated to approximately a row every three to four metres of gully length. A total of 18 rows were established from the point source of the erosion down to the intersecting gully at the bottom of the slope.
Growth was rapid. However, as before, because of the spacing between plants, there was a time lag between planting and the beginning of effective control of soil movement by the hedges. This time , because of the more fertile site, the period was about six months or half that at Toolara.
Weed and sown cereal growth was prolific at the time of Vetiver establishment, to the extent that all vegetation was contributing to erosion control, and competition appeared to be leading to a degree of suppression of vetiver development. Never-the-less, once the Vetiver grass gained control of the site, rapid and permanent stability of the gully was achieved.
Because most erosion occurs within the first three months following site preparation, when full site stability may not have yet occurred, a closer plant spacing would be more effective in achieving an early influence on the site. (eg <10 cm instead of 20 cm.). A subsequent small planting of Vetiver grass at Toolara utilised a virtual shoulder-to-shoulder placement of slips, and this resulted in a more immediate effect.
General Observations
1. Vetiver grass does not seem to tolerate long periods of soil saturation as can frequently occur in coastal Wallum sites.
2. Vetiver grass appears to be sensitive to shading competition so could be expected to die out in the plantation situation after canopy closure has occurred and before the next rotation.
3. For early erosion control, plants need to be spaced more closely than in the above trials.
4. Future plantings will need to dispense with the intermediate nursery potting step so that slips are established directly into the field.
5. Fertiliser application at the time of planting in order to achieve earlier establishment would appear to be beneficial.
The Future
We feel that Vetiver grass has a positive role to play in stabilising point sources of erosion such as gully heads road turnout drains and unstable spots aggravated by harvesting and establishment machinery in both our hoop and exotic pine plantations.
Planning is in hand to establish our own resource of Vetiver grass at the exotic pine
nursery complex at Toolara so that a prompt and comprehensive response to any future
erosion problem can be activated.