Practices which counteract loss of soil strength
Pastoral Hill Country
There are several ways to stabilise pastoral hill country where forest clearance has weakened soil, increasing the risk of slips, earthflows and slumps. Affected areas range from the easy downlands formed on sedimentary rocks in northern districts, to the rugged hills along eastern coastlines of both Islands. The techniques never completely eliminate mass movement but they substantially reduce its occurrence, and its impacts on farming, during storms and wet winters.
Spaced tree planting (Figure 8)
| Spaced tree planting - close enough for roots to interlock across slope |
The main preventive measure, on easier slopes that are valuable grazing country, is spaced planting of broadleaved exotic trees such as poplars, willows, gums and wattles. Their lateral roots interlock for distances of up to 12 metres from the trunk, and form very dense nets for about 5-6 metres out. As long as trees are no more than 12 metres apart, unstable subsoil is bound by a mat of roots (the same principle as reinforcing mesh holding concrete together). The root mat even anchors weathered rock beneath, in much the same manner as a reinforced concrete coating can be used to restrain loose rock. Trees also reduce frequency of saturation in the soil, by "pumping" water out through their roots and leaves, Even deciduous trees can appreciably delay onset of waterlogging in winter, because they have pumped so much water out of soil in autumn that it takes a long time to build up again. The traditionally used species - poplars and willows - establish best on moist sites, such as earthflows on footslopes. In wet climates they can also be used to stabilise slip-prone upper faces, but in hill country districts prone to summer drought, better results are sometimes achieved with gums and wattles, Spaced plantings of broadleaved trees18:
- reduce mass movement in pasture by 50 to 80%, where mature trees are maintained at spacings of 12 metres or closer;
- reduce mass movement by 10 to 20% where unstable slopes have been partly planted, or where older plantings have not been maintained (both situations can be quickly upgraded by blanking gaps).
Individual deciduous broadleaved trees suppress annual pasture production by 20% or less. Collectively, space-planted stands suppress it by about 40%. These reductions are relative to pasture growth on stable ground in the open. Relative to unstable ground, there appears to be no net loss of feed. This is because pasture suppressed by the trees is counterbalanced by residual growth on areas that would have been lost to erosion, had the trees not been planted. So the main impacts of spaced (18) are not on pasture growth but on other aspects of farm management:
- More efficient feed utilisation, through ability to graze stock on formerly unstable areas. Increases of about 7 stock units a hectare in annual stock carrying capacity have been recorded on space-planted ground in the East Coast and Wairarapa.
- Cost savings through reduced expenditure on fence and track repair. For instance, reductions of 52% in damage to fences and 35% to tracks were measured among space plantings in an East Coast catchment after Cyclone Bola. On Rangitikei farms during the 1992 winter, spaced plantings reduced costs of damage repair from $76 to $28 a hectare.
Figure 9: Earthflow-prone terrain, stabilised and grazed throughout year, beneath space-planted poplars and willows. This slope was previously ungrazeable in winter. East Coast. Photo: D Miller
Close Planting (Figure 10)
| Close planting - a continuous tree canopy as well as a continuous root mass on very unstable slopes |
On very steep faces, close planting with pines or other commercial timber species is an alternative stabilisation technique. Even when stable, these areas have only half to two thirds as much annual pasture growth as foothills. If repeatedly eroded, pasture growth drops to a third or less:
Close planting (19) reduces mass movement by 90% or more, relative to the levels that can be expected if steep, erodible faces are left in pasture.
Figure 10: Steep erodible faces, stabilised by close-planting with pines and other conifers. Gentler, less erodible slopes have been retained in pasture. Wairarapa.
- This level of reduction is obtained from young trees after canopy closure, at densities greater than 1000 stems per hectare.
- It is maintained by mature trees at densities as low as 300 stems per hectare.
If 20% of a farm is steep, eroded land growing pasture at a third of the expected level, its retirement constitutes a 7% drop in total pasture growth. The amount can usually be compensated by better feed utilisation elsewhere on the property. Costs of planting can be spread over a number of years, to minimise impact on the farm budget, and ensure an eventual income stream from timber harvest. The impact? of close planting on farm operations are :
- minimal loss of overall stock carrying capacity, so long as retired slopes are less than about 20% of farm area;
- fence and track repair savings around the margins of retirement blocks, proportionate to the reductions in mass movement;
- supplementary income from timber, if commercial species have been used.
A disadvantage of close planting is that economic returns take 30 years to eventuate in the case of pines or other conifers, and presently there is little commercial demand for fast-growing broadleaved species. However, some poplar, eucalypt and acacia species are made into joinery and furniture overseas, and a few New Zealand manufacturers are starting to use high-quality 20-30 year old logs where available. A small market (domestic and export) is developing for eucalypt and poplar pulp, for manufacture of paper and fibreboard from small-diameter eucalypt and poplar logs grown on a 10-20 year rotation. Close to towns, income can also be obtained within 5-10 years by sale of thinnings as firewood, Management of stands solely for firewood production, by coppicing, is also an option with eucalypts and acacias.
Reversion (Figure 11)
| Reversion - exclude stock and control pests, to encourage regeneration of young trees |
If very steep faces are still in bush, or reverting to scrub, leaving them that way will minimise slips and flows. The amount of extra grazing obtained by clearing them is so small (see comments under the heading "Close planting", about pasture production from steep faces) that it probably will not repay the expense of pasture, establishment. If it is essential to bring a steep face into commercial use, afforestation with pines will provide a better return, but at a higher cost in terms of site clearance and silviculture. Provided farmers wish to retain areas of native cover, there is probably a better case for harvesting individual high-value logs of native timber and "enriching" the bush remnants by replanting native timber seedlings. Maintaining an erodible face in native cover20:
- keeps mass movement down to 10% or less of the level that could be expected if it were converted to pasture.
The impacts (20) on farm operations are :
- a proportionate reduction in damage repair costs to fences and tracks round the margins;
- supplementary income, if logs can be individually harvested without clearing the stand.
A sustained return from native forest is likely only if the stand is managed. At least a few native species (beech, totara, rimu, kauri) are capable of producing small but merchantable sawlogs after 50 to 100 years’ growth, if select seedlings are planted, or if natural regrowth is selectively thinned. Given typical stand densities of 50 to 300 stems a hectare (depending on species), these growth rates will be enough to sustain a 10 hectare native stand’s potential timber yield, provided existing natural-growth trees are removed at a rate no greater than 5 to 30 each year. Before logging any trees for commercial gain, landowners would now have to prepare a sustainable forest management plan, and obtain the Ministry of Forestry’s approval for it (in terms of the Forests Amendment Act 1993).
Figure 11: Slope prone to gullies and slips, stable where reverting
scrub has been retained (left), but eroding under pasture (right). East Coast.
Photo: D Miller
Remedial techniques
Three techniques are occasionally used to repair mass movement scars on hill country. However unlike the farm management practices previously described, they are restorative, not preventive.
Oversowing and topdressing help restore production on freshly eroded slip scars. These techniques will generally be impractical where many slips are scattered through a farm, but become economic to use on any residual areas that open up among space-planted trees. Oversowing and topdressing can raise pasture growth on slips from less than 20% up to more than 50% of the level on adjacent stable ground within 1 to 2 years, provided stock are excluded during this time. The technique is remedial, and does not protect against future slipping.
Cultivation and drainage are mainly applied to earthflows (which usually occur on low-angle slopes accessible by machinery). In some places, they are also used where slip debris has accumulated on footslopes. Surface cultivation and subsurface drainage, if followed by oversowing and topdressing, raise pasture growth from less than 60% up to more than 90% of the level on adjacent stable ground, within 1 to 2 years. This technique is also remedial, and will not prevent future flow movement or debris accumulation.
Engineering measures are used to stabilise large, deep-seated slumps in bedrock. Examples are surface cut-off drains at the head, sub-surface drains and horizontal bores to drop water pressure in the subsoil and rock, and gabions at the toe. These expensive techniques are rarely used except where slumps threaten farm buildings or roads. In individual instances where they have been implemented, apart from saving assets they have also enabled increases in carrying capacity of about 7 stock units a hectare on the stabilised ground. The technique, although initially remedial, can also be preventive providing drains are regularly maintained.
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