Timothy Potter Forestry Student Southern Cross University
September 2001

Introduction

Australia's forests and woodlands have evolved over the past 60 million years. Following the breakup of the ancient land mass, Gondwana, Australia became hotter and drier and the once extensive areas of rainforest were displaced by hard-leafed vegetation adapted to withstand drier conditions. Poor soils, drought and fire have all played a role in the evolution of Australia's distinctive and highly endemic vegetation. The incidence and spread of fire increased, initially with the onset of drier conditions and later with the arrival of humans over 40 000 years ago. Fire played a dominant role in the evolution of an enormous diversity of species within the genus Eucalyptus that now dominates our native forest (Attiwell 1994).

Australia has 155 million hectares of native forests, including 43.8 million hectares of closed forest and open forest. Some 22.3 million hectares of closed and open forest is either privately owned or lease hold while the balance is multiple use forest (11.0 million hectares), conservation reserves (8.4 million hectares) or other categories of public ownership (2.1 million hectares) (BRS 1998). Australia's 158,570 hectares of hardwood plantation is almost all eucalypt species (National Plantation Inventory 1997). The rate of hardwood planting increased from about 2500 hectares a year in the late 1980's to over 10000 hectares a year in the early 1990's and further increases are likely.

Australia has 2.3 million hectares of rainforest, 5.6% of the total native forest estate (ABARE 1995). Rainforests in Australia are mainly found in Northern Queensland (tropical), and Western Tasmania (temperate), but there are also pockets of rainforest along the Great Dividing Range. Rainforest cabinet timber plantations are present but are, as yet, not commercially viable on a national level. There are however 45300 ha of hoop pine plantations in NSW and Queensland which are logged on a continual basis.

Australia's first commercial pine plantations were established about 1880. The rate of planting increased rapidly during the 1960's and the total area of softwood plantation (predominantly pine plantations) is 883,840 hectares (National Plantation Inventory 1997). Radiata pine is the dominant species planted in southern Australia and slash and caribean pine are the dominant species planted in Queensland. In 1997 the Federal, State and Territory Governments joined with industry and launched The Plantations for Australia: 2020 Vision which aims to treble Australia's hardwood and softwood plantation forest estate from about 1 million hectares to 3 million hectares by 2020. Some $3 billion would need to be invested in new plantations, mainly by the private sector, to achieve the vision. Soil erosion and land degradation as a whole has long term implications for the sustainability of the industry

On a world scale if less than 10% of forest area was committed to forestry plantations then the forestry operations associated with these plantations could supply 90% of the worlds timber consumption (Nichols 2000).

Soils and Their Susceptibility to Erosion

Soils are formed by the combined influence of climate, topography and living organisms working on rock, whether it be of a volcanic or sedimentary nature. This process happens over a great deal of time and depending on the type of parent material available an almost infinite variety of soils may be formed by different combinations of these factors. These different soil types have both different structures and chemical makeup. The structure of the soil has much to do with its inherent erodibility level, well-structured soils (soil with small peds and moderate cohesion) provide easy root penetration, good drainage and resistance to erosion. The chemical makeup of the soil determines the available inorganic nutrient in the soil and affects the pH of the soil.

The percentage of stone or gravel in the soil also effects the way soil behaves with regard to moisture availability (for plantation growth), rooting conditions and soil erodibility. Soil with a moderate amount of gravel within it would tend to be more permeable and therefore less prone to erosion.

Fine sandy soils or silty soils with poor structural development can be particularly prone to compaction and may form pans or cemented horizons. These compacted horizons are not conclusive to good plantation growth and can lead to erosion by not allowing water to penetrate through the cemented horizon intensifying water flow in areas where otherwise the flow would not have been so great. This panning can be produced by inappropriate use of machinery or it can occur in land used for either present or past intensive agricultural practises (Grant 1999).

The differential weathering of soils with the addition of organic materials results in the development of layers in the soil profile. These layers, or horizons, have different properties to the horizons above or below them. The surface of the soil (the O horizon) is made from almost totally organic material and may be covered by a layer of mulch (leaf and twig litter). This mulch aids in erosion control by disallowing rain and wind to act directly on the soil's surface. The next horizon, being the A horizon (top soil), is derived from the parent material but has been darkened by organic matter and tends to be rich in nutrients. The B horizon is next and it is comprised of higher levels of clay than the A horizon and is of a lighter colour. The C horizon, that underlies the B horizon, is comprised of mainly of weathered or partially weathered soil parent material lacking pedological organisation (Grant 1999). Any of these horizons can be further sub-divided to indicate colour changes, leaching or changes in structure. If the A horizon is lost due to erosion it is possible, even likely, that it will not be replaced, by natural means, in our lifetime.

Land Degradation Potential

Pine plantations can be shown to not cause excessive damage to the soil when compared to other agricultural practices. Studies show that native forests and pasture can grow well on land that has grown pine trees. Because pine plantations usually only use herbicides and fertilisers during initial establishment they use less chemicals than most other types of farming, and are less likely to pollute the air, soil or water. This is typical with most plantation forestry operations as after the initial impact of clearing and planting operations the use of fertiliser, herbicides, manual pruning and periodical thinning are the only major management practises entered into for the duration of the plantations growth. However the use of heavy machinery during these procedures tends to compound the effect of the land degradation that is caused during these operations.

The initial clearing and planting of the plantation has been found to cause some significant land degradational problems associated with erosion, sedimentation, compaction and nutrient loss. These areas of cleared land are often subject to increased erosion due to direct action from rain and wind. This form of erosion, termed splash erosion (in the case of rain), is the process by which soil particles are detached by raindrops on the soil surface. The eroded particles are then subject to further transport downhill by overland flow (sheet erosion) (Bruijnzeel 1997). This will occur until such a time as ground covers or a mulch layer develops on the soil surface and/or the root systems and canopy of the plantation are developed enough to hold the soil together and to reduce the action of rain on the soil. Erosion caused by wind is similar in that bare soil or soil that is not well structured or held together by root action is susceptible to wind based erosion. Once a forest plantation is well established wind will have little affect on soil erosion unless windthrow (trees blown over by strong wind) comes into play.

The harvesting operation, either thinning or final harvest, can been shown to be the cause for most of the possible land degradation or erosion potential of the land used for plantation forestry operations. This is due to the intrusive nature of timber harvesting procedures, ie. the use of heavy machinery, the snigging of logs, the creation of roads , creek crossings and log dumps all add up to a procedure that has a high potential for land degradation, in particular, erosion. Thinning operations have been shown to be less dramatic than the full harvest operation at the end of the plantations growth cycle.

Soil disturbance from plantation operations can lead to erosion in differing ways. Firstly compaction of soil by heavy machinery leading to an increase of soil bulk density and a resultant potential for an increase in water runoff as opposed to infiltration of the water into the soil. This increased runoff can cause the greater possibility of rutting and the formation of gullies and gully type erosion. The increased runoff can add to sedimentation of streams and watercourses and therefor loss of the A horizon. The mixing of the upper and lower soil horizons, also caused by the use of heavy machinery, can lead to a reduction in nutrient availability and therefore a loss in future productivity. This is particularly noticeable in Australian soils where the prevalence of shallow topsoils makes them prone to damage due to soil mixing. Land slips, slumps and earthflows can be a problem after clearing land in areas of hummocky terrain, wet soils (blue/grey, shrink/swell subsoils), springs or areas where previous growth had trees with a common down slope lean. Steeper slopes are more prone to landslip but slumps and earthflows can occur on very shallow slopes(Grant 1999).

In all cases where erosion may be found to be a problem the major underlying factors adjudicating over the levels of erosion experienced are:

· The degree of slope where the erosion is occurring. The greater the slope the greater the potential for erosion.
· The amount of rainfall in the area is also a major contributing factor to the erosion potential for an area.
· The soil erodibility of the area that may experience erosion due to plantation forestry operations.

Other forms of land degradation that can be found to be associated with plantation forestry are:

· The exposure of soils with the potential to become acid-sulphate soils (Grant 1999).
· Decreases in nitrogen, sulfur exchangeable calcium and depletion in effective CEC (Merino A. Edeso JM Gonzalez MJ. Marauri P 1998).
· Potential for toxic levels of Mn occurring in pine plantations (Merino A. & Edeso JM. 1999).
· Changes in groundwater flow leading to karst erosion (below ground erosion) which could lead to loss of topsoil or the creation of sinkholes (Grant 1999).

Methods of Controlling Land Degradation in Australian Plantations

Clearing operations

In many cases the clearing of an area for the establishment of a timber plantation will involve the clearing of native or exotic vegetation that may be advanced in growth. This procedure must be undertaken while conforming to local and State government legislation within the bounds of the Plantations and Reafforestation Act 1999. The planning guidelines for private hardwood plantations within the NSW Northern Rivers Region suggests that all clearing activities must be undertaken in a manner that will minimise site disturbance and erosion/sedimentation potential (Fletcher 2000).

The methods adopted to arrest erosion potential in clearing operations are as follows (Fletcher 2000):

· Clearing should not be carried out during wet conditions or when the ground is saturated.
· Sedimentation control structures such as swales, catchment drains, jute mesh drop down structures settlement ponds etc should be in place prior to any clearing operation.
· Removal of felled timber should be undertaken in a manner so as to avoid downslope snigging and the crossing of drainage depression lines.
· Stockpiles/windrows of debris are to be situated sufficiently away from drainage depression lines and vegetational buffers so as to not impact upon retained vegetation or increase sedimentation within the drainage depression.
· Stockpiles/windrows of debris are to be situated cross slope to assist with erosion control.
· Clearing is be undertaken over the shortest possible time period and only when weather conditions are favourable.
· Clearing should be timed to coincide with periods of high growth of pasture species to assist in site stabilisation.
· Dust caused be clearing operations should be avoided during windy conditions.
· Burning of debris should be undertaken with the appropriate licensing agreements.
· No vegetation (including grasses) is to be removed from filter strips, drainage depression lines, buffer zones or vegetation retention areas.

Site Preparation

Plantation forestry planting operations undertaken in an appropriate manner can reduce the risk of soil loss or adverse water quality effects during these operations. As for clearing operations legislation is in place restricting planting operations to procedures that are less likely to promote land degradation (Protection of the Environment Operations Act 1997). This relates to the times when planting or ripping of tree lines can be undertaken and to the method of operation.

The methods adopted to arrest erosion potential in planting operations are as follows (Fletcher 2000):

· Ripping/cultivation of tree lines in areas of sloping topography should follow the contours of the land (cross cultivation).
· In areas of high erodibility alternatives to ripping should be adopted. For example auger boring or manual digging.
· Surface water from upslope should be directed away from the disturbed area and redirected into the natural drainage system.
· Site preparation is to be avoided in periods of wet weather or when the ground is saturated. Work is to cease if surface water runoff from the site occurs.
· Sedimentation control structures such as swales, catchment drains, jute mesh drop down structures settlement ponds etc should be in place prior to any planting operation.

Harvesting

Harvesting operations are the practises that are most likely to produce land degradation. The removal of trees in, a presumably, stable environment can produce all manner of land degradational problems from erosion to salinity of soil. The use of heavy machinery adds to the problem. Legal constraints apply to harvesting operations in NSW.

Legal Conditions

The harvesting operations must comply with the following regulations and codes of practice :
· Licence Conditions issued by State Forests under the Forestry Act (1916)
· Forest Practices Code (FPC), Timber Harvesting In Native Forests (1999)
· Pollution Control Licence (PCL) number 4017 issued by the Environmental Protection Authority
· NPWS Section 120 licence number TS0006, Threatened Species Conservation Act (1995) and National Parks and Wildlife Act (1974)
· Regulatory and Public Information Committee Determinations (RaPIC).

Thinning

It can be shown that thinning operations have no significant effect on forest hydrology unless > 50% of the stems are harvested at any one time (Bruijnzeel 1997). Thinning of this extent, however, is an unlikely thinning regime. The mechanical operations involved in thinning procedures are likely to have an adverse effect on soil stability. The procedure for alleviating this affect are the same for thinning operations as for final harvesting operations.

Final Harvest

The final harvest of a forest plantation is in effect a twofold problem as it is both a clearing and a harvesting operation. Therefor the guidelines outlined in section 4.1 will apply. The difference being that further prescription with regard to revegetation, log dumps, timber trucks, roads and increased snig tracks will be an issue.

The methods adopted to arrest erosion potential in harvesting operations are as follows (Fletcher 2000):

· Disturbed areas are to regrassed immediately to assist in site stabilisation. Harvesting should coincide with periods of high pasture growth for quick establishment of these grasses
· Any damaged erosion control structures, roads or fire breaks are to be repaired as soon as possible after harvesting
· All roads and snig tracks should be constructed with the appropriate drainage features according to slope angle and length of slope outlined in the Integrated Forestry Operations Approvals of 1 Jan 2000 (IFOA).

Roads and Snig Tracks

Roads in plantation forests are standardly roads with a natural surface as opposed to a gravel or sealed type. This makes the possibility of erosion a real threat. The constants for roads are basically the same as those for snig tracks excepting that drains must be made in such a way as to allow the safe passage of loaded logging trucks. Along with this some roadside clearing may be necessary so as to not obstruct the same logging trucks. Debris from this clearing must not obstruct water through mitre drains. The minimum of disturbance to ground covers on the batters created by this clearing is essential to allow the speedy recovery of these ground covers to arrest any possible erosion problems.

Any and all crossings of mapped or un-mapped drainage features must be consistent with the constraints provided by the PCL and FPC.

Rollover and spoon drains will be required on some sections of road and snig tracks where concentrated water flow is likely. The distances between these drainage features can be seen in table 1.

These drainage features, either spoon or rollover drains leading into mitre drains must be retained after logging operations are finished. Where water drainage structures discharge onto a batter of greater than one metre in height, a jute mesh drop down structure and rock dissipater must be used. Where required drop down structures of jute mesh matting must be placed and secured into dished out smoothed and seeded fill surface. Rock dissipaters must be locked together in such a way so they will not be washed away or shifted by a high water flow. Rehabilitation of minor roads following harvesting operations is started by seeding with grasses (IFOA).

Table 1. Maximum distance of water flow or potential water flow along road or snig track surfaces and table drains before further drainage feature is required (mitre or rollover drain).

Road/Snig Track Grade (degrees) Drainage Feature Spacing (m)
0 - 5, 100m
5 - 10, 60m
10 - 15, 40m
15 - 20, 25m
20 - 25, 20m
> 25, 15m

Alternative Snigging Methods

The use of suspended cables for log extraction (skyline logging), walkover extraction techniques, balloon single log extraction and helicopter extraction can all reduce the impact of harvesting practices on the soil in a plantation area. These practices are not, however always finically viable.

Nutrient Deficiencies

Without fertiliser application intensively managed tree plantations generally have a negative nutrient balance (Mackensen J. Folster H 2000). This situation could be improved by rotational type use of the land used for the plantation ie harvesting in rotational regime. Perhaps by introducing legumous type ground covers on the plantation floor or the inclusion of nitrogen fixing trees within the plantation may also help. It has been hypothesised that in some instances the root systems of trees may draw nutrients from deep soil layers (C horizon) (Nichols 2000). If these plantations are left for a long enough period then the breakdown of organic matter the improved soil structure from root action and the possible transfer of assimilates between root systems may keep soil nutrient levels at a constant.

Benefits of Plantation Forestry

The planting of trees is generally accepted as a good thing for the environment. The benefits of improved soil structure produced by root growth, the filter action and reduction of surface water runnoff, the lowering of the water table and increased organic matter in the soil as a whole must be recognised as a good thing. Other issues such as the increased use of wood products for power generation as apposed to fossil fuels leads to less coal mining and the land degradation issues associated with that industry. The use of salt tolerant species in areas degraded by saline soil could reclaim land rendered basically useless by previous farming and irrigation practises.

Conclusion

There is a large area of land being used for plantation forestry. The use of this land does create some land degradation problems. The use of land for plantation forestry rather than intensive agriculture is sure to have an eventual positive effect on the land within the plantation. With appropriate consideration to environmental land degradation issues and adherence to the guidelines and legislation in place to reduce these problems the benefits of using plantation forestry to produce wood and non-wood products will show itself as a constructive step toward the reduction of degraded land in Australia and the world as a whole.

References

ABARE 1998, Australian Forest Products Statistics, June Quarter, Canberra.

Attiwill, P. 1994, 'Ecological disturbances and the conservative management of eucalypt forest in Australia's, Forest Ecology and Management, vol. 63, pp 301-34.

BRS 1998, Bureau of Resource Sciences at www.brs.gov.au/nfi/forestinfo/
Brunijnzeel L. A. (Sampurno), 1997, Hydrology of Forest Plantations in the Tropics. (In) Sadanandan N. & Brown A. G., 1997, Management of Soil and Water in Tropical Plantation Forests, CSIRO Canbera

Fletcher S. 2000, Planning Guidelines for Private Hardwood Plantations within the NSW Northern Rivers Region, School of Resource Science and Management Southern Cross University.

Grant J. 1999, State Forests Hardwood Plantations Technical Manual Part 1 Basic Soil Management, State Forests NSW

Integrated Forestry Operations Approvals of 1 Jan 2000

Mackensen J. Folster H. 2000, Cost-analysis for a sustainable nutrient management of fast growing-tree plantations in East-Kalimantan, Indonesia Forest Ecology & Management. 131(1-3):239-253, 2000 Jun 1.

Merino A. Edeso JM. Gonzalez MJ. Marauri P. 1998, Soil Properties In A Hilly Area Following Different Harvesting Management Practices Forest Ecology & Management. 103(2-3):235-246, 1998 Apr 20.

Merino A. Edeso JM.1999, Soil fertility rehabilitation in young Pinus radiata D. Don. plantations from northern Spain after intensive site preparation Forest Ecology & Management. 116(1-3):83-91, 1999 Apr 12.

National Plantation Inventory 1997, National Forest Inventory and Bureau of Resource Sciences, Canberra.

Nichols D. 2000 Land Degradation and Rehabilitation lecture Notes, School of Resource Science and Management Southern Cross University.
State Forests Of New South Wales Northern Rivers Region, Harvesting Plan

Back to the Home Page