Fig. 1: Damage to the soil such as this should be avoided at all costs (Pictures: LWF).
Harvesters und forwarders are an indispensable element of modern forestry in both private, state and corporate forests. However, many private forest landowners have reservations. For many it seems that the conscientious and careful handling of the forest, especially the forest soils (which is the basis for successful forestry), is not achievable with mechanized forestry.
Soil protection as a business objective
If ground based forest harvest operations are not carried out properly then changes in soil structure can occur from the machinery. The most essential aspect with regard to modern mechanized forestry is a well contemplated and permanent forest road and logging trail infrastructure. If such a system exists, and the machines only drive on the logging trails, than more than 80% of the forest soils will remain unaltered. As a result, the preservation of the natural soil condition will be maximized on a large portion of the forest area. Additionally, damage of the logging trails is to be kept at a minimum. The prevention of damage to the soil in the first place is clearly more preferable than the restoration of the soil after damage has occurred. At least from a forest technical standpoint, the ability to drive over the soils should be maintained. The preservation of the natural soil condition on the logging trails must be the main goal if it is desired that trees adjacent to logging trails should be able to take full advantage of important soil functions in the future (i.e. rooting ability, aeration, water storage capacity).
Fig. 2: The entire weight is diversely distributed over the various axles.
Fundamentals of Soils
Forest soils are comprised of solid particles (gravel, sand, silt and clay) as well as pores that contain air or soil water. The pores are a variable feature of soils and can be compartmentalized into three classes in a natural, undisturbed forest soil. On average a third of the soil is composed of macropores (drainage and aeration), mesopores (water storage), and micropores (nutrient storage). If this aggregate is destroyed, then the volume of the pores is often insufficient. The pore size distribution ceases to be balanced and the pores are no longer connected with one another. Thus, important soil functions are permanently lost.
Fundamentals of the machinery
Damage to the soil during harvesting is usually the result of the forwarder and not the harvester. This arises since the forwarder is disproportionately heavier than the harvester due to the heavy load of stems on bed during harvest operations. This weight is distributed onto the axles and wheels. The weight of the load is highest under the loading area and wheels and lowest under the drivers cabin. (Fig. 2). The number of axles and wheels is important, but the tire contact point also plays a decisive role. With an increase in the size of the contact point there is a decrease in the average pressure on the soil. This pressure distributes itself in a so called "pressure onion" (Fig. 3). The behavior of the "pressure onion" is the reason why there is an increase in soil damage through wheel slip on steep slopes (Fig. 4).
Fig. 3: The distribution of pressure in the soil resembles an onion.
Fig. 4: Wheel slip can arise on steep slopes as a result of the pressure onion.
Soil protection – the source and impacts
Fig. 5: An elastic deformation as shown here is absolutely problem free.
Fig. 6: The first damage to the soil arises with plastic deformation.
Fig. 7: Deep ruts with bulges on their edges characterize viscoplastic deformation.
If the contact pressure on the soil from the machine and the load bearing capacity of the soil is in equilibrium then the ground can be traversed without a problem. When the contact pressure on the soil is higher than the load bearing capacity of the soil then damage occurs (Fig. 8). The risk of soil damage increases with a heavy payload, high tire pressure and a slim tire width. The load bearing capacity of the soil is at its lowest on soils with a fine soil type (i.e. high silt content) and high water content as well as on steep slopes.
Depending on the given conditions at hand, there are three possible outcomes for the soil after it has been traversed with forest machines:
- Elastic deformation: under ideal circumstances the wheels of the machines will only elastically deform the soil. Occasionally indentations from tire tread can be seen and the original soil condition is mostly left intact. (Fig. 5).
- Plastic deformation: If the contact pressure on the soil is higher than the load bearing capacity then a plastic deformation occurs. Ruts from the tires can clearly be seen and are deeper than the surface of the adjacent forest soil. This compaction results in a reduction of pore volume, especially macropores (Fig. 6).
- Viscoplastic deformation: Under very poor conditions when the pressure on the soil is higher than the load bearing capacity a viscoplastic deformation occurs (i.e. soil displacement). Here the rut isn’t just deep and easy to see, bulges on the edges of the rut have also been created. Through this so-called "shear failure" soil compaction isn’t so significant, however a reorientation of the pore volume takes place. The pore network is destroyed and negatively influences the hydrological conductivity. This damage is long lasting and negatively impairs regeneration even decades after the damage has occurred. (Fig. 7).
Decisive for the load bearing capacity is, among other things, the water content of the soil. The water content can be determined roughly with this simple test: soils with low water content cannot be formed into shapes in the hand and crumbles. This is the ideal condition for the usage of machinery. If the soil can be formed into a sausage then the "plastic limit" has been exceeded. The water content is high and a plastic deformation can arise. If the soil cannot be rolled and is slimy, then the "liquid limit" has been reached and the water content is very high. Viscoplastic deformation will most likely occur.
| Deformation | Characteristic | Duration | Soil function | Assessment |
| elastic | slightly visible, only indentations from tire tread | Short term, reversible | intact | no risk |
| plastic | rut is easily to see, depression not so deep | long term | limited | moderate risk |
| viskoplastic | rut of considerable depth, bulges on the sides of the rut due to soil displacement. | permanent | non existent | irreversible soil damage |
Possibilities for soil protection – machine technology
Wide tires, low tire pressure and a low payload are gentle on the soil. All three adaptations help to reduce the contact area pressure, however, decreasing the weight of the payload produces the best results.
In order to reduce the load on the soil, one can decrease the weight of the machine or increase the number of tires. With respect to soil compaction, lightweight machines with eight wheels are preferred. Also the reduction of tire pressure results in a considerable improvement since the contact point of the tires with the soil increases with lower tire pressure. For example, with a decrease of 2.0 Bar per tire the area of contact can increase up to seventy percent. Wider tires also have a positive effect since the area with the highest load (directly under the wheel hub), is distributed over a larger area. However, the contact area increases only ten percent with a thirty percent increase in tire width.
Many alternatives are offered in order to increase the contact point of the machines and reduce the pressure. Possible technical aids available are bogie tracks or track harvesters/forwarders themselves.
Possibilities for soil protection – planning and organization
Fig. 8: Damage to the soil can occur if ground pressure and load bearing capacity are not in balance. Good planning and organization of operations helps to avoid the usage of machines at such times.
On top of the implementation of soil protection measures, it is imperative to safeguard the natural soil functions on the long term through forest operations that have been well planed and organized (e.g. utilization of good weather, arrangement of back up stands/areas to harvest etc.). The following points are to be considered:
- Define soil protection standards and require their adherence (e.g. through controls, incentives, sanctions)
- The forest contractor (or specialist) must constantly be provided with work
- Appropriate payment of the forest contractor (soil protection costs money!)
- Flexible timber delivery contracts (without obligatory target dates)
- Utilization of favorable weather, flexible harvest operations
- Arrangement of back up stands to harvest in case of inclement weather
- Transfer the time of harvests on difficult sites to late summer or fall
Surely many of these points are difficult to implement in the small structures of private forests as compared to larger private, corporate or public forests. Nevertheless, this should not lead to a decrease in the quality of mechanized harvesting or an increase in the risk for soil damage.
