Ground-saving work in timber harvesting

Supplying Mercer mills with wood as a raw material requires highly mechanized timber harvesting. At Mercer Holz, 20 forestry machine operators currently work on the latest generation of seven harvesters and six forwarders. They harvest around 200,000 cubic meters of wood annually, moving it to the forest roads for later transportation. These machines are particularly suited for harvesting in the pine and spruce forests where the softwood for the Mercer Germany’s pulp mills and sawmill are produced. 

Forest management and harvesting should always be carried out in accordance with good professional practice, thus ensuring sustainability in all respects. The term “sustainability” even originates from German forestry. It was first used in 1713 by Hans Carl von Carlowitz when, in the face of a looming timber shortage, the aim was to cut only as much wood as could grow back through planned reforestation. Today, sustainability in forestry has evolved, with attention given to the ecological, economic and social functions of the forest and ensuring they are maintained.

Special attention is given to the protection of the forest floor. Here forms the basis for all life in the forest, as well as the future supply of the renewable raw material wood. Its protection through the avoidance and/or minimization of damage is extremely important.

Driving heavy machinery into the forest and off the paved roads poses the greatest risk to the soil. Soil compaction, erosion, and contamination can cause significant damage to the ecosystem and degrade forest productivity. Mercer’s harvesters weigh up to 24 tonnes, and a loaded forwarder can exceed 30 tonnes. Such high weights can cause long-term structural damage, especially on heavy, wet soils. When the water content in soil is high, the soil loses its load-bearing capacity. Under heavy or loaded machinery, this water then flows and is pushed sideways out of the machine’s track resulting in track ruts. These areas are susceptible to waterlogging and erosion (Figures 1 and 2). However, rutting tracks also occur when tires slip excessively.

So what can timber harvesters do to prevent or minimize this damage?

The simplest and most important measure to protect forest soils is to keep the travelled portion of forest stands as low as possible. The same lanes are driven over again and again, so as not to damage new areas that were previously undamaged by the weight of the machines. For this purpose, the well-known certification systems FSC® and PEFC® prescribe the installation of permanent fine closure systems according to certain criteria, such as minimum distances between the back alleys. The distances between alley centers are at least 20 meters so it is possible for harvesters with ten-meter-long cranes to fell trees in the center of the blocks. This crane length represents the minimum length for harvesters. The Mercer Holz harvesters even have cranes over eleven meters in order to be able to work in a way that is gentle on the stand even with somewhat larger lane spacing.

If lanes are created with significantly larger distances because the terrain does not permit closer access or the forest owner wants to reduce the loss of timber ground area, combined methods are used to conduct timber harvesting. Trees that can no longer be reached by harvesters are felled by forest workers using chainsaws. Additionally, Mercer regularly works in FSC®-certified Berlin forests, where lane spacing averages 40 meters.

The tires used on forestry machines are also of great importance. Certification systems prescribe certain tire characteristics, such as a width of at least 600 millimetres. All Mercer harvesters and forwarders have very wide, ground-saving tires with a width of 710 millimetres. Bogie belts (Fig. 3) are available for two harvesters and for two forwarders at a time to increase the contact area and traction. These are usually used when forest soils are very wet and prone to damage after long periods of rain. 

Figure 3: bogie belts

Prolonged rain and wet conditions are one of the biggest challenges for forest machine operations. Both the forest owner, who wants to meet planning and delivery commitments, and the forest contractor, who needs to utilize expensive machinery to do so, quickly find themselves in conflict with soil conservation goals. Last year, Mercer Holz reached its operational limits while timber harvesting in Thuringia. On steep slopes, it was often necessary to work exclusively downslope. Employees had to take long detours with only a slight incline to get back to the top. Even brushwood mats that harvesters placed on the alleys from the crown material (Fig. 4) were of no help in some cases. In the end, work had to be stopped and postponed, with the machines moved to other locations.

Figure 4: A harvest mat

Possible contamination of the forest soils should not go unmentioned, especially due to the potential leakage of oils. There are more than 200 litres of hydraulic oil in the hydraulic system of a harvester, and more than 120 litres in the forwarder. Thus, there is potential for significant damage to forest soils through timber harvesting. Mercer Holz uses only biodegradable hydraulic oil and chain grease on its own harvesting machines, with oil spill kits and oil binders on each machine and company vehicle.

Mercer Holz is aware of the great importance of soil-friendly timber harvesting for sustainability in the forest and sets a good example with its timber harvesting using qualified personnel, modern machinery, and biodegradable oils.