Strength components in the forest floor restricting maximum tolerable machine forces

In forestry operations, machines used should have a high practicability, yet damage the stand as little as possible. Machine forces are undoubtedly too high in many cases, causing deep ruts, soil compaction and cutting off the tree roots. The knowledge of what forces a forest stand can withstand is still quite limited. One of the aims of our project is to supply information about the strength of the forest floor. Since the forest floor of podzol-type is a complex layered structure, we have started by studying the components in the system. A sand sediment can bear high pressures, whereas a moist fine sandy till soil should not be loaded to more than 30 kPa if severe soil compaction is to be avoided. However, the effect of shear forces and slippage is poorly studied. The humus layer above the mineral soil is armoured by tree roots, shrub roots, grass roots, etc. Intact bark is the best guarantee for healthy trees. During the sap period the shear strength of tree root bark is at least 40 N cm −2. The armouring effect of roots and rhizomes on the humus layer is considerable, as the total length of roots may be 200–500 m m −2. The strength of the humus layer is being measured.