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58. HARVESTING WOOD FROM SPRUCE SWAMP STAND
DURING SUMMER
Kalle Kärhä & Asko Poikela
Metsäteho Oy, P.O. Box 101, FI-00171 Helsinki, Finland
Kalle.Karha@metsateho.fi
Introduction
Metsäteho Oy conducted a joint study with its cooperation partners into soft-ground harvesting of
Norway spruce (Picea abies L. Karst.) swamp stands in Pälkäne, Finland, in September 2009.
The study investigated the effects of forest machine equipment on cross-terrain mobility. The
focus of the study was on the effects of the use of different tyre inflation pressures and track types.
The study covered four harvester models: a 4-wheel harvester (ProSilva 910), 6-wheel harvester
(John Deere 1270D Eco III), 8-wheel harvester (Ponsse Fox) and a rigid-tracked harvester
(ProSilva 810 T). The study also included three forwarders: an 8-wheel forwarder (John Deere
1110D Eco III), 10-wheel forwarder (Ponsse Wisent 10w) and a rigid-tracked forwarder (ProSilva
810 T). For the study, nine different harvesting systems were formed and a research sample plot
was established for each harvesting system. Each sample plot was first felled and then the empty
forwarder was driven through the plot. The forwarder was then loaded and driven in ten passes
through the sample plot.
The sample plots were staked at three-metre intervals after felling. Each stake served as a
measurement point at which the vehicle rut formation after the drive passes and the peat layer
thickness were measured. In addition, the felled volume at each stake was calculated, and the
quality of ground reinforcement with residues (good = ample logging residues on rut; average;
poor = no logging residues on rut) was assessed for each vehicle rut. The number of damaged
trees and stumps along the vehicle ruts were also recorded, as well as whether or not a strip road
bend occurred at the staked measuring point. The groundwater depth was also measured at each
sample plot.
Results
Rut formation was significantly reduced when the harvester (John Deere 1270D Eco III and
ProSilva 910) tyre inflation pressure was lowered. With high tyre inflation pressure the average rut
depth was over 3 cm, whereas with the lowest tyre pressures used in the study the average rut
depth was just 0.5 cm. Tracked harvesters produced the same average rut depth as harvesters
with the lowest tyre inflation pressures. The results show that reduced tyre inflation pressure
significantly improves harvester cross-terrain performance.
The study showed that lowering forwarder tyre inflation pressure enabled the number of passes
over the plot to be increased: With high tyre pressure only two loaded passes were possible,
whereas with the lower tyre pressure used in the study four loaded passes were possible, and
with the lowest tyre pressure five passes were achieved. Reduced tyre inflation pressure however
showed no significant effect in the study on average forwarder rut formation.
With the wheeled forwarder the quality of the ground reinforcement with residues had a highly
significant impact: With good ground reinforcement virtually no rutting occurred. Conversely, with
average or poor ground reinforcement, rutting became rapidly more severe as the number of drive
passes increased. On the basis of the study, it can be concluded that harvesting on soft-ground
sites is viable if a wheeled forwarder and effective ground reinforcement with residues are used.
Insufficient or uneven application of residues for ground reinforcement is correspondingly
detrimental. In the absence of effective ground reinforcement, the optimal operating scenario for
wheeled forwarders is summer thinning of spruce stands on load-bearing mineral soils.
When forwarders (John Deere 1110D Eco III and Ponsse Wisent 10w) with “traditional” flexible
tracks were used, the execution of a full test series, i.e. ten loaded passes, was possible and rut
formation was minimal. Even with poor ground reinforcement, the average rut depth remained
below 2 cm until the fifth load. Significant rutting began to occur only after the tenth loaded pass.
The significance of ground reinforcement with residues was considerably less with flexible tracked
forwarders than with wheeled forwarders. The ground reinforcement quality had no influence on
flexible tracked forwarder rut formation during the first few passes. Only from the sixth loaded pass
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