The Ponsse Bear harvester.
Research on the effect of tree and harvester size on productivity and harvester investment decisions demonstrates that the difference in productivity rates largely negates the difference in ownership and operating expenses between larger and smaller harvesters.
Simon Ackerman, Bruce Talbot and Rasmus Astrup collected long-term machine-derived data sets of 140,000 trees from four fully-mechanised cut-to-length harvesters of equal age and similar working conditions in two machine size classes. The harvesters were two Ponsse Bears and two smaller Ponsse Beavers.
The study took place in a plantation forestry setting. Site factors other than tree size were kept as uniform as possible, making it possible to isolate the effect of harvester size on the optimal tree-size range.
The study’s first objective was to investigate how well the optimal tree-size ranges can be distinguished for two different harvester size classes. The second objective was to evaluate harvester investment decisions based on the information obtained on the optimal tree-size ranges.
The researchers modelled productivity functions for each size class using a nonlinear mixed-effects approach. The unit costs and their sensitivity to utilisation rates and cost of capital were assessed based on these functions.
Results show that within the range of tree sizes observed and despite considerably higher capital costs (32%) on the Bear, a 50% higher mean productivity resulted in a:
- Unit cost only 17% higher than the Beaver in a disadvantageous scenario (high interest rates and low utilisation)
- Unit cost 6% than the Beaver in an advantageous scenario (low interest and high utilisation),
The smaller Ponsse Beaver harvester.
Between these extremes, there were only marginal differences in unit costs. It demonstrates that the difference in productivity rates largely negates the difference in ownership and operating expenses between larger and smaller harvesters.
The results provide insight into timber harvester investment decisions. Harvesters from two adjacent size classes can be used interchangeably at the same unit cost within a wide range of tree sizes despite productivity differences.
Note that the study did not consider increased repair costs and an eventual reduction in the expected economic lifetime on the Beaver. It also did not consider the negative effects of using a larger harvester in thinning operations.
Source: SA Ackerman, B Talbot R Astrup (2021): The effect of tree and harvester size on productivity and harvester investment decisions, International Journal of Forest Engineering, DOI: 10.1080/14942119.2021.1981046