Paul N. EFTHYMIOU, Laboratory of Forest Utilization,
Aristotle University of Thessaloniki, GREECE
The goal or principal criterion of an efficient and successful wood utilization system in a free market economy is to maximize the overall value added of the wood production chain, from the tree-felling site up to the end-use point of the final products. The lowest acceptable limits of this criterion for a rational wood utilization process is at least that the price of each final assortment should cover the cost of wood harvesting, transport and processing. Otherwise we are led to two alternatives: first, a real deficit and, second, to the need for subsidization of the utilization process by removing money from other investment possibilities or alternatives.
Since we live in a very competitive market economy with various options and social needs, it is not possible to function with subsidies for wood production in the long run. This means that apart from very few special or exceptional cases, we are obliged to structure all the wood harvesting and utilization systems with high productivity and economic efficiency.
This goal is rather easy to achieve in operations that harvest thick wood assortments because the figures of cost effectiveness, of wood prices and that of final results (in monetary units per cubic metre) are more or less positive. In this line of thought the harvesting and utilization of small-sized assortments represent in practice a real problem. This fact has a major universal application in all harvesting systems and working methods (manual, motor-manual, animal power, tractors, skidders, chipping and cable systems). The impacts of this reality can be seen if we mention that:
These serious efficiency problems have to be studied in depth and probably we should coordinate our efforts in order to find harvesting solutions, which could be proved of high practical and economic value. Cable systems, which are the main focus of this workshop, also have to deal with this problem because the high cost per cubic metre and the respective efficiency are very much influenced by the dimension of wood. This is the case especially in shelterwood or selective cuttings of young stands, where the wood harvested has an extended dispersion with low volumes per stem and per load extracted.
The law of mass per piece (Das Stückmassegesetz)
The problem of increased time and cost per cubic metre as the dimension of wood decreases is not new and was described 70 to 80 years ago by the father of Forest Work Science, H. H. Hilf, in Germany. I am referring directly to “The law of mass per piece” or in German “Das Stückmassegesetz”, which is very well known to all colleagues with German forestry culture. The first mathematical formulation for tree felling and processing times was given by the late Professor G. Speidel in 1952, and for wood skidding with two independent variables (load volume and distance per turn) by P.N. Efthymiou in 1973 (see Figure 1).
|(A):||Working time/m3 (y) depending upon wood-load in m3 (x) and distance in m (z) per turn.||(B):||Work productivity in m3/h depending upon extraction distance (z) and given for various load sizes per turn.|
Fig. 1: Depicting the influence of the “law of mass per piece” (Stückmassegesetz) in wood extraction (Efthymiou 1973).
Figure 1. The influence of the “law of mass per piece” (Stückmassegesetz) in wood extraction (Efthymiou, 1973)
The influence of this law of mass or volume per piece or unit (tree, log or load) has been taken into account by many authors and researchers in Europe and North America in the last 30 to 40 years (Gläser, 1959/60; Anderson, 1976; Häberle, 1979; Sirois, 1981; FPRS - Roberts, 1982; Grammel, 1988; Löffler, 1991; Efthymiou, 1973 and 2000; Meek, 2001 etc.).
As a result of the large impact of this law on all phases of harvesting operations we must find improved methods and system combinations for minimizing the effects of the law, either by forming bundles or bigger loads, or preparing the small stems and assortments in larger units. On the other hand, the training of forest workers and operators should lead them towards a more conscientious attitude in order to become even more productive and efficient in material handling.
Table Illustrative harvesting and transporation cost of whole tree chips versus tree size. (Taken from Klunder and Plummer 1980).
|Cost per ton (green)|
|Cost per cord1|
1 Based on green weight of 5,400 pounds per cord.
Fig 2: Harvesting and transportation cost vs. tree size. (Sirois, 1981)
In this framework, all time and piece-rate standards - as well as the productivity, cost and efficiency figures in practical use or in publications by scientists and practitioners - have to be set up or provided under specific consideration of tree/stem-dimension/volume or load-size. Only in this way can we ensure the comparability and credibility of data or research results for specific working conditions of the various harvesting systems implemented.
Fig 3: Costs, revenue and profit in utilization of
a larger part of the biomass
There is really no sense in comparing the productivity and efficiency of work of the same machine under similar forest or terrain conditions if, for example, one average assortment size is 0.15 m3 and the other one ranges at 0.40 m3. A very characteristic example is provided by Sirois (1981) as follows:
“The cost of wood (chips) harvesting and transportation of tree size one inch at dbh is 107.31 dollars per ton (green). If the tree size ranges at 6 inch dbh (15 cm) the respective cost is 8.32 dollars per ton.
And finally if the average tree size becomes 11 inch (28 cm) the cost is only 6.00 dollars per ton” (see Table 1 and Figure 2).
Naturally there is also a corresponding evolution of revenues and profits, if we take into account the prices and revenues for each assortment of the various tree or stem sizes. This simultaneous development of costs, revenues and profits in a very characteristic schematic figure (see Figure 3) is provided by Anderson (1976). The development of utilizing wood assortments and forest biomass reaches a critical value, which is the end of a profitable wood/biomass utilization. After that point it is not possible to make a profit. This applies for the small wood dimensions which may be different depending upon the material size and the systems applied.
The tree, stem and load size have a substantial impact on time, productivity, cost and efficiency per cubic metre of wood harvesting operations in all cases, as well as on working methods and systems. These effects have an increasingly negative development as the wood dimension decreases, and therefore this situation becomes menacing for the efficiency and the utilization potential of small-sized wood.
This influence refers to the “law of mass/volume per piece/unit” or, as it is called in German. “Das Stückmassegesetz” which is valid in all harvesting systems and is known for many decades.
In this framework, we should make systematic and coordinated efforts for better training of workers, as well as to design new and more successful wood harvesting systems.
Finally, we must pay attention to the comparison between the various data of time, productivity, cost and efficiency figures based on explicit tree, stem and load sizes, in order to achieve more credibility and reliability of the systems comparisons.
Efthymiou, P.N. 1973. Das Stückmassegesetz beim Holzrücken mit Zugtieren in Gebirgswald. Forstarchiv 44(10): 209 – 216
Efthymiou, P.N. 2000. Main technological, ecological and economic constraints in utilizing small-sized wood. Internat. Seminar on Forest Residues, Problems and Possibilities in Southern Europe, CBE 9 – 11 November 2000, Miranda do Corvo, Portugal.
FAO/ECE/ILO. 1976. Technical, organizational and economic problems involved in the includsion of a larger part of the forest biomass in harvesting systems, by S. Anderson. In Symposium, Vol. II 238–255, Hyvinkää, Finland. FAO, Rome.
FAO/ECE/ILO. 1996. Wood transport in steep terrain, by Trzesniowski, A. In Proceedings of Symposium, 16–20 June 1996, in Sinaia, Romania. FAO, Rome.
FPRS - B. Roberts (Ed.). 1982. Harvesting small timber: waste not, want not. Kendall/Hunt. Publ. Co. Dubuque, Iowa.
Gläser, H. 1959/60. Die Ernte des Holzes. 3. Aufl. Forstverlag Euting KG, Neuwied/Rh.
Grammel, R. 1988. Holzernte and Holztransport - Grundlagen. Paul Parey Verlag, Hamburg
Häberle, S. 1979 Das Stückmassegesetz. Forstarchiv.
Kuusela, K. 1994. Forest resources in Europe, 1950–1990. EFI - Res. Report No. 1. Cambridge Univ. Press, Cambridge
Löffler, H. 1991. Forstliche Verfahrenstechnik. 2. Aufl. Univ. München.
Meek, P. 2001. The influence of three factors on harvester productivity in commercial thinning. FERIC - Advantage, Vol. 2, No. 12, Point Claire, QC, Canada.
Sirois, D.L. 1981. A mobile harvester for utilization of weed, trees and residues. USDA, Forest Service, Auburn, AL.
Speidel, G. 1952. Das Stückmassegesetz und seine Bedeutung für den internationalen Leistungsvergleich bei der Forstarbeit. Diss. IFFA Nr. 5, Reinbek/Hamburg.