3.5. Logging scenarios
• For the simulation of logging operations, assumptions about logging damage have to be
made first. Widely varying figures are cited on the damage caused by building logging roads, skidding paths, and felling lanes.
• The extent to which a forest is impacted by logging operations primarily depends on the
intensity of felling and the methods used. When only one or two trees are extracted per hectare and the work is done as carefully as possible e.g. by skyline yarding, only a
small fraction of the forest is impacted. Intensive logging and careless felling e.g. with tractors can impact a large area of the forest.
• In the following the author investigates two different logging methods: conventional
logging with high damage, and reduced impact logging with low damage. The damage percentages were estimated based on the range of literature values s.a..
• Table 5 shows the values used in the simulations.
Table 5. Assumptions on the fraction of damaged trees in the residual stand after logging as used in the simulation for two different logging methods
a
• The damages depend on the size of the damaged trees, but are independent of the
logging intensity as a matter of simplification. •
In the next step, the authors have to define which trees can be logged. All trees above 60 cm diameter belonging to the species groups dominated by Dipterocarps 1 2 are
harvested.
• Dipterocarp trees with a diameter below 60 cm might also be of commercial interest but
they are not logged. Thus, the simulated logging scenarios represent a selective logging system as used in Malaysia e.g. Whitmore, 1990.
• The number of felled trees in one logging operation is limited to between 5 to 30 trees.
• The upper limit can be defined to exclude logging practices which modify the forest
structure too strongly. If the number of harvestable trees is too low 5 trees, no trees are felled as these kinds of logging operations cause costs that are too high and not
economically feasible.
• Logging scenarios are defined by the combination of a certain logging method with various
assumed logging cycles. •
The logging cycle is a fixed time interval determining the timing of the logging operations. It defines the period between two successive logging events. Because different cycles are
in use officially and unofficially, we simulate a wide range of cycle lengths from 20 to 100 years.
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• The currently practiced logging system in Malaysia envisages cycles between 20 and 30
years using the `Malaysian Selective Management System Whitmore, 1984; Whitmore, 1990; Aiken and Leigh, 1993.
IV. Results and Discussion 4.1. Result
4.1. 1. Results for selected logging scenarios
• In the following the authors discuss the simulation results obtained for different logging
scenarios. At the beginning of the simulation we assume an undisturbed forest. •
Fig. 1 Appendix 2, shows the results of the simulation of conventional logging with a cutting cycle of 20 years.
• Fig. 2 appendix 3, visualizes the simulated forest immediately before and after the first
logging operation at time 0. •
Each logging operation can be recognized in Fig. 1a appendix 2, due to the sudden
decrease of total standing volume. This decrease is composed of the harvested stem volume and the lost volume due to damages. The cutting cycle of 20 years is so short that
20 years after the first logging operations there are not enough harvestable trees in the forest to allow a new logging operation. Thus, the next logging is carried out later, at Year
40.
• The same situation occurs at the Years 80, 100, 120, 160, 200, 220, 300, 320 and 380.
The high variation of yield per operation is strong evidence that the forest is over-exploited Fig. 1d, appendix 2. After each logging operation the forest re growth is constituted
largely by increased growth of Macaranga species group 3, open circles in Fig. 1c compared to the composition in primary forest state at time 0 in Fig. 1c, appendix 2. But
also the species composition of the Dipterocarp species group 1 2 shifts to smaller and more light-demanding Dipterocarps group 2, Fig. 1c, appendix 2.
• Looking at the trees with a diameter above 60 cm harvestable trees, Fig. 1b, appendix 2.
During the whole simulation, the volume of these trees remains distinctly below the value for unlogged-forests value at the beginning of the simulation, forest state at time 0 in Fig.
1, Appendix 2.
• The scenario for a conventional logging at a cycle of 60 years shows the same trend Fig.
3 Appendix 4. Here the large Dipterocarp trees have more time to recover, but still they do not reach their pre-harvest volume until the next logging operation Fig. 3a, Appendix
4.
• The growth of Macaranga after logging is not as high as for the shorter logging cycles but
still considerably increased. The logged volumes per cut are higher than for the short cycle and fluctuate between 100 m
3
ha and 250 m
3
ha Fig. 3d, Appendix 4. Figs. 4 and 5 Appendix 5,6, shows the results obtained for low impact logging. The scenario with a
logging cycle of 20 years again displays strong fluctuations in the logged volume per cut including cycles without any logging Fig. 4d, appendix 5.
• The harvestable volume remains very low during the whole simulation. In contrast to
conventional logging, the growth of Macaranga is moderate Fig. 4a and c, appendix 5. Nevertheless, species composition shifts to smaller Dipterocarp species group 2.
• In the low impact scenario with a cutting cycle of 60 years Fig. 5, Appendix 6. The
volume of all trees and the volume of harvestable trees reach nearly the same value as in
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