for the MERLYN harvesting model
Forest management, in the MYRLIN harvesting model, can be varied through the following factors:
Felling cycle This is set at cell B2 on the {models} sheet. It determines the area of forest felled during any one period, and the return period to a given coupe. Longer felling cycles allow more growth to accrue above the felling limit, but cut smaller areas; the two effects approximately balance, so the influence of felling cycle on average yield is not strong, and simulations across a range of values may not show any definite optimum.
Diameter limits The minimum size below which a species may not be felled is set in column E from row 11 downwards, on the {models} sheet. Increasing the diameter limit will reduce the number of trees which may be felled, and will therefore directly reduce yields.
Harvest % The percentage of trees above the minimum diameter limit which may be felled is set in column F from row 11 downwards, on the {models} sheet. This will directly influence available yield: Reducing harvest % will proportionately reduce yield. This factor can simulate conservation measures to protect seed trees, and may also reflect the reality that for many species, form and decay defects do not permit 100% of available stems to be economically harvested.
Logging damage The logging damage factor set at cell B3 of the {models} sheet can be used to simulate the effect of RIL. However, because of the dilution effect of the non-commercial and small size trees, the impact on future yields of even quite large changes in the logging damage factor will be found to be small.
A stand projection model such as MYRLIN is not sensitive to some dynamic aspects of stand behaviour. For example, lower diameter limits may reduce seed trees on species under heavy commercial pressure. The model does not account for this. Heavy logging may induce more frequent fires, erosion, nutrient losses, and pioneer re-growth that impedes valuable trees. Some important commercial species require disturbance in order to regenerate properly. None of these factors are accounted for in the model. It should therefore not be seen as a decision-making tool about silviculture. It is rather, a useful calculating system for yield estimation given moderate, mainstream management practices. It will estimate yields over time subject to species distribution by blocks and through size classes, and will account for the main effects of increment and mortality on yield.