Effects of harvesting technology upon optimal stocking regimes of forest stands in mountainous terrain

Graduation date: 1979Although one of the most common problems facing the forest manager is the determination of management regime, there has been little effort to explicitly recognize the effect of harvesting technology and topography in the analysis. This study introduces a unified theory in harvesting in mountainous terrain which brings together silvicultural method, harvesting technology, product yield, and product price to identify the optimal path through time for a forest stand managed for the objective of maximization of net present worth. Techniques for predicting harvesting costs as a function of the specific diameter distribution to be removed from the stand have not been available. The first part of the research fills this gap by the development of a harvesting simulator for mountainous terrain. Considerable detail is devoted to discussing the validity of model assumptions including log distributions, heuristic rules for log gathering, and cost sensitivity with respect to the shape of the diameter distribution. The harvesting simulator is tested against two detailed time studies of Douglas-fir thinning in mountainous terrain and is found to compare favorably with field observations. To develop the relative harvesting costs for illustration in the stocking level analysis, two skyline yarders typical of the range expected to be operating in second growth Douglas-fir are evaluated using the simulator. Analysis of the harvesting cost results indicates that over the range of values analyzed, the elasticity of harvest cost with respect to volume removed is constant for a given mean diameter of material removed. Costs from the harvest simulator are combined with a Douglas-fir growth model in a three descriptor dynamic programming structure. The potential effects of diameter growth acceleration are modeled through biometric relationships between the three descriptors; stand age, trees per acre, and basal area per acre. The optimal thinning regime and optimal rotation age are determined simultaneously for a medium site Douglas-fir example under a predetermined set of average conditions. The sensitivity of optimal stocking level to harvest technology variables of yarding direction, yarding distance, truck transport cost, and log gathering strategies is examined. Under assumed cost differentials between uphill and downhill yarding, bare land values for downhill yarding are lower than for uphill yarding and the optimal management intensity is lower with less frequent, heavier entries. Increases in yarding cost with distance indicate that optimal stocking levels not only depend on traditional concepts of prices and costs, but that management intensity is also spatially oriented. It is demonstrated that under certain conditions substantial increases in net present worth can be made by treating portions of the stand in the same skyline road and with the same rotation age with different thinning regimes. Haul costs are exogenous to the harvesting cost simulation. However, reductions in haul cost increase bare land values by at least the magnitude of the present value of the haul cost decrease and may increase the optimal level of management intensity. The sensitivity of management regime to log gathering technique is examined by formulating a prebunching model which stratifies the log handling activity into two components. Logs are first gathered into bunches along the skyline corridor, and then the bunches are forwarded up the corridor to roadside. Prebunching and forwarding under the model assumptions is found not only to increase bare land values but in some circumstances to reduce the cost of handling early thinnings sufficiently to justify noncommercial entries to accelerate diameter growth. Constraints eliminating noncommercial thinning opportunities are shown to reduce present value