Simulating skyline yarding in thinning young forests

Graduation date: 1974Cable logging systems are being applied frequently in thinning young-growth forests, but the dynamics of these systems are relatively unknown. This study is part of a research project to develop aerial logging systems for thinning and to analyze them for economic, silvicultural, and environmental implications. The objectives of my study\ud were: 1) To measure the influence of thinning intensity, slope steepness, load size, skyline distance, lateral rigging\ud distance, and number of workers on yarding times. 2) To develop a model to simulate these relationships under varying working conditions of the skyline system and to\ud identify proposals for improved logging methods. I observed a cable-thinning operation on a 35-40 year old Douglas-fir stand on gentle and medium steep slopes. Three thinning intensities of 70, 50, and 20 percent, respectively, were applied. The skyline yarding work cycle was separated into six regular and three irregular e1ennts. I used the multimoment time study method to observe these elements and obtained values for the variables that\ud influenced these elements. Stepwise multiple linear regression analysis was used to analyze the effect of these variables on tine required to finish an element. I developed regressions for four regular elements, carriage out, lateral out, lateral in, carriage in, and for total regular yarding tine. Regressions with the observed variables did not produce adequate results for other elements. I analyzed only their frequency distribution. I tested the results of the analysis against a sample from the same yarding system in a different operation. Close correspondence in a chi-square and t-test, respectively, suggests\ud the reliability of the regressions. Several suggestions were made for modifications that may improve the skyline system. They concern hooking and unhooking, the cable\ud configuration, and lateral yarding. I developed a simulation model in the GPSS-computer language to simulate skyline thinning. The elements of the model can be modified to adjust to distinctive situations. I tested the influence of lateral yarding distance on total yarding time and simulated a regular approach with limited lateral yarding distance and a "line thinning" method