Graduate Project
 

Soil compaction from ground-based thinning and effects of subsequent skid trail tillage in a Douglas-fir stand

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  • Although soil tillage has been used successfully to alleviate compaction of forest soil after logging in a number of management contexts, little is known about the feasibility of tilling in residual stands. In an attempt to weigh the benefits of tillage against possible damage done to residual trees during tillage, a study was set up to look at both immediate and long-term effects of various treatments in a second-growth Douglas-fir thinning. Designated skid trails were laid out 46 m apart prior to ground-based logging with a rubber-tired skidder and a small crawler tractor. Pre-logging analysis consisted of penetrometer transects used to obtain a general indication of soil strength; during logging, machine movements along trails and turn characteristics were recorded. After logging, four of seven trails were tilled and approximately 40 trees were selected as study trees in each of three populations: (1) trees near untilled trails, (2) trees near tilled trails, and (3) trees away from any trails (control). Bulk density changes were measured in soil near trees in each population using a dual probe nuclear densimeter. In addition, tree and site characteristics, root and stem damage from logging and adjacent competition were recorded for each tree. Untilled trails showed average increases in soil bulk density over control of 17.8 and 11.2 percent for 10.2- and 20.3-cm depths, respectively, but a decrease of 2.7 percent at the 30.5-cm depth. Tilled trail average bulk density showed a difference of 6.1 percent greater than control at 10.2 cm and 2.2 and 3.6 percent less than control for 20.3 cm and 30.5 cm, respectively. Stepwise multiple regression used to explore associations between slope, number of vehicle turns, cumulative ground pressure, slash characteristics on trails, and interactions of all these variables showed no significant associations with bulk density changes due to logging. Analysis of Variance did reveal greater mean bulk densities at the 10.2- and 20.3-cm depths near trees on untilled trails where slash did not exist as compared to where slash did exist. Root damage due to tillage was assigned to low, moderate or high damage classes based on length and diameter of roots exposed above ground after tillage. Analysis by regression showed that these root damage classes had significantly different tree diameters, revealing a possible trend of higher root damage for larger trees along trails being tilled. A rough economic analysis estimated tillage for this project cost the Bureau of Land Management approximately $600/mile or $400/acre of tilled trail area. These figures are unusually high due to several factors including the small amount of work done for high move-in costs, complications brought on by research demands, and operator inexperience.
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