Graduate Thesis Or Dissertation

 

The effects of compaction on hydrologic properties of forest soils in the Sierra Nevada Public Deposited

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  • This study evaluated the effects of a crawler tractor, a rubber-tired skidder, and a torsion suspension vehicle on several soil and hydrologic properties in the western Sierra Nevada Mountains of California. Four sites, with soil textural classes ranging from a loam to a loamy sand and elevations between 680 and 2180 m, were studied at three moisture contents. Compaction was monitored with a double probe nuclear densiometer and an air permeameter after 1, 3, 6, 10, 15, and 20 round trips. Density measurements were made at five different depths. Infiltration capacities were determined with a small, portable rainfall simulator. Soil core samples were taken to observe porosity and conductivity levels before and after disturbance. Compaction, or the change in bulk density, was significantly greater on the crawler tractor trails at all depths. When only the plots established in the outer positions of the trails were included, this distinction between vehicles could not be made. The crawler tractor generated more uniform compaction across its trails that either the torsion suspension vehicle or the rubber-tired skidder. The greatest change in bulk density for all the vehicles took place in the surface five centimeters. Infiltration capacities on the undisturbed sites were found to exceed predicted maximum precipitation rates. Mineral soil at the three higher elevation sites was very hydrophobic. This was thought to be due to the coating of soil particles with the metabolic products of fungal mycelia. The crawler tractor reduced infiltration capacities by 78 percent, while the torsion suspension vehicle and the rubber-tired skidder caused 67 percent decreases. Suspended sediment concentrations determined from runoff collected on the infiltration plots served as an erosion index for the skid trails. The crawler tractor trails had runoff with 40 percent higher sediment loads than that found on the trails of the other two skidders. Organic matter content was inversely correlated with the sediment levels. Air permeability readings showed that the three vehicles produced very similar reductions in macroporosity in the surface 2.5 cm layer as the number of trips increased. Approximately 75 percent of the decrease in macropore space occurred by six trips. The air permeameter provided a relatively good index of compaction. Porosity and conductivity levels determined from the soil core samples showed that machine differences did not exist in the track position. Macropore space was r2duced 43 percent and the conductivity decreased 80 percent. Site differences were much more evident than vehicle differences. Observations made after one over-wintering period showed that measureable recovery had not occurred.
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