TIMBER HARVEST TRAFFICKING AND SOIL COMPACTION IN WESTERN MONTANA

Little evidence is documented regarding the effects of timber-harvest traffic on volcanic-ash-mantled glacial till soils and clay-rich Tertiary volcanic-derived soils of northwestern Montana. We identified and characterized differences in the soil physical properties between trafficked and nontrafficked areas. Soil-clod bulk density, infiltration, and soil-clod water retention were measured on 54 pedons from nontrafficked and moderately and severely trafficked sites. Soil mechanical properties were characterized using 18 nontrafficked pedons. Compared with nontrafficked areas, bulk densities in severely trafficked areas at the 15-cm depth were 76, 21, and 21% greater in ash over limestone till, ash over quartzite till, and Tertiary volcanic soils, respectively. Water retention at 0.002, 0.010, and 0.033 MPa was significantly lower at the 15-cm depth in trafficked than in nontrafficked areas of ash over limestone till. Similar differences were observed in ash over quartzite till. Compared with nontrafficked areas, cumulative 1-h infiltration in severely trafficked areas was 81% less in ash over limestone till, 79% less in ash over quartzite till, and 87% less in Tertiary volcanic soils. Similar but smaller differences were detected in moderately trafficked areas. Three independent measurements demonstrated significant differences in surface horizon physical properties between trafficked and nontrafficked areas. We detected significant physical manifestations of traffic-induced soil compaction below 30 cm.