Skidder Traffic Effects on Water Retention, Pore‐Size Distribution, and van Genuchten Parameters of Boreal Forest Soils

Compaction by harvesting equipment has a potential to decrease the quality of root environment through alteration of soil pore space. Effects of skidder traffic on soil water retention and pore‐size distribution were studied on medium‐textured Inseptic and Oxyaquic Haplocryalfs, and Typic Dystrocryepts at 14 sites in Alberta foothill and boreal forests. Treatments included 3, 7, and 12 cycles by skidders equipped mostly with wide tires. Soil cores were collected at four random points of each treatment at an average depth of 5 and 10 cm. Soil water content of each core was measured at six potentials. Tempe cells were used to measure water retention at potentials from −2 to −30 kPa; a conventional pressure plate was used in the range of −100 to −1500 kPa. The four‐parameter water retention function developed by van Genuchten (1980) was fitted to the data. The pore space of trafficked soil was not significantly affected when soil was drier than field capacity. At higher soil water contents, the first few skidding cycles caused a decrease in θ s and α parameters, which reflected flattening of water retention curves in the high potential range and a simultaneous shift of the steepest part of the slope to a lower potential. The result of compaction was a decrease in air‐filled porosity below 0.10 m 3 m −3 , which restricted aeration without changing field capacity or available water holding capacity. Most modifications of soil pore space by wide‐tired skidders can be avoided if the soils are drier than field capacity.