Snow dynamics and soil moisture in commercially thinned lodgepole pine silvopastures on the Okanagan Plateau of Southern British Columbia

Forest harvesting systems may be strategically designed to manipulate snow accumulation and soil moisture, not only to improve forage and timber productivity, but also to improve flood, wildfire and climate change mitigation. The objectives of this study were to investigate differences in snow accumulation, snow ablation, and soil moisture across various widths of commercial strip-thinning in silvopasture systems. Silvopastures were established near Kelowna, British Columbia, Canada. Stands of 45-year-old Pinus contorta (lodgepole pine) were commercially thinned in the summer of 2018. Strip-harvest treatments were linear and of various widths: 10 m, 15 m, and 20 m. Treatments were allocated in a two-factor randomized block design; strip width and orientation. Adjacent forested areas and a clearcut were also examined as controls. Strips were harvested with either east-west, or north-south orientations. Snow water equivalent was manually sampled during winters of 2019-2020. From May 2019 to October 2020, volumetric water content was continuously sampled at two depths (0-10 cm and 25 cm below the soil surface) and soil temperature was continuously measured at 5 cm depth. Snow accumulation increased with strip-width, and 20 m strips retained a mean peak accumulation of SWE which was 11.4 % greater than the clearcut (P=0.001). Snow accumulation and ablation-rates were directly associated with canopy cover and estimated solar radiation. The widest 20 m strips maintained greater soil moisture and soil temperatures throughout the growing season, compared to the narrower 10 m strips (P<0.001). However, 10 m strips did retain more soil moisture during drier months with limited precipitation (P<0.001), and strips orientated east-west retained the most soil moisture. Soil temperature was greater in both 10 m and 20 m strips compared to forested areas. This resulted in greater cumulative growing degree days (GDD) above the critical threshold of 5 °C in strips relative to non-harvested areas. Overall, strip-thinning and orientation of strips directly influenced snow accumulation, ablation dynamics, soil moisture, and temperature.