抄録
Climate change often causes unexpected problems for agricultural production. In the Tokachi region of Hokkaido, northern Japan, soil-frost depth has been decreasing since the late 1980’s, facilitating the winter survival of small potato (Solanum tuberosum L.) tubers that remained unharvested and emerge as weeds in the following cropping season. Soil frost control by snow plowing (yukiwari), which removes snow cover and allows deep soil frost to kill the potato tubers, is spreading as a practical countermeasure. However, the technique has largely been empirically based, and the level of expertise varies among farmers. We present the final limit for yukiwari based on a numerical model of soil temperature for Tokachi and other potato-producing regions in Hokkaido, with annual fluctuations taken into account. An average of 60800 unharvested potato tubers was present per hectare, mostly distributed in the top soil to a depth of 0.15 m. As the survival probability of potato tubers was almost zero where the maximum soil-frost depth reached deeper than 0.30 m, the objective frost depth was set to 0.30 m. In the Tokachi region, soil-frost depth reached 0.30 m by yukiwari in most areas except Hiroo on the coast in the southernmost area. The final limit of yukiwari in a 30-year return period was estimated to be around 20 February in mountainous areas in the north and from late January to early February in other areas. For other potato-producing zones in Hokkaido, soil froze to 0.30 m deep by yukiwari in the inland areas from the Pacific Ocean, the Sea of Okhotsk, and the Sea of Japan, but soil frost was not as deep in the southwestern part of Hokkaido. The numerical model for estimating soil temperature facilitates the decision on the work schedule of the practice of yukiwari in order that the damage from volunteer potatoes may be minimized.
