Frost Damage Increased by the Windbreak

A large orchard of satsuma mandarin located nearest the sea coast of the southern end of Kyushu was completely destroyed by the early frost on the two successive nights in November, 1968. The orchard of 72 hectare area in an artificially flattened dune, 5m above sea level, is divided into 60 rectangular sections; each section, 100m wide and 120m long, is surrounded by a pine-tree hedge 6-8m high and is partitioned further into nine small parts by 1.8m high plastic-net windbreaks set up in two lines and two rows. Under such a perfect protection against wind hazard, about 200 thousand young mandarin trees were planted in the autumn of 1967 and the spring of 1968. In September of 1968 a typhoon, passing off the sea coast to the north, swept the orchard; however, wind hazard was very slight, only faint discoloration was recognized on leaves during a few weeks after the typhoon. Unusual severe frost occurred on 15th and 16th of November after a warm spell of about ten days. Nearly 85 percent of mandarin trees in the orchard were injured; most of their trunks were frozen to death. Thus the orchard was destroyed and abandoned one year after establishment. Awful damage due to the early frost, however, was restricted within the orchard with windbreaks on the dune. Significant hazard was hardly recognized in many other orchards on the hillsides adjacent to the destroyed one. To make clear the reason for severe frost damage in the orchard with cautious windbreaks, observations were done for several nights both in the ruined orchard and on another flat ground with model windbreaks. We come to the conclusions that the night land-breeze, generally called cold air flow, lessens in reality the night chilling of the air near the ground surface, and that much obstruction to the breeze by windbreaks such as in the above mentioned case will cause fabulous coldness on the leeward side of the windbreaks on nights when radiation cooling is prevailing.