Abstract
Spiraea thunbergii, which grows naturally on rocky sites in river and mountainous areas, is regionally vulnerable in Japan. However, its conservation has not been studied in detail. This study determined safe sites for the germination of S. thunbergii by clarifying the effects of light, temperature, and water on its germination. The seeds used for the tests were collected in the upper reach of the Tama River. Germination was measured under different light exposures (brilliant, shade, leaf canopy shade, and dark), constant (10, 15, 20, 25 and 30 degree C) and alternating (5/15, 10/20, 15/25, 20/30 and 25/35 degree C) temperatures, and alternating wetting and drying on a regular schedule (wetting for 1 day, drying for 1 day (W1D1), W2D2, W4D2, W6D2, W2D4, W4D4, W6D4, W6D2, and always wet). The tests of light and temperature suggested that it is very likely that S. thunbergii does not have a light requirement, sensitivity to leaf canopy shade, or a requirement for fluctuating temperatures. The wetting and drying tests showed that S. thunbergii requires continuous wetting for 4 - 6 days to germinate. Consequently, to conserve S. thunbergii, it is important to conserve the existing populations and conserve microenvironments that allow continuous wetting for germination, such as mosses and sand deposited in the interspaces of rocks.
