Changes in pollen production rates in Sugi stands with increasing stand age and soil fertility.

自1975年以来，日本柳杉森林种植园通过昂贵的植树造林政策扩大了种植面积，目前空气中传播的花粉数量增加，助长了杉木花粉病的传播。因此，我们研究了杉木林的花粉(雄花)产量随年龄的变化。用凋落物诱捕法测定了林分雄花的开花率。研究林有11个种植园，树龄从12岁到90岁不等，具有共同的立地条件，以及一座佛教寺庙地面上的一个约600岁的林。在1991年丰产年，林分开放雄花数与林分年龄的关系呈天花板曲线，表明在50年生后几百年内有一个恒定的上限。由于林分之间的差异，40年生以上的林分之间没有发现花数的差异。另一方面，在1992年非丰产年，花朵数量减少到前一丰产年的1%左右，不同林分之间没有差异。上述结果还表明，如果杉木人工林的终伐年限延长到120-150年，单位土地面积上的花粉散布就不太可能增加。许多关闭的雄花在开放之前就掉了下来。在约600年的古林分中，大多数在12月以后花粉形成结束时倒下的花都已经败育了。而在不到90年生的其他林分中，大多数花朵都有完整的花粉囊和花粉粒。前者是生理原因造成的，后者则是冬季大风、降雪等物理因素造成的。

The current increase of air-borne pollen from Sugi (Cryptomeria japonica D.Don) forest plantations, which were expanded through the policy of expensive afforestation after World War II, has contributed to the spread of Sugi pollen disease since 1975. Therefore, we studied changes in pollen (male flower) production of Sugi stands with ageing. Production rates of male flowers in a stand were measured by the litter trap method. The study stands were 11 plantations ranging in age from 12 to 90 yr and having mutual site conditions, and a stand about 600 yr old in the ground of a Buddhist temple. In the productive year 1991, the relationship between the number of open male flowers in a stand and stand age showed a ceiling curve, indicating a constant upper limit for several hundred years after the age of 50 yr. No difference in flower number was recognized among stands over 40 yr old because of the variation among the stands. On the other hand, in the unproductive year 1992, the numbers of flowers decreased to about 1% of that in the previous productive year, and did not vary among stands. The above results also indicate that an increase of pollen dispersal per unit land area is improbable if the final cutting age of Sugi plantations is increased to as much as 120-150 yr. Many closed male flowers fell before opening. In the ancient stand about 600 yr old, most flowers that fell after December at the end of pollen formation had aborted pollen sacs. However, in the other stands less than 90 yr old, most flowers had sound pollen sacs and pollen grains. The fall in the former stand was due to physiological causes, and that in the latter, to physical forces such as strong wind and snow-fall in winter.