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

自1975年以来，由于第二次世界大战后昂贵的造林政策，杉（柳杉D.Don）人工林的空气传播花粉不断增加，导致了杉花粉病的传播。因此，我们研究了杉林花粉（雄花）生产与老化的变化。用凋落物诱捕器法测定了林分雄花的产生率。研究林分为11个年龄从12年到90年的人工林，具有共同的立地条件，和一个大约600年的林分在佛教寺庙的地面。在丰产年1991年，林分雄花开放数与林龄的关系呈天花板曲线，表明50年生以后几百年内有一个恒定的上限。40年生以上的林分，由于林分间的变异，花数没有差异。另一方面，在非生产年1992年，花的数量减少到前一个生产年的1%左右，并没有在林分之间变化。上述结果还表明，如果杉人工林的最终采伐年龄增加到120-150年，单位土地面积的花粉散布量增加是不可能的。许多关闭的雄花在开放之前就掉了。在600年生的古林中，花粉形成末期12月以后落下的花大多数花粉囊败育。90年生以下的林分花粉囊和花粉粒结构完整。前者的凋落是由于生理原因，后者的凋落是由于冬季大风、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.