Pollen yield per male inflorescence in a young Alnus sieboldiana community was measured in 1987 and 1988. The seven to eight year old community was growing on freshly dumped detritus with little humus. Male inflorescences from four trees in the study community were sampled at suitable dates before pollen release on 27 March 1987 and 8 April 1988. Number of bracts per inflorescence, number of stamens per bract, and number and dry weight of pollen per stamen were measured, and pollen yields per inflorescence were estimated by multiplying the quantities of pollen grains per stamen by the number of stamens per inflorescence. The number of pollen grains per stamen, taken from trees bearing a large number of stamens per inflorescence, was small whereas for trees with a small number of stamens, the pollen number per stamen was large. No relationship between pollen weight per stamen and the number of stamens per inflorescence was recognized. For trees showing the minimum dry weight for a single pollen grain, the number of pollen grains per stamen was maximal in both years. Comparison of pollen yields per inflorescence between the study years showed that the number in 1987 was less than that in 1988, while the weight was approximately equal in both years; in 1988 a large number of small, light grains were produced. Mean numbers of pollen grains per stamen for the four sample trees in 1987 and 1988 were 10,720 and 14,410, respectively, whereas pollen weights were 0.142mg and 0.126mg, giving a mean single pollen grain dry weight of 13.5×10^<-6>mg and 8.99×10^<-6>mg, respectively. Mean pollen quantities per inflorescence in 1987 and 1988 were 9.94×10^6 grains and 16.5×10^6 grains, and weighed 133mg and 145mg, respectively.
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