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The mating system of Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] was examined in two genetically similar seed orchard blocks with the aid of allozyme polymorphisms at six loci. The pollination environment (time of pollination and length of pollination season) of one block was manipulated by means of an overhead water spray cooling treatment. The cooling treatment temporarily delayed the pollination season in the treated block by 12 days relative to the untreated one. In addition, the length of the pollination season in the cooled block was shorter than the uncooled one by 12 days (18 vs. 30 days). The rate of inbreeding was significant in the uncooled block, while a perfect outcrossing rate was obtained for the cooled one. This difference was attributed to the low outcrossing rate obtained within the lower crown of the control block. It was concluded that the observed significant selfing in the uncooled block was caused by the firstpollination primacy that permitted related pollen to out-compete unrelated pollen in the extended pollination season. A shorter pollination season produced a dense pollen cloud of related and unrelated pollen, hence, early zygotic selection favoured unrelated embryos. An extended pollination season, on the other hand, permitted related pollen to succeed in pollination and fertilization without any challenge by unrelated pollen. Controllable variation in the rate of inbreeding reported in this study could be useful to improve the genetic quality of seed produced from seed orchards and consequently to maximize genetic gains from selections.

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