Parental rank changes associated with seed biology and nursery practices in Douglas-fir

The impact of container-nursery management practices on the genetic composition of seedling crops was evaluated in an experimental study designed to determine the cumulative effects of: (1) differences in parental reproductive output in bulk seedlots, (2) parental variation in germination parameters (percent and speed), and (3) the interaction of these parameters with container-nursery practices of thinning and culling, and their effect on the genetic representation of parents in the resultant seedling crops. Results from the experimental study were compared to predictions of a stochastic simulation designed to estimate the consequences of differential parental seed contribution, and seed-germination percent and germination speed on indicators of crop performance. The experimental study was conducted on a Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) bulk seedlot that was representative of the differential seed contributions from 19 seed orchard parents. The nursery study included a total of 42,000 seeds. Seeds were sown at three seeds per cavity. Within the 14,000 cavities used, the identity of every seed was maintained throughout the study. Parents were compared based on: (1) changes in their rank order from sowing to postthinning and postculling status, and (2) relative performance from seed contribution to seedling production. Changes were observed in both assessments. Path analysis was used to determine the percent contribution of each factor to seedling production. It was determined that germination, thinning, and culling contributed 66, 20, and 14%, respectively, to seedling production, indicating the presence of three consecutive bottlenecks in the process. Single seed or individual family sowing in the nursery was recommended for seedling production to maintain genetic diversity.