Genetic diversity and background pollen contamination in Norway spruce and Scots pine seed orchard crops

Seed orchards are the key link between tree breeding and production forest for conifer trees. In Sweden, Scots pine and Norway spruce seed orchards currently supply ca. 85% of seedlings used in annual reforestation. The functionality of these seed orchards is thus crucial for supporting long-term production gain and sustainable diversity. We conducted a large-scale genetic investigation of pine and spruce orchards across Sweden using genotyping-by-sequencing. We genotyped 3,300 seedlings/trees from six orchards and 10 natural stands to gain an overview of mating structure and genetic diversity in orchard crops. We found clear differences in observed heterozygosity (HO) and background pollen contamination (BPC) rates between species, with pine orchard crops showing higher HO and BPC than spruce. BPC in pine crops varied from 87% at young orchard age to 12% at mature age, wherease this rate ranged between 27%−4% in spruce crops. Substantial variance in parental contribution was observed in all orchards with 30%−50% parents contibuting to 80% of the progeny. Selfing was low (2%−6%) in all seed crops. Compared to natural stands, orchard crops had slightly lower HO but no strong signal of inbreeding. Our results provide valuable references for orchard management.