Abstract
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 3300 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 (<i>H</i><sub>O</sub>) and background pollen contamination (BPC) rates between species, with pine orchard crops showing higher <i>H</i><sub>O</sub> 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 <i>H</i><sub>O</sub> but no strong signal of inbreeding. Our results provide valuable references for orchard management.
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