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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.

Extensive gene flow can be detrimental to local adaptation and similarly, forestry seed sources such as seed orchards can be heavily influenced by external pollination, especially if the orchard material has been translocated a great distance. Here we conducted a coordinated genotyping-phenotyping study to examine how external pollination events and fecundity variation in a Pinus sylvestris seed orchard influence the genetic composition and the seed-lots’ autumn frost hardiness when genetic material had been translocated 630 km south. The results were then compared to those of a in situ established seed orchard. We genotyped and phenotype >1000 seedlings from these orchards, and constructed their pedigrees and scored their autumn frost tolerance in a controlled climate chamber environment. The hardiness scores were compared with a reference of nine natural stands along a latitudinal cline. We find substantial variation in fecundity and external pollination over crop years, thus unpredictable genetic composition because the contribution of some orchard clones is high in one crop but low in another. We observed that seedlings produced by mating among orchard genotypes were less hardy than expected (corresponding to an origin of −0.6°N) but the opposite in externally pollinated seedlings (+0.3 to +0.7°N). The freeze damage levels reflect the origin of parental genotypes, but to a smaller degree than expected (13% lower than expected damage levels for externally pollinate seedlings and 21% greater damage levels for internally pollinates seedlings). These results suggest that orchard parents’ origins, mating composition and orchard local environment could all affect the seed crops’ quality and their climate adaptation. Seed orchard crops are the key to realize the gain in forestry from breeding efforts. However, genetic monitoring of seed crops is necessary to improve the performance of seed orchards further and adjust deployment areas of seed crops in a timely manner for a more dynamic forestry, considering climate change and biodiversity demands.

Effective breeding requires multiplying desired genotypes, keeping them at a convenient location to perform crosses more efficiently, and building orchards to generate material for reforestation. While some of these aims can be achieved by conventional grafting involving only rootstock and scion, topgrafting is known to deliver all in a shorter time span. In this study, Scots pine scions were grafted onto the upper and lower tree crowns in two clonal archives with the aim of inducing early female and male strobili production, respectively. Their survival rates and strobili production were analyzed with generalized linear mixed models. Survival was low (14%) to moderate (41%), and mainly affected by the topgraft genotype, interstock genotype, crown position and weather conditions in connection with the grafting procedure. Survival was not affected by the cardinal position in the crown (south or north). Male flowering was ample three years after grafting and reached 56% in the first year among live scions, increasing to 62 and 59% in consecutive years. Female flowering was scarce and was 9% at first, later increasing to 26 and 20% of living scions but was strongly affected by the topgraft genotype. In one subset of scions, female flowering was observed 1 year after grafting. Overall, flowering success was mainly affected by the topgraft and interstock genotypes, and secondary growth of scions. This is one of few reports on topgrafting in functional Scots pine clonal archives.