Early rooting of dormant hardwood cuttings of Populus: analysis of quantitative genetics and genotype × environment interactions

Rooting of hardwood cuttings is under strong genetic control, although genotype × environment interactions affect selection of promising genotypes. Our objectives were (1) to assess the variation in rooting ability among 21 Populus clones and (2) to examine genotype × environment interactions to refine clonal recommendations. The clones belonged to five genomic groups ((Populus trichocarpa Torr. & Gray × Populus deltoides Bartr. ex Marsh.) × P. deltoides ʹBCʹ; P. deltoides ʹDʹ; P. deltoides × Populus maximowiczii A. Henry ʹDMʹ; P. deltoides × Populus nigra L. ʹDNʹ; and P. nigra × P. maximowiczii ʹNMʹ). Cuttings, 20 cm long, were planted in Iowa and Minnesota in randomized complete blocks at 1.2 m × 2.4 m spacing, across three planting dates during 2001 and 2002. We measured nine belowground and aboveground traits from harvested cuttings after 14 days of growth. Percent rooting ranged from 22% to 86%. Broad-sense heritability for root and top dry mass ranged from 0.09 to 0.11 and 0.31 to 0.38, respectively. There were genotype × environment interactions for most traits, with belowground growing degree-days accounting for >54% of environmental variation. Clonal rooting was stable, except at Westport, Minnesota, during 2002, when root growth was relatively poor.