Early rooting of dormant hardwood cuttings of Populus: Analysis of quantitative genetics and genotype x environment interactions

Rooting of hardwood cuttings is under strong genetic control, although genotype x 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 x environment interactions to refine clonal recommendations. The clones belonged to five genomic groups ((Populus trichocarpa Torr. & Gray x Populus deltoides Bartr. ex Marsh.) x P. deltoides 'BC'; P. deltoides 'D'; P. deltoides x Populus maximowiczii A. Henry 'DM'; P. deltoides x Populus nigra L. 'DN'; and P. nigra x P. maximowiczii 'NM'). Cuttings, 20 cm long, were planted in Iowa and Minnesota in randomized complete blocks at 1.2 m x 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 x 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.