Rootstock effects on scion growth and reproduction in 8-year-old grafted loblolly pine

Contributions of sites, rootstocks, and scion clones (and the interactions between these factors) to scion growth and reproduction were studied in loblolly pine (Pinus taeda L.). Twenty-five full-sib families were used as rootstocks. Six scion clones were grafted on them in all combinations. Study sites were three seed orchards in the southeastern United States; trees were measured 8 years after grafting. Scion elongation, diameter at breast height (DBH) of the main leader, crown diameter, cone counts, and number of male and female strobili were measured at all sites. Crown diameter and tree height were combined into a crown volume index. Highly significant differences existed among scion clones for all traits. When analysed over all three sites, the only trait with significant rootstock effects was DBH. There were significant rootstock x scion interactions for total scion elongation, crown volume index, and DBH and significant site x scion interactions for strobili counts and cone count. In analyses by close, rootstock effects were expressed differently in the different clones. There were significant rootstock effects for three traits in each of three clones and no effects at all in two clones. For all traits the largest variance component was for either site or scion clone. Components for rootstocks were small or zero. These results show that the scion clone and site have more control on scion growth and reproduction of grafted loblolly pine than the rootstock. The study did not identify full-sib families that (as rootstocks) would significantly reduce scion elongation or significantly increase cone production, for all six scion clones at all three sites.