Variation in response to nutrition in a three-generation pedigree of Populus

Response to nitrogen (N) and phosphorus (P) was examined in a three-generation pedigree of Populus in 1996 and 1997. In 1996, the first-generation parental clones female Populus trichocarpa Torr. & Gray and male Populus deltoides Bartr. and second-generation male and female F1 hybrids were grown in five treatments of varying N:P ratio. In 1997, 29 third-generation F2 clones and the first-generation clones were grown with high or low N and P. Variability in response to nutrients existed among and within generations. Growth increased with N supply but not with P supply because of N limitation. On average, the male F1 hybrid had the greatest growth and biomass per unit foliar N and P, while the female F1 hybrid and P. trichocarpa were intermediate. Populus deltoides was least productive in 1996. Most F2 clones had poorer growth than the first-generation clones, but some F2 clones were as productive and variation was large. Clones 331-1078 and 331-1122 produced at least as much biomass and biomass per unit foliar N and P as P. trichocarpa, the most productive first-generation clone. With larger screening trials, clones combining high growth rates with low nutrient requirements may be found and selected for use on nutrient-poor sites.