First-order root development from cuttings of Populus trichocarpa x P. deltoides hybrids

We studied first-order root development of field-grown plants of 44 F(1) Populus trichocarpa x P. deltoides hybrids as well as 15 clones of P. trichocarpa (T) and 20 clones of P. deltoides (D) during their first growing season. The cuttings were planted in the field on May 11-12, 1988 and destructively sampled in July, August and September. By September 12, hybrids had an average of 50 first-order roots per cutting (range = 32 to 85) with an average of 25% dead roots (range = 10 to 51%). The longest roots of hybrids averaged 107 cm (range = 58 to 182 cm) and originated at the base of the cutting. Roots originating at the side of the cutting averaged 54 cm in length (range = 12 to 96 cm). For most aboveground and root parameters measured, the values for hybrids exceeded those for both parent species clones. For all clones, aboveground weights were positively correlated with weights of collected roots (R(2) = 0.66 to 0.86). In contrast, aboveground weights were negatively (but poorly) correlated with root/shoot ratios (R(2) = 0.08 to 0.21). Roots differed allometrically depending on their location on the cutting. Total weight of roots originating along the sides of the cutting was not correlated with aboveground weight (R(2) = 0.001 to 0.27), whereas total weight of roots originating at the base of the cutting was correlated with aboveground weight (R(2) = 0.52 to 0.80). Basal roots dominated the first-order root systems of most clones in terms of maximum length and total mass, but not in total numbers of roots.