Above- and below-ground biomass and fine roots of 4-year-old hybrids of Populus trichocarpa X Populus deltoides and parental species in short-rotation culture

Mean annual aboveground leafless biomass production averaged 14.8, 11.4, and 24.3 Mg ha-1 year-1 at harvest at 4 years of age for Populus trichocarpa Torr. & Gray. Populus deltoides Marsh., and P. trichocarpa x P. deltoides hybrids, respectively. These trees were planted at 1 x 1 m spacing on a medium- to coarse-textured alluvial soil in western Washington. Branches accounted for 13.2-20.3% of the aboveground weight. Total weight of stumps and coarse roots at harvest varied from 12.3 to 29.6 Mg ha-1, or 22-33% of the weight of aboveground leafless biomass. Small and fine roots sampled to a depth of 3.17 m using soil cores amounted to an additional 6.6-11 Mg.ha-1 of roots. Stumps and all roots as a ratio of aboveground biomass (root/shoot ratio) ranged from 0.34 to 0.42, with hybrids accounting for the entire range of values present. Mass of the fine roots (less than 0.5 mm diameter) ranged from 4.0 to 6.5 Mg ha-1, or an average of 6.8% of the aboveground biomass. The smallest of the fine roots measured 0.06 mm in diameter. Specific root length of fine roots averaged 50.7 m g-1 for P. deltoides, 42 m g-1 for P. trichocarpa, and 30-47 m g-1 for hybrids. Total length of fine roots to a depth of 3.17 m ranged from 179 000 to 284 000 km-ha-1. Density of fine roots by length per unit soil volume was greatest at the surface with the range of means for clones in the top 0.18 m being 2.4-6.3 cm cm-3: at 1.0-3.17 m soil depth, density was 0.02-0.6 cm-cm-3. For two of the hybrid clones, the density of fine roots at the soil surface was half that of the other clones. The distribution of fine roots in the stratified soil profile was correlated with soil depth, Kjeldahl N, and organic matter, with the latter two parameters showing the highest coefficients of determination (0.73 and 0.71, respectively). In the more sandy but layered subsoil (0.36-3.17 m depth), soil depth, Kjeldahl N, and sand content were most strongly correlated with fine root density, with depth and sand content giving the highest coefficients of determination (0.32 and 0.31, respectively). Roots in sandy subsoil were coarser and much less branched than in adjacent finer textured layers.