Growth and biomass of two poplar clones in short rotation coppice under different densities

  Objective  In the cultivation process of energy forest, the cultivation pattern of short rotation coppice (SRC) has attracted attention from various countries due to its excellent performance in productivity and environment. However, the influence of different planting densities and management modes on its growth and productivity still needs further discussion. The purpose of this study was to investigate the difference in growth, productivity, and sustainability of two poplar clones’ short rotation coppice without later artificial water and fertilizer management.

  Method  (P. tomentosa × P. bolleana) × P. tomentosa (hereinafter referred to as “B301”) and Populus × euramericana cv.‘74/76’ (hereinafter referred to as “107”) planted in Gaotang County, Shandong Province of eastern China were used as experimental materials, with three planting densities (5 000, 10 000, 20 000 plant/ha) and three different rotation cycles (2, 3, 4 years). It took 6 years to study the survival rate, shoot diameter (D₂₂) at 22 cm aboveground, number of shoots per plant, and biomass per unit area during each cycle and during 6 years.

  Result  (1) The survival rate of the two species was higher in two-year rotation cycle, and lower in the planting density of 20 000 plant/ha. The maximum value of survival rate was 98.89%, which appeared in the second round of B301 of 2-year rotation and the planting density of 10 000 plant/ha. After six years of short-rotation management, the survival rate decreased significantly in all treatments of two- and three- year rotation cycle, with the survival rate of B301 remained above 66%, and that of 107 above 58%. (2) Both the average D₂₂ and the average stoop per stump of the two clones were larger in the lower planting density. The average D₂₂ was larger in the longer rotation and the first rotation, while the average stoop per stump was larger in the 2- and 3-year rotation and the last two rotations. The largest average D₂₂ of the two clones appeared in the first rotation of the four-year rotation and the planting density of 5 000 plant/ha. It could reach 5.83 cm in B301, while it could reach 7.32 cm in 107. The average stoop per stump of B301 ranged from 1.10 to 5.52, and that of 107 ranged from 1.14 to 9.55, and 107 has better resprout capacity. (3) Productivity of short-rotation coppice planted under this experimental conditions in North China: in a single rotation cycle, the average annual biomass per unit area was 3.36−13.42 t/(ha·year) for B301 and 4.58−14.45 t/(ha·year) for 107. During 6 years, the average annual biomass per unit area of B301 ranged from 5.10 to 9.95 t/(ha·year), while that of 107 ranged from 6.22 to 10.69 t/(ha·year). The total biomass per unit area was 30.63 − 59.68 t/ha for B301 and 37.32 − 64.17 t/ha for 107.

  Conclusion  (1) Without artificial water and fertilizer management, the productivity of 2 species of short-rotation poplar coppice is the highest at the first rotation, and declines with the increase of the number of rotations. (2) To build SRC in this region, 107 is superior to B301 on the whole in order to obtain higher productivity. (3) Considering the actual planting and management costs in operation, the rotation period of 3 years and the plant density of 10 000 plant/ha are the more suitable rotation and planting density for these two poplar clones.