Matching site-suitable poplars to rotation length for optimized productivity

Abstract

Diversity of applications, productivity potential, broad suitability and genetic variations make Populus a valuable fast-growing genus. Our goal was to assess if clonal site-suitability varies with rotation-length. We examined survival, growth (height, diameter at breast height) and estimated stem and total-wood (stem and branches) biomass of 89 clones near Fountain, North Carolina (35°42′7.52″ N, 77°34′35.04″ W) in the coastal southeastern USA at four- and eight-year rotations. The unsuitability of some clones was evident at early age while other clones became less suitable with stand age. Specifically, most mortality occurred by year-four, yet 25% clones experienced 17 to 50% mortality at older ages. Clone ‘379’ was the most site-suitable with 100% survival and 141.3 kg total-wood per tree (approximately 47.5 Mg ha−1 yr−1). Moreover, several clones with low survival produced high per-hectare biomass. Biomass (stem and total-wood) rankings changed between four- and eight-year rotations with only three top-ten clones in year-four (‘379’, ‘402’, ‘449’) in the top-ten of year-eight and two top-ten clones in year-eight (‘379’, ‘402’) also in the top-ten of year-four. Clonal productivity differences increased by 25 to 836% with age. Clones of TD (Populus trichocarpa Torr and Gray × P. deltoids Barts Ex Marsh) and DD (P. deltoides × P. deltoides) genotypes were affected by wood infection (Septoria musiva) indicating that selection based on disease resistance should be performed at clonal level. Hence, for productivity-focused stands, site-suitable clones should be selected by productivity first, then narrowed by survival and rotation length. Changes in the most ‘site-suitable’ clones can be expected between longer and shorter rotations.