Biomass Production of the Poplar Clone OP42 During the Second Rotation Plantation–The Effects of Four Thinning Treatments

Poplar (Populus species and their hybrids) plantations can produce large amounts of biomass on agricultural land during the first rotation. However, there is limited knowledge regarding plantation re-establishment through re-sprouting (second rotation) after harvest, stand management options for such plantations, and biomass production during rotation length up to 20 years. In this study, we analysed biomass production responses to thinning treatments in an 18-year-old second rotation poplar plantation in Southern Sweden. The first rotation plantation was established with clone OP42 (Populus maximowiczii A. Henry × P. trichocarpa Torr. and Gray). The thinning experiment was conducted seven years after the first rotation harvest, comprising four treatments: unthinned – 6000 stems ha⁻¹, light thinning – 3000 stems ha⁻¹, medium thinning – 1100 stems ha⁻¹, and heavy thinning – 550 stems ha⁻¹. Eleven years after thinning, standing volume/biomass reached 484 m³ ha⁻¹ (162 Mg DM ha⁻¹) in the unthinned and medium thinning plots, 443 m³ ha⁻¹ (148 Mg DM ha⁻¹) in lightly and 338 m³ ha⁻¹ (113 Mg DM ha⁻¹) in heavily thinned plots. The mean annual increment was not different among the unthinned, light, and medium thinnings, 26 m³ ha⁻¹ yr⁻¹ (9 Mg DM ha⁻¹ yr⁻¹). The total production, including living, dead and removed trees, was highest following the medium thinning, 695 m³ ha⁻¹ (233 Mg DM ha⁻¹). Gradual self-thinning in the unthinned and lightly thinned plots was increased by a drought period. Overall, this study suggests that the second rotation of poplar plantations has high biomass production and provides an alternative to planting after harvest.