Thinning effects on stand and dominant trees development in a second rotation hybrid poplar plantation

Abstract Poplar trees ( Populus species and their hybrids) are commonly cultivated on agricultural land for their high biomass production during the first rotation. However, stand development during the long-term second rotation is less understood. This study evaluated the effects of a single thinning at age 7 on growth dynamics at both stand and dominant trees levels in a second rotation poplar plantation (clone OP42) in southern Sweden. Four treatments were applied: unthinned (6000 stems ha⁻ 1 ), light thinning (3000 stems ha⁻ 1 ), medium thinning (1100 stems ha⁻ 1 ), and heavy thinning (550 stems ha⁻ 1 ). Tree mortality, height, diameter, and stem volume/biomass were monitored over 11 years post-thinning. By age 18, the above ground volume/biomass at the stand level was primarily concentrated in trees with diameters greater than 21 cm. The standing volume/biomass of individual dominant tree ranged from 0.80 m 3 (268 kg dry matter, DM) to 0.87 m 3 (292 kg DM) in the medium and heavy thinnings, and from 0.64 m 3 (214 kg DM) to 0.67 m 3 (224 kg DM) in the unthinned and lightly thinned plots. The total stand-level volume/biomass production in unthinned and lightly thinned plots was maximized by an 18-year rotation, with the mean annual increment (MAI) culminating at 33 m 3 ha −1 yr −1 (11 Mg DM ha −1 yr −1 ). In the medium thinning, MAI reached 38 m 3 ha⁻ 1 yr⁻ 1 (12.7 Mg DM ha⁻ 1 yr⁻ 1 ) at age 18 and continued to increase thereafter, a trend also observed in the heavy thinning. For dominant trees, culmination of MAI had not yet occurred under any treatment.