Nutrient Distribution Indicated Whole-Tree Harvesting as a Possible Factor Restricting the Sustainable Productivity of a Poplar Plantation System in China

We evaluated the biomass and contents of five major macronutrients (N, P, K, Ca and Mg) in 10-year-old poplar trees (Populus deltoids Bartr. cv. "Lux"), and determined their nutrient use efficiencies (NUEs) at Zhoushan Forestry Farm (32°20' N, 119°40' E), Jiangsu province, in eastern China. The above- and below-ground biomass of poplar trees was 161.7 t ha(-1), of which 53.3% was stemwood. The nutrient contents in the aboveground part were as follows: 415.1 kg N ha(-1), 29.7 kg P ha(-1), 352.0 kg K ha(-1), 1083.0 kg Ca ha(-1), and 89.8 kg Mg ha(-1). The highest nutrient contents were in stembark, followed by branches, roots, stemwood, and foliage. The NUEs of the aboveground parts of poplar for N, P, K, Ca and Mg were 0.313, 4.377, 0.369, 0.120, 1.448 t dry biomass kg(-1) nutrient, respectively, while those of stemwood were 1.294, 33.154, 1.253, 0.667, and 3.328 t dry biomass kg(-1), respectively. The cycling coefficients, defined as the percentage of annual nutrient return in annual nutrient uptake, of N, P, K, Ca and Mg for the aboveground part were 87, 95, 69, 92, and 84%, respectively. Based on the NUE and nutrient cycling characteristics, shifting from whole-tree harvesting to stemwood-only harvesting and appropriately extending the harvest rotation could prevent site deterioration and support sustainable productivity of poplar plantation systems.