Effects of Soil Water and Nitrogen on the Stand Volume of Four Hybrid Populus tomentosa Clones

With the aim of improving poplar timber production, a successive 8-year irrigation and fertilization factorial experiment with three blocks was designed to measure the response of Populus tomentosa stands to water and nitrogen in Huabei Plain, China. Specifically, we examined the responses of four P. tomentosa clones (P. tomentosa BT17, S86, B331, and 1316) to three irrigation levels (45%, 60%, and 75% above field capacity), as irrigation thresholds, and four N levels (0, 80, 160, and 240 g per plant). The results showed that both irrigation and nitrogen had significant effects in terms of improving clone stand volume. Further, we demonstrated positive interactions between irrigation and nitrogen. The stand volume increment of the four hybrid clones varied from 104.53 ± 19.84 to 191.35 ± 30.56 m3/ha in the descending order S86 > B331 > BT17 > 1316. With increasing irrigation level, the average stand volume of the four clones increased significantly from 120.46 ± 5.23 to 158.53 ± 21.72 m3/ha. When nitrogen level was increased from 0 to 240 g/plant, the average stand volume increment of the four clones increased from 126.04 ± 8.75 to 156.16 ± 26.01 m3/ha, respectively. Our results suggest that a comprehensive and specific management program is needed to improve poplar timber production.