Influence of nitrogen on growth and productivity of short-rotation Populus trichocarpa X Populus deltoides hybrids

The nitrogen fertilization response of six generally unrelated Populus trichocarpa Torr. and Gray X Populus deltoides Marsh. hybrids was examined. The nitrogen-treated trees received a total of 500 kg N.ha-1 in three equal applications at the beginning of the second, third, and fourth growing seasons. Annual growth in height and diameter were significantly increased by nitrogen only in the third growing season. Mean stem dimensions and productivity after four growing seasons for fertilized and control plots, respectively, were as follows: total height, 13.2 vs. 12.6 m; DBH, 11.2 vs. 10.1 cm; total basal area, 20.1 vs. 16.6 M2.ha-1; mean annual volume growth (inside bark). 22.2 vs. 18.4 m3.ha-1.year-1; and mean annual woody biomass production (above ground), 25.3 vs. 20.9 Mg.ha-1.year-1. Nitrogen increased yield at the end of 4 years by 21%. All values were significantly different according to analysis of variance except total height. Based on the lack of fertilizer response in the fourth year, two annual applications of nitrogen in the second and third growing seasons may have been enough to obtain the measured growth response. Competition among trees at the relatively close spacing may have limited the ability of these very fast growing trees to respond to the third nitrogen application. Nitrogen appeared to shorten the time required to reach culmination of periodic annual increment. At the wider spacing now recommended for these plantations (40% greater than used in this study), the culmination of periodic annual increment would be delayed and nitrogen responses may be of longer duration. Clones differed significantly from each other in all measures of growth except annual height growth, but generally, the clones responded similarly to nitrogen since the clone X nitrogen interactions were no significant for most of the parameters studied. Despite similar response to nitrogen treatment, the clones appear to differ in efficiency of nitrogen use efficiency