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. & Gray x Populus deltoides Marsh. hybrids was exami ned. The nitrogen-treated trees received a total of 500 kg N.ha-1 in t hree equal applications at the beginning of the second, third, and fou rth growing seasons. Annual growth in height and diameter were signifi cantly increased by nitrogen only in the third growing season. Mean st em dimensions and productivity after four growing seasons for fertiliz ed 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 m 2.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 yea rs by 21%. All values were significantly different according to analys is of variance except total height. Based on the lack of fertilizer re sponse in the fourth year, two annual applications of nitrogen in the second and third growing seasons may have been enough to obtain the me asured growth response. Competition among trees at the relatively clos e spacing may have limited the ability of these very fast growing tree s to respond to the third nitrogen application. Nitrogen appeared to s horten the time required to reach culmination of periodic annual incre ment. At the wider spacing now recommended for these plantations (40% greater than used in this study), the culmination of periodic annual i ncrement would be delayed and nitrogen responses may be of longer dura tion. Clones differed significantly from each other in all measures of growth except annual height growth, but generally, the clones respond ed similarly to nitrogen since the clone x nitrogen interactions were not significant for most of the parameters studied. Despite similar re sponse to nitrogen treatment, the clones appear to differ in efficienc y of nitrogen use efficiency. The three most productive clones produce d more without added nitrogen than the three least productive did with added nitrogen. Consequently, the top three clones appear to be more efficient in using nitrogen (and other needs supplied by the site) eve n though the top clones also responded positively to increased nitroge n.