GROWTH-RESPONSE OF YOUNG, THINNED DOUGLAS-FIR STANDS TO NITROGEN-FERTILIZER IN RELATION TO SOIL PROPERTIES AND TREE NUTRITION

Application of 224 kg N/ha to young, thinned stands of Douglas-fir (Ps eudotsuga menziesii (Mirb.) France) at 35 sites in western Oregon and Washington significantly increased basal area and volume increment ove r 8 years following treatment. However, response varied considerably b etween sites, and relative volume increment exceeded 10% at only 19 of the 35 sites. Response to applied N was evaluated in relation to fore st floor and soil variables as well as to levels of N in foliage. Rela tive responses in basal area and volume were significantly correlated with total N concentration and the C/N ratio of the soil. However, the se relationships explained only part (18-22%) of the observed variatio n in response. In contrast, relative response was strongly correlated with the level of N in the foliage of nonfertilized trees at 11 sites, accounting for 94% of the variation between sites. Use of foliar N da ta clearly has potential to predict growth responses to N fertilizatio n of young thinned Douglas-fir stands, although further work is needed to test the relationship for a wider range of sites and stands.