YOUNG TRUE FIR TREES RESPONSE TO NITROGEN-FERTILIZATION IN WESTERN WASHINGTON AND OREGON

Forest fertilization has been successful in Douglas-fir [Pseudotsuga m enziesii (Mirbel) Franco] stands in the Pacific Northwest, but little information is available on fertilizer responses of true firs in the n orth-western USA. This study was initiated to evaluate the effects of fertilizer treatments in young managed stands of true firs across a ra nge of sites. Single-tree fertilization screening trials were establis hed in 22 young stands of noble fir (Abies procera Rehder) and Pacific silver fir (A. amabilis Dougl. ex J. Forbes) in the Pacific silver fi r zone in western Washington and Oregon. Fertilizer treatments include d N applied at rates of 224 and 448 kg N ha -1 as NH4NO3 and urea, alo ne and in combination with P, S, and micronutrients. Unit needle weigh ts and foliar N and P concentrations were determined after one growing season, and tree growth was measured for 4 yr. First year foliar N co ncentrations and needle mass and 4-yr tree basal area growth was signi ficantly greater (P = 0.05) for all fertilizer treatments than for con trols for both species. Significant increases in foliage mass and nutr ient content indicate that growth increases due to added N are likely to continue for several more years. Further work is needed to extend f avorable results from these single-tree trials to stand-level response estimates.