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

Forest fertilization has been successful in Douglas-fir [Pseudotsuga menziesii (Mirbel) Franco] stands in the Pacific Northwest, but little information is available on fertilizer responses of true firs in the north-western USA. This study was initiated to evaluate the effects of fertilizer treatments in young managed stands of true firs across a range of sites. Single-tree fertilization screening trials were established in 22 young stands of noble fir (Abies procera Rehder) and Pacific silver fir (A. amabilis Dougl. ex J. Forbes) in the Pacific silver fir zone in western Washington and Oregon. Fertilizer treatments included N applied at rates of 224 and 448 kg N ha -1 as NH4NO3 and urea, alone and in combination with P, S, and micronutrients. Unit needle weights and foliar N and P concentrations were determined after one growing season, and tree growth was measured for 4 yr. First year foliar N concentrations and needle mass and 4-yr tree basal area growth was significantly greater (P = 0.05) for all fertilizer treatments than for controls for both species. Significant increases in foliage mass and nutrient content indicate that growth increases due to added N are likely to continue for several more years. Further work is needed to extend favorable results from these single-tree trials to stand-level response estimates.