Ten-Year Effect of Fertilization on Tree Growth and Mortality Associated with Armillaria Root Disease, Fir Engravers, Dwarf Mistletoe, and Western Spruce Budworm in Northeastern Oregon

In October 1988, each of four randomly selected 4 ha plots within four 16 ha blocks in a mixed-conifer forest in northeastern Oregon received one of the following treatments by helicopter: (1) urea at 350 kg/ha of N, 2) 100 kg/ha N, 25 kg/ha P, 25 kg/ha K, 25 kg/ha S, and 3) 300 kg/ha N, 75 kg/ha P, 75 kg/ha K, 75 kg/ha S, or 4) untreated control. Ten years after treatment, grand fir mortality within plots averaged 37% (range 4 to 56%) of the sampled trees. Fir mortality was associated with fir engraver beetles, flatheaded fir borers, Armillaria root disease, and defoliation by western spruce budworm. Larch mortality ranged from zero for trees with no or low dwarf mistletoe infection to 56% in trees with severe infection. After 10 yr, there were no significant fertilizer effects on grand fir and western larch with respect to (1) incidence of mortality; (2) diameter increment; (3) vigor as assessed by cambial electrical resistance (CER); (4) live crown ratio; and (5) larch dwarf mistletoe severity rating (DMR). DMR significantly affected larch diameter increment, incidence of mortality, and CER regardless of treatment. Possible reasons why fertilization in this experiment did not have the same effects as fertilization in smaller studies are discussed.