Interactions between effects of insect defoliators on tree physiology and growth and soil nutrient and water availability are poorly understood. We addressed whether the western spruce budworm influences nutrient and water relations, ectomycorrhizae, leaf gas exchange, and growth of Douglas-fir seedlings under different environmental conditions. Four-year-old, potted seedlings were grown in two soil types (basalt-derived, limestone-derived) with two levels of soil moisture (high, low) and were subjected to three levels of budworm defoliation (none, moderate, heavy) and two levels of frass and litter produced by budworm feeding (with, without) over 2 yr in a greenhouse environment. Defoliation, soil moisture, and soil type influenced allocation of seedling biomass to leaves, stems, and roots. The addition of frass and litter had no effect on any measured soil or tree characteristic. Interactions between defoliation and other experimental factors were not important except for the defoliation x soil moisture interaction where heavy defoliation reduced the negative effects of low soil moisture on seedling predawn water potential, net photosynthetic rate, and stomatal conductance. Heavily defoliated seedlings had less biomass, higher foliar concentrations of N, Ca, and Mg, less ectomycorrhizae, and higher net photosynthetic rate and stomatal conductance compared with nondefoliated seedlings. For. Sci. 45(2):280-291.