Conifers (Picea and Abies spp.) have replaced aspen (Populus tremuloides Michx.) over much of aspen's historic range in the western United States. We measured the impact of this change upon the production of understory vegetation potentially useful as forage for livestock and wildlife on two southern Utah national forests. A negative exponential relationship between conifer cover and understory biomass was demonstrated as log(biomass)  =  6.25 − 0.03787(% conifer), adjusted R²  =  0.57. Understory production in aspen stands begins to decline under very low levels (10% to 20%) of conifer encroachment. Management implications include loss of forage production capability and wildlife habitat and potential overstocking of livestock grazing allotments if the associated loss of forage is not considered.