When fertilizers increase radial growth rates, they also increase the diameter of trees that will be used to compute future basal area responses. If the mean basal area growth per tree is computed for fertilized trees that are larger than the unfertilized trees, the length of time that fertilizers directly affect growth can be overestimated. In this paper, a model that describes mean basal area responses per tree over time as a function of direct nutrient effects and indirect tree-size effects is suggested. Application of the model to a uniform field fertilization experiment shows that after the 1st year, some or all of the average basal area response of individual trees is due to the difference in size between fertilized and unfertilized trees. To evaluate direct fertilizer responses properly, mean basal area growth per tree must be adjusted for the unequal size of fertilized and unfertilized trees.