The trunk-formula appraisal method written by the Council of Tree and Landscape Appraisers (CTLA) involves measuring the cross-sectional trunk area of the tree at breast height and that multiplying that by the unit cost of trunk area sold at the nursery and three depreciating factors. In recent revisions of the CTLA method, criticism was raised that large trees increased too rapidly in appraised value with small increases in size; each unit increase in diameter of a circle results in a progressively greater increase in its area. To address this phenomenon, the CTLA wrote the Adjusted Trunk Area formula to gradually diminish the effect of increases in trunk diameter on the appraised value of the tree. This formula has since been published in their 9^th edition of the book Guide for Plant Appraisal.

This formula has one major weakness: It is a downward-sloping quadratic equation that does not behave in the same way as the data it is intended to model. In this research, I will illustrate the problems with this formula and propose two potential solutions. Rather than a quadratic formula, a logistic or a natural log formula should be used. While a quadratic formula eventually decreases in value, a logistic function approaches a maximum value and a natural log formula continues to increase but does so at a diminishing rate. Both of these alternatives are offered for the Adjusted Trunk Area formula.