Variable credit shall be allowed for preservation of heritage trees, as defined in the adopted Eugene Urban Forest Management Plan. The planning and development director shall determine the value of the Heritage Tree, according to formulas established in Valuation of Landscape Trees, Shrubs and Other Plants (International Society of Arboriculture) and shall relieve the contractor/developer from planting a number of trees having a value of up to 50 percent of the value of the Heritage Tree preserved.