Existing vegetation. Trees and shrubs which are within the required rear or side setback area of the development parcel and which are adjacent to any abutting agricultural, institutional or residential lot shall not be disturbed. The existing trees and shrubs shall be utilized to satisfy the required buffer area planting required herein; provided, however, that the performance standards regarding the height and mix of types of plantings are met. In the event the existing trees and shrubs do not satisfy the buffer requirements, the applicant shall use new plantings in combination with the existing plantings to satisfy the buffer requirements. Further, there shall be no grading of the existing slope within the required rear setback area, except for the planting of trees or shrubs required to satisfy the buffer requirements. If such grading is required, it shall not increase the existing slope or contour of the ground within the required buffer area.