Plant materials. Required plantings shall include both trees and shrubs, and may include ones existing on the site. To be credited towards meeting these requirements, trees must be at least 2 1/2 inches caliper four feet above grade, be of a species common in the area, and be ones which reach an ultimate height of at least 30 feet. To be credited towards meeting these requirements, shrubs must be at least 24 inches in height at the time of building occupancy, reach an ultimate height of at least 36 inches, and be of a species common in the area.