Single-genus groupings include apples, azaleas, boxwoods, dogwoods, hollies, magnolias and maples. The Eastern Himalayan region consists of oaks, laurels, maples, rhododendrons, alder, birch and dwarf willows. Farther north, tulip trees are gradually replaced by maples and birches. Examples include grasses, birch trees, ragweed and maples. The pea-gravel driveway is lined with maturing pin oaks and red maples. Birnam Wood was represented by sycamore maples from Scotland. Acer is a genus of trees and shrubs commonly known as maples. In addition, the dense canopy of Norway maples can inhibit understory growth. Other maples, especially smaller or more unusual species, are popular as specimen trees. Unlike many other maples, mature trees do not tend to develop a shaggy bark. Most maples are trees growing to a height of . Vermont's hills reveal red, orange, and (on sugar maples) gold foliage as cold weather approaches. Red maple produces one of the smallest seeds of any of the maples. Examples include horse chestnut, maples, ackee and lychee. The closest relatives of the maples are the horse chestnuts.