Complex sentence: 'Amsterdam' is an attractive small tree, generally columnar in shape, its leaves, which are of average size for an elm, turning a rich yellow in autumn and remaining attached to the tree for a period much longer than is usual for the species.
Split into simpler sentences: 'Amsterdam' is an attractive small tree, generally columnar in shape. The leaves, which are of average size for an elm, turn a rich yellow in autumn and remaining attached to the tree for a period much longer than is usual for the species.