Site index curves, developed for black, white, scarlet, and chestnut oaks using stem analysis data, reveal that each species has a different pattern of height growth, and polymorphic height growth patterns exist for different levels of site quality. Each species curve reveals better height growth after 60 years than predicted by presently used harmonized site index curves. White oak in particular maintains good height growth after 100 years of age. Forest Sci. 18:109-120.