Typical trees of these forests are sugar maple ("Acer saccharum"), American beech ("Fagus grandifolia"), eastern hemlock ("Tsuga canadensis"), Carolina silverbell ("Halesia tetraptera"), tulip poplar ("Liriodendron tulipifera"), red maple ("Acer rubrum"), white oak ("Quercus alba"), northern red oak ("Quercus rubra"), yellow birch ("Betula alleghaniensis"), yellow buckeye ("Aesculus flava"), and basswood ("Tilia americana"). Oaks gain numbers on drier sites. Southern Appalachian low-elevation pine forests occur on a variety of topographic and landscape positions, including ridgetops, upper- and mid-slopes, and in lower elevations (generally below ) such as mountain valleys. These forests are dominated by shortleaf pine ("Pinus echinata") and Virginia pine ("Pinus virginiana"). Pitch pine ("Pinus rigida") may sometimes be present. Hardwoods are sometimes abundant, especially dry-site oaks such as southern red oak ("Quercus falcata"), chestnut oak ("Quercus prinus"), and scarlet oak ("Quercus coccinea"), but also pignut hickory ("Carya glabra"), red maple ("Acer rubrum"), and others. The shrub layer may be well-developed, with hillside blueberry ("Vaccinium pallidum"), black huckleberry ("Gaylussacia baccata"), or other acid-tolerant species most characteristic. Herbs are usually sparse but may include narrowleaf silkgrass ("Pityopsis graminifolia") and goat-rue ("Tephrosia virginiana"). Southern Appalachian oak forests, widespread in the southeastern United States, occur on dry, upland sites on southern and western aspects and ridgetops. The composition of these forests varies throughout their range but often includes chestnut oak ("Quercus prinus"), northern red oak ("Quercus rubra"), eastern black oak ("Quercus velutina"), white oak, ("Quercus alba"), and scarlet oak ("Quercus coccinea"). Hickories such as bitternut ("Carya cordiformis"), shagbark ("Carya ovata"), and mockernut ("Carya tomentosa"), are found here, as are black tupelo ("Nyssa sylvatica"), red maple ("Acer rubrum"), white pine ("Pinus strobus"), and white ash ("Fraxinus americana").