Introduction

The term woodland is used for a variety of shrub- and tree-dominated communities on sites that are not favorable for the development of a forest, whereas the term savanna implies a grass/forb-dominated community with trees at a lower density than expected in the forests of the area. Tree-dominated woodlands are rare across the western Great Plains of North America, occurring mostly along rivers and occasionally on rocky escarpments. Various broad-leaved trees dominate the riparian zone, especially plains cottonwood (Populus deltoides) (plant nomenclature follows Kartesz 1994), boxelder (Acer negundo), green ash (Fraxinus pennsylvanica), and peachleaf willow (Salix amygdaloides). On escarpments, ponderosa pine (Pinus ponderosa var. scopulorum) is common (Figure 15.1). Associated species include Rocky Mountain juniper (Juniperus scopulorum), skunkbush sumac (Rhus trilobata), big sagebrush (Artemisia tridentata), mountain mahogany (Cercocarpus montanus), little bluestem (Schyzachyrium scoparium), and sideoats grama (Bouteloua curtipendula) (Hansen and Hoffman 1988). Bur oak (Quercus macrocarpa) is a common associate in some areas, such as in the Black Hills of South Dakota (Hoffman and Alexander 1987). Near the Rocky Mountains, ponderosa pine often occurs beyond the rocky escarpments, forming parklike savannas on the deeper, finer-textured soils of the adjacent grassland. Limber pine (Pinus flexilis) sometimes occurs with ponderosa pine, but usually it appears at higher elevations where the climate is drier and colder (Peet 1988).

Similarly, desert shrublands of the Southwest grade into woodlands near the mountains or on plateaus. Woodlands dominated by various species of oak (Quercus spp.), especially Gambel oak (Q. gambelii), are common in some areas (Dick-Peddie 1993).