Wayne A. Robbie Chapter 2: Grassland Assessment Categories and Extent Introduction_______________________ extent of these systems. Other classification systems of This chapter establishes a general framework for Southwestern grasslands exist (Barbour and Billings describing the various kinds of grasslands outlined 2000, Brown 1994, Dick-Peddie 1993, Küchler 1964) and in subsequent chapters. This framework outlines the emphasize biogeographic, ecological, and biophysical major categories or classes of grasslands that occur features that are consistent with the scale and level of as part of Southwestern terrestrial ecosystems within generalization being used here. The general distribu- National Forest System lands and provides an ecologi- tion of grasslands for this assessment is located on the cal and environmental context in regards to how they National Forest System lands in Arizona, New Mexico, differ in their floristic, geographic, spatial, and climatic Texas, and Oklahoma (fig. 2-1). settings. More detailed information about these grass- land systems is also presented in chapter 6. Grassland Categories_______________ Grasslands of the Southwest vary according to vegetation, climate, soils, and topography and dis- Descriptions of each grassland assessment category turbance regimes. They are distinctly different from follow. other vegetation assemblages in that the dominant The Desert Grassland encompasses annual and and codominant plants are graminoid species. For perennial graminoid and adjacent shrub communities example, other forbs and shrub plant species occur at low elevations adjacent to the Chihuahuan, Mohave, within the grasslands but are subordinate to grass in and Sonoran deserts. These grasslands occur between the total cover and composition. the Great Basin grasslands, chaparral, and woodland The major grassland categories used in this assess- ecosystems and have been commonly referred to as ment—that is, those categories that represent the major semidesert grasslands by Brown (1994). The distribu- grasslands formations in the Southwestern Region on tion of these grasslands are mainly within the Basin National Forest System lands (Carleton and others and Range, Sonoran-Mohave Desert, Tonto Transition 1991)—are Desert, Plains, Great Basin, Montane, ecoregion sections, and limited areas within the White and Colorado Plateau grasslands. These generalized Mountain-San Francisco Peaks, Northern Rio Grande groupings reflect the geographic and ecological differ- Intermontane, and Sacramento-Monzano Mountain ences that are determined by unique floristic, edaphic, ecoregion sections (McNab and Avers 1994). Desert physiographic, and climatic characteristics. Although grasslands intermingle with desert scrub communities not taxonomic with respect to any vegetation hierar- (Dick-Peddie 1993) and have evolved through natural chy, these categories are intended to aid the reader and anthropogenic successional disturbance processes. in understanding the uniqueness, distribution, and Grass species that are diagnostic to this category include USDA Forest Service Gen. Tech. Rep. RMRS-GTR-135-vol. 1. 2004 11 Grasslands of National Forest System lands in Arizona, New Mexico, Texas, and Oklahoma. Texas, Arizona, New Mexico, Grasslands of National Forest System lands in Figure 2-1. 12 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-135-vol. 1. 2004 black grama (Bouteloua eriopoda), tobosa (Pleuraphis the expansive Great Basin Grassland category based mutica), and curly mesquite (Hilaria belangeri). Other upon recent ecological mapping (Laing and others 1986, key graminoid species that occur within this forma- Miller and others 1995, Robertson and others 2000) tion include bushmuhly (Muhlenbergia porteri) and and what Kuchler (1970) referred to as the Galleta- burrograss (Scleropogon brevifolius). Major shrubs Threeawn Shrub Steppe. These grasslands occur on that occur in association with these species include nearly level, wind-desiccated geomorphic surfaces of creosote bush (Larrea tridentata), velvet mesquite sedimentary and igneous origin. Grass species that (Prosopis velutina) in Arizona, western honey mesquite characterize this category include western wheatgrass (Prosopis glandulosa var. torreyana) in southern New (Pascopyrum smithii), needle and thread (Hesperostipa Mexico, tarbush (Flourensia cernua), turpentine bush comata), blue grama (Bouteloua gracilis), galleta (Ericameria laricifolia), desert ceanothus (Ceanothus (Pleuraphis jamesii), and New Mexico feathergrass greggii), and soaptree yucca (Yucca elata). (Hesperostipa neomexicana), and various species of The Great Basin Grassland occurs within the White three-awn (Aristida spp). Common shrubs include Mountain-San Francisco Peaks, Saramento-Manzano big sagebrush (Artemisia tridentata), black sagebrush Mountains, Central Rio Grande Intermontane, and (Artemisia nova), fourwing saltbush (Atriplex cane- higher elevations of Basin and Range and Sonoran scens), and Mormon tea (Ephedra trifurca). Oneseed Desert ecoregion sections (McNab and Avers 1994) of juniper (Juniperus monosperma) and Utah juniper the Southwestern region. These grasslands are higher (Juniperus osteosperma) woodlands and savannas are in elevation and climatically cooler and moister than adjacent to Colorado Plateau grasslands. desert grasslands and are adjacent to and intermingle The Plains Grasslands consist of the shortgrass, with juniper (Juniperus spp.) savanna ecosystems. The midgrass, and tallgrass prairies of the National Great Basin grasslands are similar to Brown’s (1994) Grasslands. These grasslands extend throughout Plains and Great Basin grasslands and Dick-Peddie’s the Great Plains physiographic province (Fenneman (1993) Plains–Mesa grasslands except the geographic 1928) and occur within the Southern High Plains, range of this category for this assessment is restricted Pecos Valley, Redbed Plains, and Texas High Plains to the Basin and Range Physiographic province ecoregion sections (McNab and Avers 1994). Climate (Fenneman 1928). Diagnostic plant species include ranges from subhumid to semiarid as these grasslands blue grama (Bouteloua gracilis), galleta (Pleuraphis extend from east to west. The characteristic plant jamesii), Indian ricegrass (Achnatherum hymenoides), species that are abundant throughout the shortgrass and sideoats grama (Bouteloua curtipendula). Some prairie include blue grama (Bouteloua gracilis) and dropseeds, (Sporobolus spp.) and wolftail (Lycurus buffalo grass (Buchloe dactyloides). The midgrass phleoides) are codominant and add to the diversity prairie ecosystem is codominated by little bluestem of this category. The Great Basin grasslands tend to (Schizachyrium scoparium), blue grama (Bouteloua be drier than the Shortgrass Steppe grasslands and gracilis), and plains bristlegrass (Setaria vulpiseta). have a blend of warm and cool season graminoid and The tallgrass prairie is dominated by big bluestem forb species. Shrubs that are present in association (Andropogon girardii). These different prairie ecosys- with grassland vegetation of this category include tems are aggregated and reduced to one category for fourwing saltbush (Atriplex canescens), sacahuista this assessment and reflects a wide range of ecological (Nolina microcarpa), small soapweed yucca (Yucca properties and processes. glauca), skunkbush sumac (Rhus trilobata), and cat- The Montane Grasslands category includes the claw mimosa (Mimosa biuncifera). As this grassland montane, subalpine and alpine meadows, valleys, and integrades with savanna ecosystems, minor amounts high elevation grasslands that occur throughout the of trees such as emory oak (Quercus emoryi), alliga- Grand Canyonlands, Painted Desert, Tonto Transition, tor juniper (Juniperus deppeana), and Utah juniper White Mountain-San Francisco Peaks, Basin and (Juniperus osteosperma) dominated woodlands are Range, Central Rio Grande Intermontane, South- evident. Central Highlands, Sacramento-Manzano Mountain, The Colorado Plateau Grassland is located in Southern Parks and Ranges, and Upper Rio Grande northern Arizona above the Mogollon Rim and northern Basin ecoregion sections (McNab and Avers 1994). New Mexico in association with the Colorado Plateau These grasslands are similar to Subalpine-Montane and adjacent to small areas of the Rocky Mountain Grasslands described by Dick-Peddie (1993) and the physiographic provinces (Fenneman 1928). It occurs Alpine and Subalpine and Montane Meadow grasslands within the Grand Canyonlands, Painted Desert, Tonto of Brown (1994). Carleton and others (1991) classified Transition, White Mountains-San Francisco Peaks, montane, subalpine and alpine terrestrial ecosystems Navajo Canyonlands, Southcentral Highlands, South- as edaphic-fire and topo-edaphic-zootic disclimaxes Central Highlands, Southern Parks and Ranges, and with temperate continental climates. Diagnostic plant Upper Rio Grande Basin ecoregion sections (McNab species that characterize these ecosystems include and Avers 1994). Colorado Plateau Grasslands—a new Arizona fescue (Festuca arizonica), mountain muhly category of Southwestern grassland—primarily splits (Muhlenbergia montanus), Kentucky bluegrass (Poa USDA Forest Service Gen. Tech. Rep. RMRS-GTR-135-vol. 1. 2004 13 pratensis), timber oatgrass (Danthonia intermedia), typically occurred at the edges of the National Forest Thurber fescue (Festuca thurberii), tufted hairgrass System boundary. (Deschampsia caespitosa), alpine avens (Geum rossii), and Bellardi bog sedge (Kobresia myosuroides). Practical