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Deborah M. Finch
Cathy W. Dahms
Chapter 1:
Purpose and Need for a Grassland
Assessment
Purpose___________________________ the larger framework. The assessment is not a deci-
sion document because it surfaces issues and risks
This report is volume 1 of an ecological assessment to grassland ecosystems that provide the foundation
of grassland ecosystems in the Southwestern United for future changes to Forest Plans or project activi-
States, and it is one of a series of planned publications ties but does not make any site-specific decisions or
addressing major ecosystems of the Southwest. The recommendations. The report also provides a scientific
first assessment, General Technical Report RM-GTR- basis for conducting ecosystem restoration projects,
295, An Assessment of Forest Ecosystem Health in the provides a starting point for public discussion on
Southwest (by Dahms and Geils, technical editors, desired conditions for the future, and contributes to
published July 1997), covered forested ecosystems. the overall understanding of the physical, biological,
Given the complexities of grassland ecology and the and human dimensions of grassland ecosystems in
increasing number of challenges facing grassland the Southwest.
managers, the USDA Forest Service Southwestern The report is divided into two volumes. The first
Region, in partnership with the agency’s Rocky volume (herein) focuses on the ecology, types, condi-
Mountain Research Station, focused on grasslands tions, and management practices of Southwestern
in its second assessment. The assessment is regional grasslands. The second volume emphasizes wildlife
in scale and pertains primarily to lands administered and fish species and their habitat requirements in
by the Southwestern Region (Arizona, New Mexico, Southwestern grasslands.
Texas, and Oklahoma). To prepare this document, we assembled a team of
Broad-scale assessments are syntheses of current authors from the Southwestern Region and the Rocky
scientific knowledge, including a description of uncer- Mountain Research Station whose expertise focused
tainties and assumptions, to provide a characterization on or included grassland ecosystems. An outline of
and comprehensive description of ecological, social, chapter titles and chapter contents was prepared us-
and economic components within an assessment area ing a group consensus process. Authors volunteered
(USDA Forest Service 1999b). A primary purpose of to write specific chapters that were then reviewed
this assessment is to provide context to National by the team. Following team review, each individual
Forest System land management planning efforts chapter was sent to a minimum of two peer reviewers
involving grasslands, both at the Forest Plan level for critique, and in addition, the entire revised volume
for Plan amendments and revisions, and at the was sent to two reviewers. Also, the team interviewed
project level to place site-specific activities within Forest Service employees (see appendix).
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-135-vol. 1. 2004 1
We thank all the authors for writing and rewriting 1992, French 1979). Further subdivisions according
their chapters. We are grateful to Art Briggs and to physiographic province include Great Basin
Bob Davis from the Regional Office for support- grassland and Plains grassland. In general, the term
ing this project. We appreciate helpful reviews on plains refers to grasslands in areas of flat topography
the entire document by Will Moir and Rex Peiper. (Coupland 1992). Plains grassland is then subdivided
Reviews provided on individual chapters are also into short, tall, and mixed grasslands. At a finer scale,
much appreciated. This project was financially sup- grasslands are designated by vegetation commu-
ported by the Regional Office of USDA Forest Service nity/plant association as classified by Küchler (1964),
Southwestern Region and by the USDA Forest Service Clements (1920), and others.
Rocky Mountain Research Station. We thank Paulette Temperate grasslands are areas at mid-latitude that
Ford, Carol Raish and Rose Pendleton for helpful com- are dominated by perennial grasses and forbs. Climate
ments on chapter 1. We thank the Station’s Publishing is moderately dry (semiarid) with discrete wet/dry
Services staff for helpful editing and layout. seasons and temperature and precipitation extremes
(Sims 1988). Soils are predominantly characterized
Southwestern Grassland as Aridisol or Mollisol with large amounts of humus
Ecosystems________________________ (Aber and Melillo 1991, Sims 1988, Whittaker 1975).
Temperate grasslands include tall, mid and short
In the Southwestern Region, the Forest Service grasses (Odum 1971, Whittaker 1975). Tall grasses
has adopted the Soil Conservation Society (SCS) of are about 150 to 245 cm (5 to 8 feet) high, mid grasses
America’s definition of grasslands, that is, “lands approximately 60 to 120 cm (2 to 4 feet) high, and short
on which the existing plant cover is dominated by grasses 15 to 45 cm (0.5 to 1.5 feet) in height. Short
grasses” (SCS 1982). Risser (1995) defined grasslands grasses include buffalograss (Buchloe dactyloides),
as “biological communities that contain few trees blue grama (Bouteloua gracilis ), and other gramas.
or shrubs, are characterized by mixed herbaceous Mid grasses include little bluestem (Schizachyrium
vegetation, and are usually dominated by grasses.” scoparium), needlegrass (Stipa spp.), western wheat-
Supported by the National Science Foundation, the grass (Pascopyrum smithii), and Indian rice grass
U.S. International Biological Program (IBP) character- (Achnatherum hymenoides) (Odum 1971).
ized natural grasslands as climatically determined Temperate grassland biomes include prairie and
by soil water availability and precipitation volume steppe (French 1979, McKnight 1993, Odum 1971,
and seasonality, successional grasslands where for- Whittaker 1975). Prairie, including the true tall-grass
est vegetation has been removed, and agricultural prairie, mixed-grass prairie, and short-grass prairie,
grasslands where a few native or introduced spe- is dominated by grasses and forbs, has a scarcity
cies are maintained. This report addresses natural of shrubs, and has no trees. Mixed-grass prairie
grasslands. is an ecotone between tall and short-grass prairie.
This assessment includes the following South- The term steppe refers to a temperate biome that
western grassland types: is dominated by short grasses and bunchgrasses
(McKnight 1993) and is dryer than prairie. Steppes
• Montane grassland receive approximately 25 to 50 cm (10 to 20 inches)
• Colorado Plateau of rain per year and experience hot summers and cold
• Desert grassland winters; these climatic conditions support plants such
• Great Basin grassland as blue grama, buffalograss, big bluestem (Andropogon
• Plains grassland gerardi), cacti, and sagebrush.
Subalpine grasslands are discussed within the Grasslands can be subdivided using the U.S. National
montane grassland category. Alpine grasslands are Vegetation Classification system (Federal Geographic
not discussed as a separate category because they Data Committee 1997) and other methods according to
have a limited extent in the Southwest. Where they class, subclass, group, formation, regional biome type,
are mentioned, they are discussed in conjunction with alliance, plant associations, or habitat types. Grassland
montane grasslands, although they occur on a different categories for the Southwestern United States include
mountain gradient. Riparian and/or wetland inclu- the Plains grassland, Great Basin grassland, and
sions occur in all grassland types and are discussed the Colorado Plateau as discussed above, as well as
separately where appropriate. montane grassland and desert grassland. Montane
Ecologists and geographers identify broad-scale grassland can be found in small patches within the
Southwestern grasslands (that is, biome level) as mixed conifer and ponderosa pine forests. Montane or
temperate grassland. These biome classifications high-mountain grasslands consist of meadows below
are according to macroclimate conditions defined timberline (French 1979), while alpine grassland is
by Köppen, Threwartha and others (Coupland located above timberline (Whittaker 1975). Desert
2 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-135-vol. 1. 2004
View of Animas Valley, looking east toward Animas Mountains, New Mexico. Mesquite in perennial grassland.
(Photo by Ronald Bemis)
grassland occurs in arid and semi-arid climates. It savanna biome does not occur in the Southwestern
varies in composition from mixed herbaceous species United States according to broad-scale vegetation
with few shrubs to primarily a combination of shrub classification systems based on climate. The Forest
species (French 1979) such as mesquite (Prosopis spp.) Service also uses a classification system based on a
and creosote bush (Larrea tridentata). Subregions in- geographic approach, also referred to as regionaliza-
clude the Chihuahuan Desert grasslands of southern tion, which is a process of classification and mapping
New Mexico, characterized by black grama (Bouteloua to identify homogeneous map units at various scales.
eriopoda), and the Sonoran Desert grasslands of south- The National Hierarchical Framework of Ecological
eastern Arizona. Units adopts Bailey’s classification by ecoregions
Biome classification systems use the term savanna (Bailey 1995); the hierarchy consists of domain, divi-
to describe tropical grasslands that are primarily sion, province, section, subsection, landtype, landtype
located in Africa and Australia, South America and association, and landtype phase. At the regional scale,
southern Asia/India (Whittaker 1975, McKnight provinces and sections are the most useful units for
1993). Similarly, Bailey’s ecosystem classification assessments.
uses the term savanna to describe a Division within
the Humid Tropical Domain. However, some people Relationship of Assessment to
use the term temperate savanna to describe areas in Ecosystem Management____________
the Southwestern United States. McPherson (1997)
defines North American savannas as “ecosystems Ecosystem management is an evolving philosophy
with a continuous grass layer and scattered trees or that has been adopted by many government agen-
shrubs.” The woody plant overstory has approximately cies including the Forest Service. The Forest Service
30 percent cover or less with a grass understory. He has defined ecosystem management as “a concept
further defines and maps areas of the Southwest as of natural resource management wherein national
Piñon-Juniper Savanna, Southwestern Oak Savanna, forest activities are considered within the context of
Ponderosa Pine Savanna, and Mesquite Savanna. economic, ecological, and social interactions within a
Although this terminology is sometimes used, a defined area or region over both short and long term”
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-135-vol. 1. 2004 3
(Thomas and Huke 1996). National Forest activities this report, chapter 2 describes the extent and types
in this context are all activities occurring on National of grasslands in the Southwest. The general ecology
Forest System lands, including grassland ecosystems. of Southwestern grasslands is evaluated in chapter
Ecosystem management is sometimes referred to as 3. A discussion of the biological diversity, functional
ecology-based multiple-use management in that there processes, and consequences of grassland fragmenta-
is a shift from focusing exclusively on sustaining tion is provided in chapter 4. Cultural dimensions
production of goods and services to sustaining the of grassland management, both from a historic and
viability of ecological, social, and economic systems. contemporary perspective, are covered in chapter
While other agencies and organizations have devel- 5. Historic and current conditions of southwestern
oped their own definitions of ecosystem management grasslands in relation to land management are cov-
reflecting their differing missions, they typically have ered in chapter 6. Chapter 7 discusses the concept
a goal of ecosystem sustainability or maintaining of grassland sustainability and why understanding
ecological integrity (Grumbine 1994, Kaufmann and sustainability is critical for managers and stakehold
-
others 1994) while recognizing that people are part ers to collaboratively develop desired conditions for
of the ecosystem and that human needs should be grassland areas. Management decisions will need to
reflected in ecosystem sustainability (Keystone Center be site-specific based on the unique characteristics of
1996, USDA Forest Service 1994). the area. Because there is no one-size fits-all man-
A significant difference among various ecosystem agement strategy for an area, chapter 8 discusses
management philosophies may be the degree in which a wide range of tools available for use by grassland
people are included within the sustainability concept. managers.This chapter has an eye toward highlight-
Former Forest Service Chief Dombeck stated, “We will ing some of the more innovative work being done in
still track traditional outputs of goods and services grasslands rather than attempting to document all
but they will be accomplished within the ecological possible tools. Chapter 8 also covers research needs,
sideboards imposed by land health” (Dombeck 1999). since adaptive management and the utilization of the
The Keystone Center’s National Policy Dialogue best scientific knowledge are important components
Group on Ecosystem Management placed the goals of ecosystem management.
of sustaining vibrant, livable, and economically di-
verse human communities and the involvement of Ecosystem Sustainability____________
stakeholders on a par with the goals of maintaining
ecosystem integrity and sustaining biodiversity and
ecosystem processes at a regional scale (Keystone Ecosystem sustainability is the ability of an
Center 1996). The Southwestern Region’s philosophy ecosystem to maintain ecological processes and func-
embraces all these goals as well. In developing the tions, biological diversity, and productivity over time
human dimension principles and strategies for the (Kaufmann and others 1994). It was the subject of
Southwestern Region, the Human Dimensions Team the 1992 Earth Summit/United Nations Conference
recognized that human needs and wants must be on Environment and Development in Rio de Janeiro
balanced with ecosystem capabilities. However, on a and a 1987 World Commission on Environment and
practical level, they also recognized that the goal of Development report called Our Common Future (also
meeting human needs frequently conflicts with the known as the Bruntland Report; Bruntland 1987).
goal of sustaining natural ecosystems (USDA Forest Sustainable ecosystems are able to maintain their
Service 1994). When this is the case, ecosystem man- ecological integrity.
agement may be considered an optimal integration Ecological integrity is achieved when ecosystem
of ecological sustainability and human dimensions structure, function, processes, and services are pre-
(including both economic considerations and societal served over space and time (Grumbine 1994). Ecosystem
needs and desires) (Jensen and others 1996). structure is the spatial arrangement of the living and
Assessments are a tool in ecosystem management to nonliving elements of an ecosystem, for example, abiotic
develop a holistic understanding of ecological sustain- elements (temperature, light, wind, relative humidity,
ability as well as the human dimension of ecosystems. rainfall) and community structure (species richness
As an introductory chapter, chapter 1 not only describes and the distribution of heterotrophs, autotrophs, and
the purpose and need for assessments, it also gives a consumers). Ecosystem function refers to the processes
brief overview of Southwestern grassland types and whereby the living and nonliving elements of ecosys-
defines two terms in frequent use in later chapters: tems change and interact, such as biogeochemical
ecosystem sustainability and adaptive management. processes and succession. Ecological processes are the
Chapter 1 also explores and emphasizes the role of, actions or events that link organisms and their envi-
and need for, monitoring of grassland conditions and ronment. Ecosystem processes include disturbance,
trends, a topic not covered in detail in later chapters. In succession, evolution, adaptation, natural extinction
4 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-135-vol. 1. 2004
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