Chapter 2: Assessment Approach

"Under the concept of ecosystem management, planning is conducted using appropriate regional or area assessments of ecological, economic, and social effects and the interaction of these factors to enhance management..."

Jack Ward Thomas, 1995

This assessment examines the condition of forested ecosystems in Arizona and New Mexico and describes their past, present, and future at a regional planning and analysis scale. A variety of hierarchies exist to delineate these ecosystems on different characteristics (e.g., climatic, terrestrial, aquatic, and social), and the assessment team had considerable discussion on which hierarchy to use. Although it would be appropriate to structure this assessment by either ecoregions (terrestrial), river basins (aquatic), or economic-political units (social), required ecological information on past conditions, current status, and potential trends is generally not available by these geographic designations. Data are, however, available for Southwestern ecosystems by life zone characterized by biotic community. The assessment is therefore based on taxonomic categories of dominant vegetation; and differences in condition and response for geographic provinces are noted within discussions organized by biotic community.

The assessment follows the guidelines of the draft National Framework for Integrated Ecological Assessments² and includes the following elements for Southwestern forests as a whole and for each life zone in particular:

1. A description of the current and historic composition, structure, and function.

2. A description of the abiotic and biotic events, including human actions, that contributed to development of the current condition.

These elements are incorporated into the assessment on the fundamental premise that human actions change ecosystems. Past, present, and future perceptions and beliefs influence ecosystems, just as ecosystems influence the physical, spiritual, cultural, and economic well-being of people. Before managers can attempt to integrate goals, perceptions, beliefs, and values into ecosystem planning, they must be aware of the origin and evolution of these human wants and needs. This topic is addressed in Chapter 3.

An historical perspective (element 1) establishes reference conditions for estimating how current ecosystems differ from ecosystems of the past. Historical characterizations also provide insights into possible future ecosystem development by identifying significant disturbance agents and regimes, vegetation patterns, environmental constraints, and the variability of biotic patterns and processes. Characterization of historic conditions in the Southwest during Native American and Spanish Colonial times is provided in Chapter 4.

During the Territorial Period following the war of 1848, came increasing levels of hunting, grazing, timber harvest, farming, irrigation, and building of towns, cities, and civil projects. The changes in forest ecosystems (element 2) that resulted from new resource demands and management regimes are described in Chapter 5.

A summary of ecosystem conditions is provided in Chapter 6. The indicators of forest health are: 1) biological diversity, 2) biotic integrity and resilience, and 3) human needs and uses. The assessment team believes these three indicators accurately reflect the biological, physical, and human dimensions required for sustaining ecosystems. Biological diversity is a frequently used measure of ecosystem complexity; reduction of ecosystem complexity is generally considered detrimental to ecosystem health. Biotic integrity, as used in this report, refers to the ability of a community to recover and maintain system processes within historic variability. Resilience to disturbance insures maintenance of biodiversity and biotic integrity. Human needs and wants reflect not only the economic needs of society and communities, but also their cultural, spiritual, and aesthetic aspirations.

Actions the assessment team believes could improve forest health are described in the remaining chapters. Management opportunities are discussed in Chapter 7; specific tools are covered in Chapter 8; and research needs are identified in Chapter 9.

ECOSYSTEM SCALE, HIERARCHY, AND CLASSIFICATION

One of the differences between ecosystem management and previous management approaches is the recognition of the importance of scale and hierarchy. Ecosystems are defined by their boundaries, and depending on one's perspective could be a pond, a small watershed, a major river basin, or even the whole biosphere. Ecosystems can be viewed both as a landscape of similarly scaled stands (like a patchwork quilt) and as an ordered nesting of patches within stands, stands within forests, and forests within regions (a hierarchical organization). The concept of scale applies not only to ecosystem structure but also to ecosystem processes (Figure 2.1). These processes act over distinct and characteristic spatial and temporal scales and determine ecosystem structure (Holling 1992). These processes may be local such as tree fall, or landscape such as habitat fragmentation, or rapid such as plant re-establishment and succession after fire, or slow such as soil development. Some processes, such as bark beetle outbreaks, are particularly relevant to ecosystem health because they occur at spatial and temporal scales which correspond to those with human interest and importance (Holling 1992). Identification of scale and hierarchy are necessary to provide the proper context for ecosystem description and analysis.

The complexity of describing ecosystems over a range of scales and accounting for their hierarchical structure is handled through classification. Although classification is a challenging task, it produces very useful management tools. Ecological classifications range from relatively simple, using few characteristics such as species composition, to more comprehensive, integrating multiple environmental characteristics such as climate, topography, soil, and vegetation. Classification systems identify the relative degree of similarity among ecosystems and arrange these into groups ranked by spatial scale or hierarchy. One of the best uses of ecological classification is for stratifying the land base into units with similar productive capacities and responses to management. Classifications also help in the conduct of inventories for rare species (or other ecosystem elements) because these species are usually associated with a single or definable group of ecosystems. When classification is combined with a Geographic Information System ( GIS), the pair becomes a powerful tool for analysis and display of spatial data. Ecological classification systems are developed from either a geographic or taxonomic approach.

The geographical approach (Bailey 1996) is a top-down method whereby the classifier begins with a large land area and splits it into smaller units with similar vegetation, landform, or other attributes. This method, also called regionalization, is a simultaneous process of classification and mapping with the objective of identifying internally homogeneous map units. Because the method is used at a broad scale, map units often contain ecologically significant inclusions such as riparian corridors that do not fit well into the map topology. The best use of regionalization is classification and mapping at broad scales–dividing continents into domains (based on broad climatic zones), domains into divisions (based on regional climate), divisions into provinces (based on landforms, altitudinal zones and plant formations), provinces into sections (based on physiography) and sections into subsections (based on surficial geology). The National Framework of Ecological Units (Table 2.1) based on terms defined by Bailey (1995) is an excellent example of the geographical approach.

In Bailey's (1995) approach, landtypes are divided into landtype phases or aggregated into landtype associations for landscape scale planning and analysis. At these detailed levels, forest and woodland stands as well as linear riparian ecosystems are recognized. Because these units can be observed on the ground, they are especially meaningful to managers and the public. The phase, landtype, and association are the smallest recognized divisions in the hierarchy of the National Framework of Ecological Units (Table 2.1). Landtype and phase (land units) are useful for project planning and analysis and link to landscape units. Landtype associations (landscape units) are useful in forest planning and tier to the subregional units described by Bailey et al. (1994).

Geographic classification is useful for strategic planning at regional or statewide levels. For planning and management at watershed, forest, and project levels, however, a fine-grain, taxonomic classification is required. The taxonomic approach (Pfister and Arno 1980) is a bottom-up aggregation of individual, sampled sites which represent the population of all sites within an area and are intensively measured for a wide variety of ecological attributes. Aggregation uses multivariate statistical analysis to determine the similarity between units; mapping is a separate activity. The basis for classification may be potential or climax vegetation (habitat type) as developed by Daubenmire and Daubenmire (1968) and illustrated for Southwestern forests and woodlands by Stuever and Hayden (1996). Although vegetation alone well integrates numerous ecological and environmental factors, various other taxonomic systems explicitly use climatic, physiographic, edaphic, and vegetation data. An example of this approach is the Ecological Land Classification Framework for the United States (Driscoll et al. 1984).

The comprehensive ecological classification system used in the Southwestern Region for analysis and planning is the Ecological Land Classification Framework for the United States (Driscoll et al. 1984) based on the Modified Ecoclass System. From this framework, the Southwestern Region developed the Terrestrial Ecosystem Survey ( TES) procedures for classification, mapping of ecological units, and direction for interpreting the relationships of soil, vegetation, and climate. Two scales are available for mapping of ecological units. TES units are mapped as polygons at 1:24,000 and correspond to the land unit scale (Table 2.1). General Ecosystem Survey (Carleton et. al 1991) units are mapped at 1:250,000 and correspond to the landscape scale (Table 2.1). These units are meaningful to resource managers and the public in that these units are observable spatial features that repeat themselves across the landscape. Managers can use these units to evaluate cause and effect relationships among management scenarios. The landscape scale and scales immediately above and below in the National Framework (Table 2.1) are those most appropriate for a regional assessment of forest ecosystem health.

FORESTED PROVINCES OF THE SOUTHWEST

Ecoregion provinces designate regional geographic areas with similar climates and landforms (Bailey 1995). Within the National Hierarchical Framework of Ecological Units, the province is the appropriate scale for broad-level, strategic planning and assessment. The seven provinces (Bailey et al. 1994) mapped for the Southwest (Figure 2.2) are:

1. American Semi-Desert and Desert Province,

2. Arizona-New Mexico Mountains Semi-Desert - Open Woodland - Coniferous Forest - Alpine Meadow Province,

3. Chihuahuan Semi-Desert Province,

4. Colorado Plateau Semi-Desert Province,

5. Great Plains - Palouse Dry Steppe Province,

6. Southern Rocky Mountain Steppe - Open Woodland - Coniferous Forest - Alpine Meadow Province, and

7. Southwestern Plateau and Plains Dry Steppe and Shrub Province.

Because this is an assessment of forest and woodland ecosystems, the American Semi-Desert and Desert, the Chihuahuan Semi-Desert, and the Southwestern Plateau and Plains Dry Steppe and Shrub Provinces are excluded from further discussion. Provinces (and more specifically sections and subsections) delimit areas with like interactions among land units and which consequently form landscapes characteristic of the region. For example, because of differences in climate, landform, and history, succession proceeds differently in the Southern Rocky Mountain Province than in the Colorado Plateau Province. Differences in succession lead to the formation of regionally distinct landscapes, even though the same species are present in both provinces. Bailey (1995) describes the Southwestern provinces which include forest and woodland communities.

Arizona–New Mexico Mountains Semi-Desert – Open Woodland – Coniferous Forest – Alpine Meadow Province

The geomorphology includes mountains, hills, plains, and scarps across central Arizona, and western and southern New Mexico. The elevation starts at 6,000 feet and goes up to 12,500 feet. Precipitation ranges from 12 to 35 inches annually. Potential vegetation includes pinyon (certain Pinus spp.), juniper (Juniperus), Gambel oak (Quercus gambelii), ponderosa pine (P. ponderosa), white fir (Abies concolor), and Douglas-fir (Pseudotsuga menzesii). Aspen (Populus tremuloides) is an occasional species. The primary abiotic disturbance is fire; and the primary anthropological land use is forest management, including recreation, timber, aesthetics, and wildlife habitat. Portions of the Tonto, Coconino, Apache–Sitgreaves, Lincoln, Cibola, and Santa Fe National Forests are included in this province.

Colorado Plateau Semi-Desert Province

This province contains lower tablelands in Arizona and New Mexico; the Colorado River is the region's only large stream. The geomorphology of this province includes canyons, cliffs, scarps, plateaus, hills, and mountains. The elevation ranges from 3,000 feet to 7,000 feet. Annual precipitation ranges from 6 inches to 25 inches. In the lowest vegetation zone are arid grasslands and shrubs; sagebrush (Artemisia) is common over large areas. The woodland zone is the most extensive; pinyon and juniper are the dominant vegetation. The montane zone extends over the high plateaus and mountains; trees include ponderosa pine, Douglas-fir and aspen. Abiotic disturbances include wind, floods, drought, and fire. The principal anthropological land use is production of forage for livestock grazing and browsing. Portions of the Tonto, Prescott, Kaibab, Coconino, and Apache-Sitgreaves National Forests are included in this province.

Southern Rocky Mountain Steppe – Open Woodland – Coniferous Forest – Alpine Meadow Province

Landforms include mountains and valley plains of northern New Mexico. The elevations range from 7,500 to 14,000 feet. Annual precipitation ranges from 24 to 28 inches. Ponderosa pine, Douglas-fir, aspen, subalpine fir (Abies lasiocarpa) and Englemann spruce (Picea engelmannii) are common tree species. The primary abiotic disturbance is fire. Recreation, mining, and ranching are important land uses. This province includes significant portions of the Carson and Santa Fe National Forests.

Great Plains – Palouse Dry Steppe Province

Landforms include valley, lowlands, and elevated plains and hills in northern New Mexico. Elevation ranges from 6,800 to 8,800 feet. Precipitation varies from 6 to 8 inches annually, and less than half of the precipitation falls during winter. Various species of forbs and grasses ( graminoids) are found in the uplands; cottonwoods (Populus) and willows (Salix) along riparian corridors form the only forests in the province. Farming and ranching are the primary land uses. This province includes portions of the Carson and Santa Fe National Forests.

LIFE ZONES OF THE SOUTHWEST

The General Ecosystem Survey (Carleton et al. 1991) groups Southwestern ecosystems into life zones characterized by biotic community types including desert, grassland, chaparral, evergreen oak woodland, coniferous woodland, ponderosa pine, mixed conifer, spruce–fir, tundra, and riparian wetlands. Because this an assessment of forest, woodland, and associated riparian ecosystems, desert, grassland, chaparral, and tundra life zones are excluded from further discussion. The concept of a life zone is derived from a taxonomic classification system described first by Merriam (1898), revised by UNESCO (1973), and applied in the Southwest by the Terrestrial Ecosystem Survey. The General Ecosystem Survey life zones (Table 2.2) can be cross-referenced to the biotic communities described by Brown and Lowe (1977, 1980) and Brown (1994). Aspen is a component of the montane forest found mostly in the mixed conifer zone but also in the ponderosa pine and the spruce–fir zones. Riparian wetlands occupy little area but like aspen perform special and very important ecological and landscape functions within their life zone. Because of their uniqueness and value, aspen and riparian wetlands are treated here along with evergreen oak and coniferous woodlands, ponderosa pine, mixed conifer, and spruce–fir forests as forest biotic communities of the Southwest (Figure 2.3).

A specific ecosystem can be located with reference to a geographic province; and if it is defined by the dominant vegetation, it can also be associated with other ecosystems of similar biotic composition (life zone). Whereas the province scale is the correct perspective for examining landscape dynamics, the life zone is the appropriate scale for describing aspects of community development such as disturbance regime and successional pattern. Every ecosystem is a unique entity with its own particular history, composition, structure, and potential. Although ecosystems of a common life zone tend to respond in similar ways, each is different. Some of this difference can be explained by location within a landscape and province. Because most of the available information for past and current ecosystems is cataloged or identified by life zone, this assessment primarily describes Southwestern ecosystems by life zone and notes differences by province where they are known.

Evergreen Oak and Coniferous Woodlands

Woodlands generally include evergreen oak and conifer species that occupy certain areas along an elevational gradient from low-elevation desert shrub/grasslands and short-grass prairies to high-elevation montane coniferous forests of ponderosa pine and Gambel oak. Oak woodlands occur within the range of 4,000– to 9,000–feet elevation. Gambel oak occurs at higher elevations, and wavyleaf oak (Quercus undulata) occurs either below Gambel oak or intermingled with it in a transition zone. Woodlands were used extensively by prehistoric and historic populations for habitation and subsistence. Uses today include grazing, fuelwood harvest, and recreation.

Evergreen oak woodland, characterized by wet summers and mild winters, extends from the Sierra Madre of Mexico into southeastern Arizona and southwestern New Mexico. In the United States, a variety of oak species such as Emory oak (Quercus emoryi), Arizona white oak (Q. arizonica), Mexican blue oak (Q. oblongifolia), gray oak (Q. grisea), silverleaf oak (Q. hypoleucoides), and netleaf oak (Q. rugosa) are found in conjunction with the following Madrean pine species–Apache pine (Pinus engelmannii), Chihuahua pine (P. leiophylla var. chihuahuana), and Arizona pine (P. arizonica) (Brown 1994).

Pinyons and junipers, together or alone, dominate coniferous woodland communities. These woodlands occupy approximately 23 million acres in New Mexico, about 13 percent of which are on national forest lands, and 4.1 million acres in Arizona, 34 percent on national forest lands. The pinyons include Pinus edulis, the most common pinyon pine throughout the type, border pinyon (P. discolor), and Arizona singleleaf pinyon (P. californarium subsp. fallax). Junipers are frequently found at lower elevations than pinyons and typically occupy sites with deep soils. The most common junipers in the Southwest are one-seed juniper (Juniperus monosperma) found in central and southern New Mexico and much of Arizona below the Mogollon Rim, the Rocky Mountain juniper (J. scopulorum) in the higher and colder woodlands of northern New Mexico and Arizona, Utah juniper (J. osteosperma) in northwestern New Mexico and northern Arizona, and alligator juniper (J. deppeana) associated with the Madrean woodlands of southern Arizona and New Mexico (Brown 1994, Gottfried 1992).

Ponderosa Pine

Ponderosa pine (yellow pine or blackjack pine) is found from 6,500 to 8,000 feet elevation. At lower elevations, the ponderosa pine forest meets woodlands and at higher elevations transitions into the mixed conifer zone. Ponderosa pine forests of central and northern New Mexico and Arizona cover about 8.4 million acres. The predominant form throughout the Southwest is the three-needled, Rocky Mountain ponderosa pine (P. ponderosa var. scopulorum). In lower elevations of southern Arizona, however, the five-needled, Arizona pine is more common. Other species associated with ponderosa pine at low elevations are Gambel oak and New Mexico locust (Robina neomexicana); at high elevations associates are southwestern white pine (Pinus strobiformis), Rocky Mountain Douglas-fir (Pseudotsuga menziesii var. glauca), Rocky Mountain white fir (Abies concolor var. concolor), and quaking aspen (Populus tremuloides) (Brown 1994). Uses include timber harvest, grazing, camping, and other types of recreation offering cool relief from hot urban areas.

Mixed Conifer

Mixed conifer forests dominated by Douglas-fir, white fir, and blue spruce (Picea pungens) occur at elevations from 8,000 to 9,500 feet. There are about 1.5 million acres of mixed conifer forest in the Rocky Mountain and Madrean montane forests of southern Colorado, New Mexico, Arizona, and the Sierra Madre Occidental of Mexico (Brown 1994). Ponderosa pine, southwestern white pine, aspen, and a number of other tree species may occur in these forests. Uses are similar to the ponderosa pine community.

Spruce–Fir

Spruce–fir forests are found at high, subalpine, elevations in the Southwest from approximately 8,000 feet to over 12,000 feet. Spruce–fir forests are typically restricted to areas receiving more than 25 inches of precipitation from winter snows and summer thunderstorms. The predominant spruce is Engelmann spruce which is found as far south as the Pinaleno Mountains in Arizona and the Sacramentos in New Mexico. The co-dominant species is subalpine fir (Abies lasiocarpa). Some populations of subalpine fir possess a distinctive outer cortex and are called corkbark fir (A. lasiocarpa var. arizonica). Small stands of aspen or blue spruce are found within the spruce–fir forest (Brown 1994). Uses include wilderness recreation, skiing, and the enjoyment of high places. Mountain peaks have special cultural and religious significance for many Southwestern Indian tribes.

Aspen

Quaking aspen occurs at elevations above 6,000 feet as small, transient patches in ponderosa pine, mixed conifer, or spruce–fir forests. There are close to 500,000 acres of aspen in the Southwest, seventy-five percent in northern New Mexico and the remainder in the Mogollon Rim-White Mountains of Arizona (Brown 1994). Aspens can reproduce by cloning from an established root system and establish a new stand of trees quickly after a fire or other disturbance. Aspens, however, are intolerant of shade and eventually lose out to competition when they become overtopped by re-invading conifers. Aspen stands are especially valued for their scenic quality and use by traditional communities.

Riparian Wetlands

Riparian wetlands including cienegas make up less than 2 percent of the land of New Mexico and Arizona, but they are the most biologically diverse and productive ecosystems in the Southwest. Over 65 percent of Southwestern animals depend on riparian habitats during all or part of their life cycles. Millions of Southwestern residents use these areas for recreation and agriculture. The most important species of Southwestern riparian wetlands are the Fremont cottonwood (Populus fremontii) and the narrowleaf cottonwood (P. augustifolia) (Brown 1994).