2.
Terrestrial Ecosystems
Authors
The reports in this chapter were prepared by the following individuals.
Arizona's Woodlands and Shrublands
o Jeffrey M. Klopatek, Ph.D., Department of Botany, Arizona State University
Arizona's Hot Desert Scrublands
o Duncan T. Patten, Ph.D., Center for Environmental Studies, Arizona State University
o William L. Halvorson, Ph.D., National Park Service, University of Arizona
Arizona's Forests
o Duncan T. Patten, Ph.D., Center for Environmental Studies, Arizona State University.
Arizona's Grasslands
o Ward Brady, Ph.D., Department of Environmental Resources, Arizona State University
Arizona's Woodlands and Shrublands
Introduction
Woodlands and shrublands (scrublands) encompass nearly 72 percent of Arizona's landscape. They can be subdivided into two general categories based on their geographical location and centers of origin:
o Those occurring in the southern or warm regions
o Those occurring mainly in the northern (and/or higher elevations) or cold regions
The division between these two is arbitrary at best, as many of the floristic elements intergrade into one another, especially in the mountainous regions.
In principle, the division is based on winter temperatures and precipitation. The northern zones receive a considerable amount of snow and hard frost, whereas the warm desert regions experience mild winters.
The vegetative elements can be subdivided, based on their physiognomy, into scrublands and woodlands. The southern and southeastern section of the state are dominated by flora of the following warm or hot deserts:
o Chihuahua
o Sonora
o Mohave desert flora
o Madrean evergreen woodlands reaching from Mexico into the southeastern part of the state
This report concentrates on the Great Basin Desert scrublands and Pinyon-Juniper (Great Basin Conifer) woodlands characterizing northern Arizona, but will also touch on the Chaparral and southern Oak woodlands in the state.
It should be noted that most of the Pinyon-Juniper woodlands and Great Basin desert scrublands occur on the Colorado Plateau, and, although the term Great Basin Desert is frequently used, this region is actually a semi-desert due to its elevation at mostly over 5000 feet (West 1983). The terms scrublands and woodlands will be used to refer to the Great Basin desert scrub and Pinyon-Juniper woodlands, respectively.
Northern Shrublands and Woodlands
The northern shrublands and woodlands appear to possess a relatively simple physiognomic and floristic structure, but, like most other ecosystems, they have developed complex mechanisms to make maximum use of their environmental resources.
Their two-dimensional structure gives way to a complex three-dimensional organization when the below-ground or soil component is added. This includes the presence of cryptogamic or mycrophytic crusts (West 1990, Klopatek 1992), nutrient islands (Barth 1980, Klopatek 1987, 1992), and transitions between endo and ectomycorrhizal symbionts (Coe and Klopatek 1987, Klopatek et al. 1988).
Much of this complexity is based on the spatial and temporal variability of resources and the competition for those resources. Thus, it should be noted that, although some groups of vegetation like Sagebrush, Saltbush, and Blackbrush have been lumped together as Great Basin desert shrubs, they will all respond differently to stress because of:
o Differences in phenology, physiology, and morphology
o Different environmental settings to which each has adapted
An attempt has been made to distinguish between stress effects on woodlands and shrublands in addressing each of the individual stressors where data are available. In cases where data do not warrant a separation, or there appears to be no major impact, the two major types are addressed together.
Great Basin Scrublands
The majority of the lands occupied by the Great Basin desert scrub in Arizona are under the following federal ownership:
o Bureau of Land Management
o US Forest Service
o National Park Service
o Native American Reservations
The Great Basin Desert Scrub in Arizona is found on the Colorado Plateau and is dominated by three woody shrub types:
o Sagebrush
o Shadscale/Saltbush
o Blackbrush
Species diversity is characteristically low and is dominated by the woody shrub species.
Precipitation and Grazing
Precipitation is low with most regions receiving less than 10 inches of moisture per year. Because of the discontinuity of vegetative cover brought about by both grazing and soil types, much of the area is subject to erosion.
The Great Basin Grasslands have been subject to severe past grazing pressures, such that much of their original area is now encroached by shrubs and is viewed as a shrub-steppe.
o The Sagebrush type is best represented in Arizona by the big Sagebrush complex, although other species may dominate the drier sites, especially in the Grand Canyon region.
o Shadscale and Saltbush are the two species that prevail in the salt-desert shrub vegetation type that adjoins the Sagebrush, but receives less precipitation, about 150 to 200mm per year. Along with these species, Greasewood occurs in the lowlands and more saline areas.
o Blackbrush exists as an ecotonal vegetation type originating out of the Mohave Desert, and occurs principally along the Colorado River on extremely shallow, sandy soils (Bowns and West 1976, Jeffries and Klopatek 1987).
Rainfall regimes are similar to that for Shadscale, but Blackbrush has adapted to the shallow (less than 4 feet deep) and non-saline soils.
Actual areas occupied by the three vegetation types in Arizona are uncertain.
Data available from the Arizona State Lands Department list Great Basin Desert Scrub at 5.7 million acres (digitized from the map of Brown and Lowe 1980). But as with most map data at that scale, this is a rough estimate. (See Map B-5 (Cool Scrublands of Arizona), Appendix B.)
West (1983) lists the Blackbrush association covering approximately 247,000 acres in Arizona. A distribution map produced by Bowns and West (1976), however, shows an area more than ten times that size. As the area listed by West (1983) does not include any areas in western Arizona along the Colorado River and the Grand Canyon National Park where Blackbrush occurs in great expanses (Turner 1982), an estimate of 741,000 acres is used.
For the other two types, based on mapped data (e.g., Kuchler 1970, Brown and Lowe 1980, GCNP unpublished) estimates of 3 million acres for Sagebrush and 2 million acres for the Shadscale/Saltbush association are used.
Pinyon-Juniper Woodlands
The Pinyon-Juniper Woodland is dominated by two species of trees: Pinyon and Juniper. The type is characterized by one primary species of Pinyon Pine, with Mexican Pinyon occurring in the southern mountains.
Utah Juniper occurs throughout the northern and central regions, inter-mixing with one-seed Juniper in the central part of the state. The one-seed Juniper is the dominant species in eastern Arizona.
Alligator Juniper occurs from central Arizona down through the southeastern mountains, while Rocky Mountain Juniper occurs in the northeastern region.
Nielson (1987) described the area near Grand Canyon, Arizona, as the evolutionary center of Pinyon-Juniper woodlands. These Pinyon-Juniper woodlands occur in the transition zone between semiarid vegetation like chaparral, desert shrub or grasslands, and coniferous forests.
This ecotonal positioning is further supported by the co-dominants being endomycorrhizal (Juniper), similar to most arid land shrubs, and ectomycorrhizal (Pinyon), characteristic of the more mesic pines (Coe and Klopatek 1987). These mycorrhizal fungi are mandatory symbionts. Without a mutualistic relationship, neither tree nor fungus would survive.
Soils, Growth Rate, and Distribution
Pinyon-Juniper woodlands in Arizona are located at elevations from 4200 to 7500 feet, and are found on a wide variety of soils and parent materials. Soils vary in texture from stony, cobbley, and gravelly sandy loams to clay and clay loams, and vary in depth from shallow to deep (Springfield 1976). Parent materials also vary widely from granite, basalt, cinders, limestone and sandstone to mixed alluvium (Aldon and Brown 1971).
Growth rate information on Pinyon-Juniper ecosystems in the Southwest shows that productivity varies widely between sites (Lavin and Johnsen 1977). Tress and Klopatek (1987) report that it takes nearly 250 years for a Pinyon-Juniper community to reach a mature successional stage. Mature Pinyon and Juniper trees can live in excess of 300 to 350 years with increasing rates of production as they age (Meeuwig and Budy, 1979).
Pinyon-Juniper woodlands occupy between 62 and 79 million acres in the western United States (Arnold et al. 1964, Little 1977, West 1984). More than 13.8 million acres exist in Arizona and New Mexico (Springfield, 1976), with most being on public land. A recent survey by the USDA Forest Service places Pinyon-Juniper and Juniper woodlands at 13 million acres (Conner et al. 1990), and that figure will be used for this assessment.
Environmental Stressors
Accidental Releases
The effects of contaminants accidentally released are determined by the physical setting of the release site, such as drainage pattern and soil type.
Specific cases of impacts are unknown. Potential cases are a function of the length of transportation corridors through the ecosystem type, including pipelines, railways, and roadways.
The Pinyon-Juniper woodlands have been the site of numerous intentional applications of herbicides with mixed results.
Little data are available on air pollution impacts on cold desert shrubs.
Data on specific Atriplex species show that the C4 species (common in this region) are less susceptible to both ozone (O3) and sulfur oxides (SO2), because of reduced transport of these pollutants into the leaves (Winner and Mooney 1980).
Due to the relatively dry environment, some pollutants such as SO2, SO42, NOx, may actually have fertilization effects on the ecosystem (Aber et al. 1989).
Lichens have been used as early indicators of air pollution effects. A study on power plant emissions in the Four Corners region showed no effects on the lichen populations (Nash and Sommerfeld 1981), as did a recent investigation into air quality in the Grand Canyon (Boykin 1993).
All forests (woodlands) experience some degree of multiple-pollutant exposure above pre-industrial levels (Taylor et al. 1994). However, data on air pollution effects on Great Basin woodland species are limited.
Pinyon Pine and other western Conifers have been shown to be highly sensitive to effects of air pollution, and the species in Arizona can be similarly affected if levels are high enough.
High levels of ozone (O3) are normally associated with downwind areas from cities, especially at higher elevations (NRC 1991).
Data collected by the National Atmospheric Deposition Program (NADP 1992) show no significant levels of air pollution for northern Arizona. Haze from air pollution, has been shown to seriously impair visibility in the Grand Canyon region, but direct effects from the air pollutants are thought to be minimal (GCVTC 1992).
Degradation of the Cultural Environment
The area in Arizona that encompasses the scrublands and woodlands also holds a significant amount of cultural and archeological wealth.
The Anasazi and Sinagua people lived throughout this region and left behind many culturally significant dwellings like Wupatki National Monument, Navajo Monument, Walnut Canyon, and Montezuma Castle. Native Americans like the Navajo, Hopi, Apache, Hualapai currently inhabit nearly 50 percent of these areas.
Degradation of the environment stems directly from individuals destroying Native American burial grounds and historical dwellings in search of artifacts. No estimate is available on the extent (in area) of disruption, but recent newspaper articles cite extensive disturbance relative to the total area.
These disruptions result in a permanent alteration of the cultural environment. Additional degradation comes from visibility impairment. This topic has garnered a great deal of local, regional, and national attention, particularly in the Grand Canyon area (US EPA 1993).
Visibility can be improved immediately with corrective measures. As a result, a task force is currently trying to improve the visibility problems in this region (GCVTC 1992).
Physical Alteration of the Ecosystem
Grazing in the Great Basin Scrublands
The nineteenth century brought a significant increase in the amount and types of livestock into this region, and, with the abundance of federal lands, made up the prescribed formula for overgrazing. The overgrazing, combined with the partial evergreen traits of the shrubs and their seed dispersal characteristics, led to severe degradation of this type (West 1983).
The dominant vegetation types are fairly resistant to grazing, but the associated species, predominantly the cool season (C3) grasses, are most affected and often eliminated (West 1983).
o For example, in Blackbrush ecosystems, Jeffries and Klopatek (1987) and Klopatek (1992) demonstrated a pronounced shift from C3 grasses to C4 grasses with both declining in extent. Additionally, some shrub species such as Kochia and Winterfat are the preferred, palatable winter browse species, and are often greatly reduced in number and cover (Blaisdale and Holmgren 1984).
In cases where grazing is continual by a variety of livestock such as horses, cattle, sheep, and goats, ecosystems have become severely degraded and recovery is extremely slow (West 1983, Jeffries and Klopatek 1987).
o For example, the Navajo Indian Reservation lands are so depleted that goats have become the animal most likely to survive on the harsh browse (Dwyer 1977), and the tribe has recently had to purchase grazing rights on National Forest lands for their cattle (USDA NF pers. comm.).
Overgrazing affects not only the vegetation, but also the wildlife that it supports. Large mammals, such as antelope and desert bighorn sheep, both depend on this vegetation type during different periods of the year.
Grazing in the Pinyon-Juniper Woodlands
Aldon (1973) described Pinyon-Juniper woodlands as one of the most abused ecosystem types in the Southwest, with much of the abuse stemming from overgrazing. Indeed, much of the past research on Pinyon-Juniper was concerned with eradicating the dominants (trees) of this ecosystem to improve grazing for domestic livestock (Arnold et al. 1964, Clary et al. 1974, Lavin and Johnson 1977, Clary and Jameson 1981).
It has been estimated that between 2.5 and 3.7 million acres of Pinyon-Juniper woodlands were manipulated by a combination of burning, herbicides, bulldozing and chaining during the 1950's and 1960's to improve grazing conditions (McCullock 1973). The philosophy behind this destruction was that more trees meant less grass for grazing (Jameson 1967). Much of this philosophy was based on the occurrence of Blue Grama, a C4 warm season grass that has difficulty existing in the shade and litter under the canopy.
The naturally-occurring cool season grasses like Mutton Grass and Squirrel Tail, that do well both under canopy and in the interspaces, have been virtually eliminated in many areas due to overgrazing by domestic livestock (Jameson et al. 1962, Schmutz et al. 1964, Thatcher and Hart 1974, Baxter 1977, Beymer and Klopatek 1989).
Early reports suggested the "best" use for this woodland as range for grazing by domestic livestock (Springfield 1976). Also, early studies (Johnsen 1962, Tausch et al. 1981) have noted Pinyon-Junipers encroaching into the grasslands because of fire curtailment, as well as reduction of understory fuels brought about by grazing. These observations, coupled with a possible climate shift, led ranchers to decry Pinyon- Juniper woodlands as a menace to society and the regional economy (McCulloch 1973, Goodloe 1993).
Early results do show an increase in grass productivity with the elimination of the trees (Arnold et al. 1964, Springfield 1976, Clary and Jameson 1981), an increase which follows the normal successional trend (Tress and Klopatek 1987). In other words, the maintenance of this system in a subclimax state yields an increase in grass productivity.
However, it should be noted that some areas showed no noticeable improvement of forage species ten years following tree control (Clary 1971). Surprisingly, no research has gone back to check on the long-term effect of the past conversion processes after more than 25 years.
Recent research (Beymer and Klopatek 1992, Yorks et al. 1994) has contradicted earlier statements such as those in West (1984), that previously-grazed woodlands experiencing a reduction in grazing would result in the tree canopy closing in and a further reduction of understory species. On the contrary, shrubs and grasses both increased their dominance. This indicates that these systems are resilient, albeit slowly, with a reduction in grazing pressures.
Continued grazing may contribute to ecosystem deterioration. An example of this is an increase in soil erosion that accentuates the retrogression of the system without letting species become established (Price 1993).
Despite the past pattern of rampant eradication, little ecosystem-level research has been conducted to determine the factors responsible for maintaining or improving the productivity or bio-diversity of these woodlands. It has been shown that the clearing of this woodland can be very deleterious to native deer and elk populations in certain parts of the state, especially those areas north of the Mogollon Rim (McCulloch 1973).
Where intensive grazing has occurred, ecosystem productivity is reduced and the reduction is often accompanied by a loss of palatable species, particularly the cool season grasses ( Jameson et al. 1962, Baxter 1977). The present natural cryptogamic or microphytic crust is often destroyed or severely reduced (Jeffries and Klopatek 1987, West 1990, Beymer and Klopatek 1992, Klopatek 1992). Weedy species like Snakeweed, Rabbitbrush, and Cheat Grass often dominate the over-grazed ecosystems.
The woodlands have been severely manipulated for decades, primarily for the purpose of increasing forage for livestock or increasing water yield, which has also proven to be to be unproductive (Ffolliot and Thorud 1974). The effects on plant species diversity have gone unrecorded, but results indicate a significant decrease in mammal and bird species diversity (Clary et al. 1974, Swenson 1977). Reports indicate that woodlands possessing patches of different successional stages maximize animal diversity (McCulloch 1973, Clary et al. 1977).
Agriculture
Agriculture has minimum effects on these ecosystem types because it is largely lacking in these vegetation types.
Highways
The impact of highways on both scrublands and woodlands is proportional to the area covered. Unpaved roads, predominantly in the national forest lands, may produce the most impacts.
Energy Production
The extraction of energy resources, primarily sub-bituminous coal deposits, is concentrated in the scrublands and woodlands of northeastern Arizona. Approximately, 66,700 acres located on Black Mesa on the Navajo and Hopi Reservations are under contract to the Peabody Coal Company (Espey, Huston and Associates 1981).
Principal vegetation types are Pinyon-Juniper woodlands, Big Sagebrush, and Saltbush communities.
All coal mining is done by surface mining that is under the Surface Mining Reclamation Act. Lands must be reclaimed to diversity and productivity levels existing prior to the mining.
However, early reclamation efforts consisted of planting varieties of tame grass such as Russian Wild Rye, Crested Wheat Grass, and Smooth Brome, with Saltbush being the only naturally-occurring species.
o Planted Saltbush demonstrates less water stress resistance than the naturally occurring species (Wilkins and Klopatek 1983).
Additionally, native mycorrhizae propagules that are necessary symbionts for the native vegetation are eliminated in the top soil storage piles (Coe and Klopatek 1983).
These factors, combined with the high demand for grazing in the region, indicate that the return of the surface-mined areas to a healthy, natural ecosystem may be long in coming, if at all.
Recent reclamation efforts have incorporated more natural species, but no reclaimed lands have been released for use again by Arizona tribes and thus, their resilience and resistance to grazing have not been measured (F. Vest, Peabody Coal Co., pers. comm.).
Indirect effects of coal extraction may be felt by the lowering of the water table because of the massive water supply for the coal-slurry pipeline to Bullhead City. Although no direct evidence exists linking this process to a lower water table, concern has been voiced by the tribes.
In 1976, nearly 91,700 acres in Arizona, primarily in northeastern Arizona, were held by the uranium industry for mining and exploration (Yamamoto 1982). As the economic conditions for mining uranium have been depressed, much of these rights are now held in speculation. Recent efforts have been undertaken to reclaim old uranium mill tailings sites in Arizona, but the success is unknown.
Timber Management
Pinyon-Juniper woodlands are the largest of the forest types in Arizona, although their stature has restricted them to woodlands (Conner et al. 1990).
The Pinyon-Juniper woodlands of northern Arizona have a relatively low productivity (Grier, et al. 1992). However, contrary to past interests, there is an increasing emphasis on harvesting Pinyon and Juniper trees for fuel wood.
o For example, Pinyon-Juniper sales in the Coconino National Forest have risen from 1300 cords in 1970, to over 28,000 cords in 1984. A total of 169,000 cords were harvested from all of Arizona's woodlands in 1984 (McClain 1988)).
This intensive use of Pinyon-Juniper woodlands, coupled with the continued grazing, may affect the long-term interrelationships between site productivity, succession and tree growth. This hypothesis is supported by historical evidence, suggesting that excessive harvesting of Pinyon and Juniper results in the trees' failure to recover because of slow growth (Samuels and Bettancourt 1982).
Further, Samuels and Bettancourt (1982) suggest that the increasing human population of the region and an increasing harvest of Pinyon and Juniper for fuel wood may seriously deplete the woodlands. As Arizona's preference is for burning Juniper, the harvesting may lead to a type change in many areas.
Fire Suppression
Wright et al. (1979) indicate that fires have a natural return cycle in Pinyon-Juniper woodlands and Sagebrush shrublands. However, none of the woodlands or scrublands is adapted to fire, and all show a slow recovery from being burned (Bowns and West 1976, West 1983, DeBano and Klopatek 1988). Intense fires may seriously hamper regeneration efforts (Klopatek et al. 1990).
The systems that abut Pinyon-Juniper woodlands such as Ponderosa Pine, chaparral, and grasslands, are fire-adapted (see, for example, Swetnam 1990). As a result, fire suppression may allow for expansion of Junipers into more xeric environments.
Recreation
Recreation impacts of effects on Pinyon-Juniper woodlands are undocumented, although these woodlands provide significant areas for deer and elk hunting (McCulloch 1973).
Urbanization
Most of the impacts from urbanization come from the development of second or vacation homes around Sedona, Prescott, and Payson. The exact area impacted is unknown.
Biological Alteration of Ecosystems
Species Introduction
Species introduction stems primarily from the effects of overgrazing as described for the Great Basin Scrublands and the Pinyon-Juniper woodlands on page 79.
Many species, such as Cheat Grass, could not become established without the assistance of grazing. Once established, however, many species cannot be effectively eliminated once the disturbing force (stressor) has been removed.
o For example, many cool season grasses have been replaced with warm season grasses in the Grand Canyon National Park, and, even after nearly 50 years with grazing removed, still dominate the woodlands (Jameson et al. 1973, Beymer and Klopatek 1992).
Chaparral and Oak Woodlands
The interior chaparral covers approximately 3.5 million acres in Arizona (Hibbert et al. 1974), while Oak woodlands cover nearly 1 million acres (Conner et al. 1990).
Chaparral occurs at elevations of from 3200 to 6000 feet, generally in a northwest to southeast distribution through the center of the state, and along the face of the Mogollon Rim (Pase and Brown 1982).
Precipitation varies between 13.8 mm and 23.6 inches per year.
Oak woodlands replace chaparral in southeastern Arizona where the precipitation pattern shifts from a summer and winter bi-modal precipitation to a summer dominated precipitation pattern.
In contrast to California's coastal chaparral, Arizona's interior chaparral is dominated by scrub Live Oak, Manzanita, Desert Ceanothus, Mountain Mahogany, and Silktassel.
Chaparral ecosystems, similar to the Pinyon-Juniper woodlands, were subjected to treatments such as mechanical manipulation, and herbicides in the 1950's and 1960's to increase water yield and grazing potential.
Because of their high accessibility and relatively gentle terrain, these ecosystems were heavily grazed by goats, especially between 1880 and 1920, and until 1940 (Pase and Brown 1982). Many of the important range grasses were eliminated from most of the sites and, as a result, have been confined to rocky protected areas.
The elimination of the dominant Chaparral vegetation was the focus of a major water yield effort in the 1950's and 1960's. It was not considered cost effective (Pfolliet and Thorud 1973, Hibbert et al. 1974), and was eventually found to reduce potential ecosystem productivity because of the loss of nitrogen (as nitrate) from the system (Longstreth and Patten 1975).
Dominant in the Oak woodlands are such species as Emory Oak, Arizona White Oak, and Mexican Blue Oak. The main contact that the Oak woodlands have at the lower elevations is grasslands in a savanna zone.
Most grasses are warm-season bunch grasses that are adapted to the summer precipitation. Although many herbaceous and shrub species may increase with grazing, the usual result is an increase in bare ground (Brown 1982).
Harvesting of fuel wood from Oak woodlands in the Coronado National Forest increased from 900 cords in 1976, to over 5100 in 1979, forcing the National Forest to set an annual limit of 1500 cords to permit sustained harvesting (Bennet 1992).
Reduction of fires, or cessation of the episodic burns of historical records, appears due to livestock grazing and reduction in amount and continuity of surface fuels (Swetnam et al. 1992).
Conclusions
This report briefly summarizes realized and potential impacts from the stressors highlighted by ACERP.
Because attention was paid primarily to the direct effects of the stressor on the ecosystems, many questions remain un-addressed on a variety of levels.
o At the population and community levels, the Committee is concerned with trophic interactions and food-chain support.
o At the ecosystem level, questions on long-term sustainability arise.
o At the level of landscape, the structure and function of the mosaics of ecosystems have not been dealt with. This includes, for example, the question of how this patchwork quilt of different ecosystems occurring throughout the landscape interacts.
Arizona's Hot Scrublands
Introduction
The hot desert scrublands of Arizona are located primarily in the southwestern quarter of the state below about 3300 feet elevation, and include about 27.5 million acres. These scrublands primarily include that portion of the Sonoran Desert located in Arizona, about 25 million acres.
Limited scrublands outside the Sonoran Desert bio-geographic region are found in lower elevations in the southeastern and west-central parts of the state. Those scrublands in the southeastern part of the state include western extensions of the Chihuahuan Desert, about 1.2 million acres, while the west-central scrublands include eastern extensions of the Mojave Desert, about 3.5 million acres. Many of the southeastern Arizona ecosystems are desert grasslands or higher elevation, mountain- related systems (Bahre 1991). (See Map B-6 (Hot Scrublands of Arizona), Appendix B.)
Sonoran Desert
The southwestern Arizona scrublands, or Sonoran Desert, is divided into two vegetation types. Based on Shreve's (1951) classification, the Sonoran Desert in Arizona includes the Arizona Upland and the Lower Colorado-Gila geographic areas.
o The Arizona Upland portion of the Sonoran Desert is a crassicaulescent type characterized by Cercidium Opuntia (palo verde cactus) located in the higher elevations of the desert (500-3000 feet).
o The Lower Colorado-Gila portion is a microphyllous type dominated by Larrea Ambrosia (creosote bushbursage) located in the lower elevations of southwestern Arizona (less than 500 feet).
Brown and Lowe (1977) also described two geographic subdivisions of the Sonoran Desert essentially similar to Shreve's, but in their mapping of the subdivision types, the Arizona Upland Subdivision, at about 10.5 million acres, includes the lower areas on mountains throughout southwestern Arizona.
Their Lower Colorado River Sonoran Desertscrub covers about 11.8 million acres.
The extension of the Chihuahuan Desert in southeastern Arizona is characterized by shrubs, primarily Creosote Bush (Larrea tridentata) and Tarbush (Flourensia deltoidea).
The Mojave Desert extension is characterized by the presence of Joshua Tree (Yucca brevifolia).
Arizona Upland Subdivision
The Arizona Upland subdivision includes an unusually large number of life forms due to the heterogeneity of the environment (Hastings and Turner 1965).
A large percentage of the surface area of the Arizona Uplands consists of hills and mountains. Rainfall in this subdivision is bi-seasonal ranging from 4 inches to over 12 inches per year.
This subdivision includes many shrubs, especially Creosote Bush and Triangle Leaf Bursage (Ambrosia deltoidea), but it is characterized by the number of tree species and succulents. Both foothills and Blue Palo Verde (Cercidium microphyllum and C. floridum) are common, along with Ironwood (Olneya tesota), and Mesquite (Prosopis velutina). Succulents include various species of prickly pear and cholla cactus (Opuntia spp.), barrel cactus (Ferocactus spp.) and saguaro (Carnegia gigantea).
Lower Colorado-Gila Subdivision
The Lower Colorado-Gila subdivision in Arizona has many fewer life-forms than the Arizona Upland, and is characterized primarily by low stature shrubs.
The characteristic shrubs are Creosote Bush and White Bursage (Ambrosia dumosa). Individuals of the tree species found in the Arizona Upland occur only along the floodplains and larger washes of this subdivision, which also has bi-seasonal rainfall of about 4 to 6 inches, but it is winter season dominant.
Both Sonoran Desert subdivisions produce extensive stands of annual plants following seasonal rains.
Biota
Biota of the Sonoran Desert in Arizona have been described by Lowe (1959, 1961, 1964). Because of the habitat diversity of the Sonoran Desert scrublands, faunal diversity is high. Examples include:
o Desert bighorn sheep, pronghorn, deer, and javelina
o Coyotes, skunks, raccoon, and fox
o Mice, wood rats, kangaroo rats, and ground squirrels
o Jackrabbits
o Bats
o Insects
o Birds
o Lizards, rattle snakes, other reptiles and amphibians
This diversity, along with the heterogeneity of the vegetation, makes the Arizona scrublands vulnerable to a wide range of perturbations.
Several researchers, such as Bahre and Shelton (1993), Hastings and Turner 1965), Martin and Turner 1977), Gehlbach (1981), Goldberg and Turner (1986), and Humphrey (1987), have recorded changes in Sonoran Desert scrublands in repeat ground photography and permanent plot studies. Except for the extirpation of the native vegetation for settlement, the decline of native grasses, and the invasion of exotics, most of the changes appear to be related to plant life cycles and/or short-term cycles linked to climatic and other environmental fluctuations.
o For example, Goldberg and Turner (1986) have demonstrated both short- term and long-term cyclic changes in the dominants such as Bursage, Brittlebush (Encelia farinosa), White Rattany (Krameria grayi), Janusia gracilis, Menodora Scabra, Staghorn Cholla (Opuntia versicolor), Lycium spp, and even long-lived species such as Ironwood, Foothills Paloverde, Creosote Bush, Saguaro and Blue Paloverde.
Bahre (1991) found no evidence that desert scrubland communities have invaded extensive areas of former grassland in southeastern Arizona or that the distribution of Chihuahuan and Sonoran desert scrubland communities has changed during the historic period.
Humphrey (1987) notes that many areas of desert scrubland that had supported grass or a grass-shrub mixture up to 1830, now support only shrubs. He attributes this change to long-exerted grazing pressures and a slight, but consistent, trend toward increased aridity.
It appears that the boundary between grassland and scrublands has not changed significantly, nor has the range of desert scrubland species, but the numbers and importance of shrubs has increased dramatically. The mechanisms by which these shrub increases have taken place are not well understood, but the increases are generally attributed to a combination of overgrazing and wildfire exclusion (Wright 1980). The other major directional change in desert scrubland is the rapid expansion of alien (introduced) species.
Environmental Stressors
Accidental Spills
Accidental spills occur along transportation routes.
The number of spills is low, and in most cases, spills are limited to the highway right- of-way. When spills of toxic materials are extensive enough to affect scrublands beyond the right-of-way, the effects can be serious and long term, as most components of the shrubland ecosystem lack resistance to such toxins. Often the soil will be contaminated, and if it is not removed, the resilience of the shrubland species is reduced.
The uncertainty of the consequences of spills is high because there is limited literature on this subject.
Air Pollution
Air pollution in Arizona is primarily point source outside of urban areas. In areas close to these point sources, as near copper smelters for example, effects can be serious because the pollution is chronic.
There are two types of ecosystem response to air pollution.
o The first, exemplified by high chronic levels of pollution like SO2, as has occurred at smelters with no controls, results in a loss of non-resistant species (Wood and Nash 1976).
o The second, exemplified by lower, but chronic levels of pollution, causes seasonal necrosis of leaves on woody plants, and probable reduction in herbaceous plant cover (Gabriel and Patten 1990, 1994a, 1994b).
Woody plants demonstrate a moderate level of resistance to air pollution, and when the pollution source is reduced or eliminated, they recover quickly (Gabriel and Patten 1994b).
Lichens have been used as indicators in the study of the effects of air pollutants on desert ecosystems, and their reduced occurrence in areas with chronic air pollution demonstrates that air pollution in desert scrublands has some general effects.
There is little uncertainty about the effects of air pollution on the desert scrublands even though the number of studies is limited.
Physical Alteration of Ecosystems
Grazing has had a major influence on the hot desert scrublands of Arizona, with the major physical alterations coming in the last half of the 19th century when there were few controls on number of grazers. During this period, the desert was so heavily grazed that when a drought occurred in the early 1890s, few cattle were able to survive (Hastings and Turner 1965, Bahre 1991). The condition of scrublands in Arizona today is in large part due to this early period.
In recent years, there has been better control and less physical alteration, but there are still many areas that are particularly sensitive to trampling of the vegetation, compaction of soil, disturbance of the soil surface, increased erosion, and other physical damage.
Although many desert plants, such as creosote bush, are not palatable to cattle, the desert ecosystem as a whole lacks resistance to grazing. The scrubland did not evolve with large ungulates such as cattle, and, as a consequence, is not adapted to this type of disturbance (Martin 1963).
One reason the scrubland now supports some grazing is the introduction of exotic grasses and forbs like Mediterranean grass and Erodium (parrots beak) that produce forage following periods of rainfall.
Grazing has also altered the scrubland/grassland transition areas by expanding the distribution of Mesquite (Humphrey 1968).
Removal of grazing allows a gradual recovery of the desert scrubland, but it will never recover to its original state because of the introduction of exotic plants. This is shown, for example, by studies at the Santa Rita Experimental Range.
Literature on the effects of grazing on hot desert scrublands is limited, although some of the extensive literature on effects on desert grasslands is applicable, for example, the many studies by Humphrey (1968).
Agricultural impacts to desert scrublands include complete removal of vegetation, serious soil disturbance, and massive changes to the area's hydrology. For these reasons, desert scrublands lack any resistance to agricultural development. Where agricultural fields have been abandoned, there are usually several conditions of soil disturbance to contend with, such as elevated salinity and groundwater drawdown.
Resilience of the scrublands is limited since it takes a long time for site physical and chemical conditions to return to normal. Recovery usually takes decades and possibly may take centuries (Karpiscak 1980, Conrad 1981). Total recovery may never be achieved because some species in the scrubland ecosystem may have been dependent on shallow groundwater.
Mesquite, once a common species in the Gila River floodplain near Coolidge, was lost to agricultural removal and groundwater drawdown (Judd 1971). The groundwater, once only a few feet below the surface, is now hundreds of feet deep, a depth well below the root zone of any of the deep-rooted desert scrubland woody species.
There is little uncertainty about the effects of agriculture on desert scrublands.
Highway construction leads to habitat destruction and isolation of animal populations. The desert ecosystem has no resistance to this type of disturbance, which obviously is long-term. There is also the immediate biological effect of road kills which can be a serious degradation of populations close to the highway, for example, desert tortoise populations (Nicholson 1978).
Isolation and fragmentation of desert animal populations can have long-term effects on the health of populations and ecosystems (Simberloff and Abele 1982). As long as the highway exists, the ecosystem will exhibit no resilience or recovery. Removal of the highway and restoration will have limited success because of the altered soils and other conditions, a condition also described for agricultural recovery on page 91.
Highways also produce a certain amount of toxics that affect the vegetation adjacent to the highway.
o For example, lead from gasoline and heavy metals from exhaust and brake linings enter the soils and may be taken up by the plants (May 1976), although increased use of unleaded gas will change this effect.
Except for studies on animal populations, long-term effects of highways are not well studied.
Energy Production
Energy production within the hot desert scrublands includes a limited number of fossil fuel power plants, one nuclear thermal power generator, and hydroelectric dams.
The fossil fuel plants produce some air pollution, but apparently at insufficient quantities to affect the desert ecosystem, although this is not well studied.
The thermal nuclear facility uses cooling towers that "spray" high salinity mist out onto the adjacent desert. This has been monitored by the power facility operator and apparently no significant changes have taken place thus far.
Hydroelectric dams form an impoundment which totally destroys the terrestrial desert scrubland it covers, but enhance development of riparian vegetation along the impoundment margin. Refer also to the discussion of riparian stressors beginning on page 2. This effect of hydroelectric dams is long-term, and there have been no cases within Arizona where a dam has been dismantled to permit a study of resilience or recovery of the desert ecosystem to this type of disturbance.
Fire Suppression
Fire was not common in desert scrublands prior to expansion of grazing and urban development (Shreve 1951, Humphrey 1974, Patten and Cave 1984, Cave and Patten 1984, Loftin 1987). These human activities brought in exotic annual vegetation which produces sufficient amounts of fuel to carry desert fires (Patten and Cave 1984, and others).
Thus, fire suppression is not an activity that has altered the scrubland ecosystem. On the other hand, keeping fires from the desert scrubland is a positive action in most cases.
Mining
Mining in upland scrublands has left major impacts, especially those associated with the large open-pit mining operations. These have left vast acreages devoid of any vegetation and without sufficient soil for recovery to take place.
The tailing ponds of these large mining operations also cover large amounts of scrubland.
Across many acres of Arizona there are also many scattered, small-scale mine shafts and waste piles.
The total use of the land by mining operations creates a situation in which the ecosystem lacks any resistance. None of Arizona's large open-pit mines have been totally abandoned, so the ability of these areas to recover is in question.
Attempts have been made to "restore" mine tailings by surfacing them with soil and planting grass and other xerophytic plants. This represents neither resiliency nor restorability of the site, because the species being used for restoration are usually non-native. With some extra irrigation, restoring tailings with non-natives has had some success, as at the Anamax Mine south of Tucson.
Some areas on mine tailings where surface water drains onto the site have shown some resiliency; however, the species that are recovering must be tolerant of the chemical conditions of the tailings. Tamarisk (Saltcedar), an invasive exotic species, has invaded these wet, stable areas on tailing deposits.
Long-term resilience or restorability of open-pit mines is uncertain.
Timber Management
Impacts of timber management on the Arizona Uplands portion of the desert scrubland have been acute and, for the most part, localized. From the 1890s to the late 1920's, cutting of Mesquite near Tucson created a radical shift in the ecosystem which is still in evidence (Clemensen 1987, McAuliffe 1993).
Today there are still areas where cutting of Mesquite and Ironwood for fuel and charcoal is a serious threat (Suzan et al. 1994).
The hydrology of streams that run through the desert scrubland may be altered by changes in the density of trees that line those streams.
Interbasin Water Transfer
Moving water from one hydrological basin to another may impact the hot desert scrubland if the transfer is pumped groundwater that once was used for agriculture. Tucson is allowed to transfer 60% of the original groundwater used for agriculture in the Avra Valley, the remaining 40% considered by the courts to equal groundwater recharge when the pumped groundwater was applied to agricultural lands.
Major interbasin transfer canals, such as those of the Central Arizona Project, may create a barrier to migration of desert animals similar to the impacts of highway construction.
Channelization
Channelization has immediate impacts on the localized hydrology and therefore, in addition to the physical alteration of the stream channel and surround lands, alterations may take place that have long-term chronic effects on the upland ecosystem.
In most cases, channelization most seriously affects the riparian ecosystem with little effect on the uplands. Refer also to the discussion of riparian stressors beginning on page 2.
Water Diversion
Water diversion may impact hot desert scrublands through changing the local hydrology. The effects, however, may be indirect and difficult to detect. Many animals from the scrubland may be dependent on the water source that is diverted, thus this activity may reduce the populations of these animals.
There is little literature on the effects of diversion on Upland ecosystems, but alteration of the river and riparian ecosystems where the water is removed is well known and discussed in the Rivers and Streams section on page 54, and in the Riparian section on page 2.
Removal of diversion structures and canals should allow the system to recover, but this has not been studied.
Groundwater Pumping
Reduction of groundwater has caused the physical change of the ground surface through subsidence and development of fissures (Laney 1976).
Except for surface cracks, pumping probably has had little effect on the hot desert scrubland. Of greater concern is removal of the groundwater source for many scrubland plants that once depended on the shallow groundwater (Judd 1971).
Reduction or elimination of groundwater pumping will not cause Upland groundwater to rise because the rate of groundwater recharge is too small to replace the hundreds of feet of groundwater decline.
No one knows how resilient Upland vegetation is to reduction of groundwater pumping, although Upland vegetation that is dependent on shallow groundwater lacks much resistance to the level of decline occurring in Arizona.
Recreation
Recreation causes the physical destruction of habitat through trailing and trampling. Especially destructive are off-road vehicles (Estabrook 1981). These activities lead to increased erosion and loss of vegetation canopy which results in loss of shade- requiring species (Suzan 1994).
The ability of the scrubland ecosystem to resist the effects of recreation is dependent on the magnitude and type of recreation. Continued off-road vehicle use is very destructive and the system lacks much resistance, while the ecosystem is quite resistant to change from limited hiking.
Resilience follows the same pattern. Heavy off-road vehicle use causes such extensive changes in soil cover that resilience is long-term, while the system is highly resilient to removal or changes in hiking trails.
The impact of recreation in desert scrub has had only limited study, but the studies that do exist show how easily the ecosystem can change (Estabrook 1981).
Urbanization results in the total loss of acres of desert scrublands to asphalt and concrete, people and cars. There is no returning to the natural ecosystem once any lands are utilized in this fashion.
Limited attempts at "restoring" desert scrubland through landscaping has had questionable success.
Biological Alteration of Ecosystems
Grazing
Grazing has led to a change in species composition and changes in the dominant plants of the southwestern deserts.
Exotic plants such as Mediterranean Grass, Red Brome, Parrots Beak and Lehman's Lovegrass have been accidentally and purposefully introduced. Often, less palatable species such as Burrobrush increase at the expense of more palatable species.
In some cases, overgrazing has resulted in an increase in scrublands into areas that were previously desert grassland. Mesquite has greatly increased in many grazed areas of southern Arizona (Humphrey 1968, Bahre and Shelton 1993).
Many acres of scrubland have been targeted for management which would lead to wholesale type conversion to rid the land of unacceptable species.
o An example of this is the chaining of Mesquite and the introduction of Lehman's Lovegrass.
The ability of the scrubland to resist the above activities is dependent on the magnitude of both grazing level and area. Unfortunately, if active management is removed along with grazing, the existence of exotic species reduces the resiliency of the ecosystem.
Restoring the ecosystem to a "natural" state following abandonment of grazing is essentially impossible because of the existence of exotics. There is sufficient literature on effects of grazing on biota of the desert scrubland to promote understanding of the biological effects of grazing.
Hunting
In most cases, hunting has short-term consequences to the hot desert scrublands. The major threat is that over-hunting may lead to threatened population viability. This would not be a concern for non-charismatic fauna.
Illegal Collecting
Illegal collecting threatens the hot desert scrubland ecosystem by concentrating on those charismatic species that have financial value. For the most part, this involves species of herptofauna like the desert tortoise and various rattlesnake species, and succulent plant species like the saguaro.
Takings of protected species or species with limited viability may have a long-term impact on the population with the species having little ability to resist this activity. For this reason, laws and regulations such as the Endangered Species Act and the Native Plant Protection Regulations guard against this type of stressor.
If collecting is terminated, resilience of the species depends on the population's level of degradation. There is extensive literature on those species that are threatened or endangered found in the documents used to establish their status, as well as the open literature.
Species Introduction
Species introductions, especially plant species introductions, are affecting nearly all of the hot desert scrublands.
Many of these species have altered the ecosystem, making it more susceptible to other human activities.
o For example, introduction of exotic annual vegetation has made the desert scrubland much more susceptible to wild fire (Cave 1982, Loftin 1987).
The ecosystem has little resistance to species introductions if the species are selected to tolerate the hot, dry conditions of the desert scrublands. If introductions are stopped, the existence of introduced species, many having become naturalized, is such that the ecosystem can never recover to its original structure and species composition.
Restoration is not a viable option. There is sufficient information on species introduction to show that this is a problem that can never be corrected. It is probably best to accept the changes and manage the ecosystem to maintain its functions, regardless of its altered composition.
Food and Drinking Water Contamination
This is not applicable to the hot desert scrubland ecosystem.
Global Climate Change
Global climate change may occur in the future. If it does, the global circulation models project that Arizona will be wetter and cooler, or drier and hotter.
This uncertainty leads to lack of consideration of this stressor as a major issue because little or nothing can be done about it. The ecosystem will respond to whichever direction the climate change takes.
Land and Soil Contamination
Contamination of land and soil is limited to highly developed areas within the hot desert scrubland. These areas include dump areas within and adjacent to cities, areas adjacent to industries which have used the land for waste deposition for decades, and agricultural areas. Refer also the Urbanization discussion on page 96, and the Agriculture discussion on page 91.
All of these locations no longer support a functional scrubland ecosystem, thus resistance and resilience does not apply to this issue.
Local contamination from accidental, and perhaps purposeful, "spills" of materials such as auto oil may create some very localized problems. This is only important if these types of spills or deposits are allowed to accumulate, in which case the contamination may cause long-term changes in the soils.
Natural Hazards
Floods
Sheet flooding across the surface of the desert is a normal phenomenon which has little impact on the desert ecosystem. Often connected with these events are sudden floods in desert washes. These may cause short-term disturbances which rapidly recover
Fire
Natural fire is a hazard to most hot desert scrublands (Patten and Cave 1984, Cave and Patten 1984, Loftin 1987). These systems are not fire-adapted and therefore fire is a major source of destruction.
Because of the increased presence of exotic annuals, the scrublands are not very resistant to fires.
Resilience to fires is limited as successional processes in the desert following fire are quite long because of the long-lived nature of the species that inhabit this area.
Restoration is possible, but not a recommended procedure because the desert will recover in time.
Information on response of desert scrublands to fire, and the concerns of increasing fires due to introduction of exotics, is adequate to show that this will be an increasing ecosystem problem in the future (Bahre 1985, McLaughlin and Bowers 1982).
Insect Infestations
The natural hot desert scrubland ecosystem is not affected in an unnatural fashion by natural insect outbreaks.
Radiation
Not applicable.
Surface Water Contamination
Not applicable to upland hot desert scrublands.
Groundwater Contamination
Not applicable to Upland hot desert scrublands.
Impediments
Lack of Education
The desert scrubland ecosystem, although revered by some, has been considered a wasteland and dumping ground by many. Until this attitude is changed, altering the continued degradation of the desert will never occur.
Equity Issues
The scrubland ecosystem, because it is extensive and not as structurally diverse as forests and riparian areas, is considered a low form of ecosystem and therefore one for which the public has little concern.
Transboundary Issues
The hot desert scrubland ecosystem extends into Mexico and California, and is managed within Arizona by many agencies and land holders, including many Native American Tribes.
For this reason, problems that may affect this ecosystem should be of interest to all groups. Unfortunately, all groups do not have equal concern and thus manage this ecosystem in different ways, often for top profitability rather than ecosystem sustainability.
Conclusions
The hot desert scrubland is one of the most extensive ecosystems in Arizona. It is diverse, extending from mid-elevation mixed tree, shrub and succulent communities to lower elevations characterized by shrubs.
Of all of the stressors, grazing has had the most extensive impact on this ecosystem. Associated with grazing has been the introduction of exotic species and the spread of woody native species.
The scrubland ecosystem is sensitive to disruptive forces that alter the soil or change the biota. Because most of these impacts were not part of the ecosystem's evolutionary history, the scrubland's ability to resist these stressors is slight.
Resilience and recovery of this ecosystem are slow because factors such as rainfall and temperature often create conditions unsuitable for rapid plant recruitment, animal reproduction, or soil recovery.
Arizonans should be concerned with any perturbation to the desert scrubland, but the extensive nature of this ecosystem reduces the ability to control these perturbations.
Arizona's Forests
Introduction
Forest ecosystems of Arizona are located in elevations generally greater than 5,000 feet in three broad regions:
o Mountains in the eastern part of the state
o Mogollon Rim
o Kaibab and Coconino plateaus
Forests also occupy the upper elevations and north slopes of numerous "sky islands" atop isolated peaks.
Forest types include, with increasing elevation, the Ponderosa Pine forest, the mixed Conifer forest, and the subalpine Spruce-Fir forest.
Forest establishment and maintenance are dependent on local climatic conditions and topography.
The ratio of precipitation to evaporation generally increases with elevation, resulting in a gradient from warm dry Ponderosa Pine forests at lower elevations, mixed Conifer forests at intermediate elevations, and Spruce and Fir forests at the highest elevations. Alpine tundra occurs above the Spruce-Fir forests only in the highest elevations of the San Francisco Peaks. (See Map B-7 (Forests of Arizona), Appendix B.)
Cooler conditions found on north-facing slopes or in drainages will allow forest types to grow at lower than normal elevations (Merriam 1884, Whittaker and Niering 1965).
Annual Precipitation
Precipitation in the Ponderosa Pine zone is about 20 inches per year, while that in the sub-alpine Spruce-Fir zone is 27 inches. Annual precipitation is bi-modal with summer storms formed by orographic events, and winter precipitation, normally as snow, the result of winter cyclonic events.
Ponderosa Pine Forest
The lowest elevation forest type considered in this document is the Ponderosa Pine forest ecosystem found from about 5,000 to 8,500 feet. The most extensive of the forest types in Arizona, the Ponderosa Pine forest ecosystem covers about 4.8 million acres, and is most widely used for commercial timbering. It is characterized by Ponderosa Pine (Pinus Ponderosa), but also includes species of Oak, for example, Arizona Oak (Quercus arizonica) at lower elevations, and Gambel Oak (Q. gambellii) at higher elevations.
Historically, the Ponderosa Pine forest was more open than at present, often appearing as a savanna with extensive grass cover among the scattered trees. The grass cover fueled frequent fires that reduced Pine recruitment and maintained the open nature of the forest (Cooper 1960, Covington and Moore 1994). Management of these forests over the past century has included fire suppression which has allowed increased Pine recruitment and reduction of grass cover.
The old-growth Ponderosa Pine forests support threatened or endangered species such as the Goshawk and Mexican Spotted Owl.
Logged stands of Ponderosa Pine create a wide variety of habitats and ecotones between forest and clear-cut areas. These may support a large diversity of species, some being selective to logged stands (Franzreb 1975).
Mixed Conifer Forest
The mixed Conifer forest occurs in a zone just above the Ponderosa Pine forest, at about 8,000 to 9,500 feet. Lower elevations may include some Ponderosa Pine, but the mixed Conifer forest is characterized by Douglas Fir (Pseudotsuga menzesii var. glauca), White Fir (Abies concolor), and southwestern White Pine (Pinus flexilis var. arizonica) (Lowe 1964, Brown and Lowe 1977). Quaking Aspen (Populus tremuloides) is a common successional species in this forest.
This forest type is also used commercially, but not to the extent of the Ponderosa Pine forest, Douglas Fir being the primary commercial species of the mixed Conifer type.
Today, the forest is a closed canopy forest with recruitment of the characteristic species occurring in the shaded, moist conditions on the forest floor.
Before Euro-American settlement, fire was a common occurrence in this forest. As evidenced by tree ring data, fire had a 5 to 10 year recurrence on the Pinaleño Mountains prior to fire suppression (Swetnam 1990), keeping the mixed Conifer forests much more open than they are today.
Sub-Alpine Spruce-Fir Forest
The sub-alpine Spruce-Fir forest occurs above the mixed Conifer zone, starting at about 9,500 to 10,000 feet and extending upward to timberline at about 12,000 feet. For this reason, Spruce-Fir forests are found only on the highest peaks in Arizona and cover very little acreage. The most common occurrences of Spruce-Fir forests are the San Francisco Peaks, the White Mountains, and the higher peaks, or sky islands, in the Madrean Archipelago such as Pinaleños or Mt. Graham.
The sub-alpine forest is characterized by Engelmann Spruce (Picea engelmanni) and Corkbark Fir (Abies lasiocarpa var. arizonica). The transition from this forest to the mixed Conifer may include Douglas Fir. The forest canopy is generally closed and recruitment occurs on the cool, moist forest floor.
Major fires have not been as common as in the other forest types, recurring about every 30 to 60 years (Stromberg and Patten 1993, 1994; Patten and Stromberg 1995).
The Spruce-Fir forests found on the sky islands in southwestern Arizona are considered to be relict forests from the Pleistocene, isolated by climate change and development of more arid ecosystems at lower elevations (Martin 1963). This long- term isolation has allowed speciation to take place in the associated animal and plant species, thus there is a high level of endemism in these forests.
The relative isolation of these forests, and low quality of the timber have prevented these forests from being commercially exploited.
Environmental Stressors
Accidental Spills
Accidental spills occur along transportation routes. The number of spills is low, and in most cases, spills are limited to the highway right-of-way. When spills of toxic materials are extensive enough to affect forests beyond the right-of-way, the effects can be serious and long-term as most components of forest ecosystems lack resistance to such toxins. Often the soil will be contaminated, and if it is not removed, the resilience of the forest species is reduced.
The uncertainty of the consequences of spills is high because there is limited literature on this subject.
Air Pollution
Air pollution in Arizona is primarily point-source, such as smelters, or urban non- point source.
Areas immediately adjacent to air pollution point-sources, such as locations of smelters and fossil fuel power plants, generally are not forested in Arizona. Thus, the high-level fumigations that might occur from these sources occur in lower elevation ecosystems such as desert scrubland. As a result, forests are spared high-level fumigation but may receive chronic low-level fumigation of air pollutants such as SO2 and ozone.
Urban non-point source pollution, especially from automobiles, is most common in the large urban centers of Arizona which are not located within forests.
With the exception of limited studies, especially near smelters (Nash 1976), the effects of air pollution on forest ecosystems in Arizona are not well understood. Nash studied only changes in the lichen flora near smelters, showing that there was some reduction in the number of species.
Studies in other western states have shown that, with increasing urban air pollution that may travel distances into surrounding forests, the forest vigor declines. An example of such a study occurred in the Sequoia National Park (Pronos et al. 1978; Williams and Williams 1986).
The ability of forest species to resist air pollution damage is based on the level and magnitude of the pollutant, but once damage begins, individuals will rapidly decline in health and vigor.
Removal of air pollution, if stopped before extensive damage occurs, will allow recovery of the forest. However, studies of the effects of air pollution and acid precipitation in forests of the eastern United States show that effected ecosystem processes are not readily reversed (see National Research Council 1989).
Physical Alteration of Ecosystems
Grazing
Because of their relatively high elevation and cold winters, forests are grazed mostly in summer. Of the three forest types discussed here, grazing occurs primarily in the Ponderosa Pine forest and in most of the area covered by this type.
The mixed Conifer and Spruce-Fir forests have limited herbaceous cover, except in forest openings and meadows.
Direct impacts of grazing in forests are mainly on the herbaceous cover, although there is evidence in some forests that heavy grazing may damage tree seedlings.
Grazing of domestic livestock in the higher elevation forests often conflicts with use of the areas by wildlife, especially elk. The combination of cattle and elk has had some negative impact on forest ecosystems, although, some studies show that elk tend to utilize areas previously grazed by cattle because of the palatability of herbaceous regrowth and higher quality forage (Miller et al. 1993)
If cattle grazing were the only grazing impact in the forests, ecosystem resistance might be high, but the combination of elk and cattle can cause heavy utilization and low resistance.
Resiliency of the forest herbaceous ecosystem following reduction in or removal of grazing, is dependent, as with other ecosystems, on the original level of grazing. Resilience is lower if the system was overgrazed (Jeffries and Klopatek 1987; Covington et al. 1994).
Agriculture
In Arizona, few forest areas have been cultivated for agriculture. The climate and water availability in the forested areas is not conducive to extensive agricultural development.
Essentially no natural ecosystem is resistant to agricultural cultivation. On the other hand, abandoned agricultural fields in a forested area can be expected to recover relatively quickly, although few studies have been done to demonstrate this. For example, only limited evidence documents Anasazi agriculture on the Kaibab and Coconino plateaus, but this occurred nearly 800 years BP and primarily in the Pinyon-Juniper type.
Highways
Highway construction leads to habitat destruction and isolation of animal populations.
Forest vegetation has no resistance to this type of disturbance, which obviously is long-term.
There is also the immediate biological effect of road kills, which can pose a serious threat to populations near the highway such as deer and other ungulates that may dash across the highway or utilize roadside plantings. Isolation and fragmentation of animal populations can have long-term effects on the health of populations and ecosystems (Wilcove et al. 1986).
As long as the highway exists, the ecosystem will exhibit no resilience or recovery. Removal of the highway and restoration will have some success because disturbed soils will allow rapid seedling establishment.
Except for limited studies on animal populations, long-term effects of highways is not well known.
Energy Production
Except for potential effects of air pollution from coal-fired thermal facilities, energy production is not relevant as a stressor to forest ecosystems in Arizona. This is because hydroelectric dams are constructed at elevations below the forest zones.
Fire Suppression
Of the three forest types discussed in this report, the Ponderosa Pine and mixed Conifer forests have been negatively impacted by fire suppression.
o For example, the Ponderosa Pine forests are dependent on surface fires to limit the density of forest recruitment, control fuel accumulation, and rejuvenate herbaceous and shrub understory (Covington et al. 1994).
o Fire suppression thus has allowed the forest to become denser than normal, increasing the fuel load and the potential for more destructive fires.
o Fire suppression or exclusion has also resulted in modification of the hydrological regimes of forested areas. Soil water use is higher because of the increased forest cover and stream flows may be reduced (Covington and Moore 1994).
Ponderosa Pine and mixed Conifer forests have little resistance to fire exclusion, responding with increased tree density and resulting in a closed forest (Swetnam 1990). If fire suppression were stopped, high fuel loads might cause more damaging fires than under the natural park-like forest structure, reducing the resiliency of the forest to elimination of this stressor.
Ponderosa Pine forests can be restored, but through careful forest management practices, primarily using thinning, removal of heavy fuel accumulations and controlled burns (Covington and Moore 1994).
Spruce-Fir forests are not as dependent on fires for seedling recruitment as Ponderosa and mixed Conifer forests, thus fire suppression does not greatly affect this aspect of Spruce-Fir forest dynamics. However, fire suppression has allowed greater fuel loads to accumulate. These fuel loads increase the potential for more destructive crown fires. In the Spruce-Fir forest type which is not very responsive to fire suppression, discussion of resistance to fire suppression is not particularly relevant.
Mining
Most open-pit mining in Arizona occurs in the desert scrublands or chaparral and Pinyon-Juniper zones. Thus, except for air pollution from smelters, mining has little or no impact on the forest ecosystems. (Refer to the Air Pollution discussion on page 78 for additional information.)
There are small mining claims throughout the mountains of Arizona, but these are old and affect runoff and stream water quality rather than the surrounding forests.
Timber Management
Timber management relates to the various practices used to utilize the forests for lumber, wildlife habitat, water yield and recreation. In most cases it entails modifying the forest from its natural state. Fire suppression is just one form of timber management. Harvesting, thinning, planting, and vegetation modification that, for example, replaces trees with grass, are all timber and associated watershed management practices.
Some of these practices may affect forest wildlife by, for example, altering forest canopy structure for avian habitat, or creating forest openings for increased herb and browse habitat. Because these management practices are designed to alter the forest, the forest ecosystem has little resistance.
On the other hand, if all of these practices ceased, it probably would take many decades or even centuries for the forest to recover to a state resembling the original conditions. Thus, resiliency is moderate on a long-term basis, but low on a short-term basis. There is little information about abandoned commercial forests.
Ponderosa Pine forests are the primary commercial forest in Arizona, thus mixed Conifer and Spruce-Fir forests are managed on a limited basis, and only when marketable timber exists.
Interbasin Water Transfers
Not relevant to forests in Arizona.
Channelization
Channelization is practiced primarily on the lower elevation, high-order rivers, and thus is not relevant in this discussion of higher elevation ecosystems.
Water Diversion
Small water diversions may occur in forest ecosystems but these primarily affect the rivers and not the forest ecosystem itself.
Groundwater Pumping
Small or local wells are common throughout the forest ecosystems wherever human activity occurs. Groundwater recharge in these mesic, higher elevation zones is high, thus the effects of groundwater pumping on shallow rooted trees is negligible.
Impoundments
Small impoundments, primarily stock tanks, are found in many forested areas.
In Arizona, most large impoundments used for water storage are located at elevations lower than the forest zones, with the exception of reservoirs for Flagstaff and other high elevation towns.
Forests are removed to make room for impoundments, and thus forests have no resistance to this activity.
Removal of the impoundment will allow forest re-establishment, which probably will be short-term because of the suitable silt to loamy mineral soil in the impoundment basin. Abandonment of large impoundments is not common, but observations of small abandoned stock tanks support this concept, unless the impoundment berm is left in place. Then water will still impound and prevent recovery of the site.
Information on effects of impoundments on forests is very limited.
Recreation
All forest ecosystems throughout Arizona are heavily used for recreation. Hiking, camping, off-road vehicle use, and winter sports are common. Many forests are managed for specific recreational activities.
Forests show little resistance to active management practices such as campground development and recreational road construction. Unfortunately, recreational use can have major impacts if campfires escape, or if off-road vehicles invade non-use areas.
Most forests have a relatively high level of resilience if recreational activities are removed, but this is dependent on the type of recreational activity. Sites with extensive soil disturbance for such activities as parking areas will show a prolonged resilience period. Curtailment of camping for only a few years in specified areas is often used to allow vegetation recovery.
The US Forest Service has compiled considerable literature on recreational impacts, using it to improve recreational management. For additional information, refer to the extensive Research Memoranda from the USFW Rocky Mountain Forest and Range Experimental Station.
Urbanization
Where urbanization occurs within forest ecosystems, it causes a total loss of the ecosystem. Although small, this area is increasing as towns in the cool forests expand due to second-home development and retirement migration.
The forest has no resistance to this type of activity and there is no information on recovery or resilience to modern urbanization. Abandoned mining towns have recovered over a period of about a century. This might indicate a moderate level of resilience if non-organic construction materials were removed from the town site, an unlikely occurrence today.
Biological Alteration of Ecosystems
Grazing
Grazing in forests has led to changes in species composition as cattle selectively graze more palatable species (Covington et al. 1994). The interaction of elk and cattle compounds the impacts, making it difficult to place the cause of change strictly on cattle grazing.
Composition changes and loss of species are not as great within the forest community as in the riparian ecosystems associated with the forests, as also described in the Riparian section beginning on page 2.
If grazing is low or moderate, forests are moderately resistant to species change, but heavy grazing reduces this resistance, especially when grazing is combined with altered fire regimes (Covington et al. 1994). Fire exclusion alone can produce similar and extensive changes in vegetation composition with woody plants invading and reducing the amount of herbaceous cover.
Resilience is not well understood because the exclosure studies have been done with exclusion of fire and thus the forest response produces abnormal high density stands.
Hunting
Management of game animals, especially elk, for hunting has had a negative impact on forest ecosystems as the populations are allowed to expand to meet hunting demands. Conflicts with cattle grazing also occur as described in the Grazing discussion on page 91.
Illegal Collecting
Not applicable to forest ecosystems in Arizona.
Species Introduction
All ecosystems have been altered to some extent by purposeful or inadvertent introduction of nonnative species.
Species introductions to forest ecosystems are no exception.
o For example, forests have experienced the introduction of exotic herbivores like cattle, horses, sheep, and Rocky Mountain elk, and exotic herbs like Cheatgrass, Timothy, and Kentucky Bluegrass. (Covington and Moore 1994).
This demonstrates a low resistance of the system because the exotic species have become functional components of the forest ecosystem.
Often, when a competitive species is introduced, the whole ecosystem may not change, but the native species may be lost or greatly degraded.
o An example of this occurred with the introduction of the Tassel-Eared Squirrel to the Pinaleño Mountains. This species forced the Mount Graham Red Squirrel to a limited range on the mountain top, where it now is considered endangered. This example also indicates a low resistance of the native species, but not much alteration of the ecosystem. There is no information to determine how the species or ecosystem might respond if the introduced species were removed.
Food and Drinking Water Contamination
This is not applicable to forest ecosystems.
Global Climate Change
Speculation about global climate change has produced many conflicting models.
Warming and/or drying can be expected to cause long-term upward migration of forests, while cooling and/or increased precipitation will cause the opposite effect. Continued high CO2 concentrations will favor woody vegetation expansion over herbaceous vegetation (Covington et al. 1994).
Land and Soil Contamination
Contamination of land and soil is limited to highly developed areas . In the forest ecosystems these areas primarily include dump areas within and adjacent to cities and towns.
These locations no longer support a functional forest ecosystem, thus resistance and resilience do not apply to this issue.
Local contamination from accidental, and perhaps purposeful, "spills" of materials such as auto oil may create some very localized problems. This is important only if these types of spills or deposits are allowed to accumulate. Consequently, the contamination may cause long-term changes in the soils.
Natural Hazards
Floods
Floods occur in the elevational range of forest ecosystems in Arizona. Flooding, however, affects floodplain ecosystems and not Upland forests. Thus flooding is not a critical stressor to forest ecosystems.
Fire
A natural phenomenon within forest ecosystems, fire removes accumulated organic debris within the forest and triggers a successional process that creates a variety of habitats while the forest is rejuvenated.
Fire suppression may create fires of greater magnitude than normal which result in an unnatural modification of the forest system.
Sub-alpine Spruce-Fir forests tend to resist fires, primarily because of the mesic conditions within the forest.
Because forests evolved with fires as a natural process, forests tend to be highly resilient following natural fires.
Controlling fires only prolongs the inevitable, while creating a condition that enhances seedling and sapling survival and accumulation of litter. These conditions tend to produce a greater potential for high-intensity fires, often extensive crown fires. Today's fires, as well as insect outbreaks, are unprecedented in the evolutionary history of the biota.
Careful restoration treatments are necessary before the practice of fire exclusion is modified. There is extensive literature on effects of fire in southwestern forests, both from academic research and research within the USFS Rocky Mountain Range and Experiment Station.
Insect infestations
Periodic insect outbreaks are normal occurrences within forest ecosystems. Outbreaks occur either on a cyclical basis, or when the forest is over-mature and the trees are senescent.
Healthy trees generally are more resistant to insect invasion than unhealthy or senescent trees, but this is not always the case, especially in extensive insect infestations.
Most forests are highly resilient following insect infestations, recruitment occurring in openings created by the death of infected trees. In a few cases, shrub invasion during the infestation may reduce the resilience of the forest and its ability to reestablish a cohort of young trees.
Forest entomology is a well-developed science and our understanding of commercial forest responses to insect damage is good.
Radiation
Not applicable to the natural ecosystem.
Surface Water Contamination
Little applicability to upland forest ecosystems.
Groundwater Contamination
Little applicability to Upland forest communities with shallow rooted species.
Arizona's Grasslands
Introduction
The high landscape diversity which characterizes Arizona is well illustrated by the diversity of its grassland types. These grasslands differ in species composition, physical site characteristics and evolutionary history. They also, therefore, differ in response to physical and biological stressors.
While generalizations can be drawn regarding the response of grasslands to various ecosystem stressors, such generalizations may obscure the range of this ecosystem's responses to disruptions, responses which may characterize this general ecosystem type. General discussions of vegetation change on Arizona ecosystems, including grasslands, are given by Bahre (1991) and Hastings and Turner (1965).
Lowe (1964) identifies three grassland types in Arizona:
o Desert grassland, which occurs principally in the southeastern corner of Arizona and is considered by to be a transitional grass-dominated landscape between deserts below and grassland or shrub lands above.
o Plains grasslands, which occurs both in the eastern portions of the state and in Santa Cruz county in south-central Arizona.
o Mountain grasslands, which occur in small openings in the coniferous forests in northern Arizona.
Brown (1982) subdivides Arizona grasslands into the following categories:
o Arctic-Boreal: These grasslands are represented in Arizona by Alpine and Sub-alpine Grasslands.
Alpine Grasslands are very limited in Arizona and are confined to the highest elevations in the Pinaleño, Escudilla, and San Francisco mountains. Sub-alpine Grasslands are also limited in extent but occur over a larger area, particularly in the White Mountains.
o Cold-Temperate: These grasslands are represented by the Montane Meadow and Plains and Great Basin Grasslands types.
Montane Meadows occur in natural openings at higher elevations and are usually restricted to flatlands with heavy, poorly drained soils.
Plains and Great Basin Grasslands occur over much of the state excluding the southwestern quadrant but are more common in eastern and southcentral Arizona.
o Warm-Temperate: These grasslands are represented by only semi-desert grasslands which occur in southeastern Arizona above the Chihuahuan Desert.
o Tropical-Subtropical: These grasslands are again very limited in extent and are represented only by the Sonoran Savanna Grassland found in the Altar and Santa Cruz valleys of southern Arizona.
The distribution of Arizona grasslands can be seen in Map B-8 (Grasslands of Arizona), Appendix B.
Environmental Stressors
Accidental releases
Accidental releases of toxic materials in grassland ecosystems are expected to be local events near such areas as highways, railroads, other transportation corridors, or mines.
The most likely releases would be petroleum products like oil or gasoline. The consequences of such releases may be severe on a very local scale, but have little impact on the larger ecosystem.
Risk is expressed as a function of both the impact of the stressor and the likelihood of exposure to that stressor. Risk to grasslands from this stressor, therefore, is low because of the limited spatial area impacted and the limited probability of exposure to accidental spills, even though consequences of a spill can be severe on a very local scale.
Accidental spills and accidental releases and their effects on Arizona's grassland ecosystems are not documented in the literature. In spite of this lack of documentation, the uncertainty associated with this stressor is low.
Air Pollution
Data does not yet suggest that air pollution places Arizona's grassland ecosystems at risk.
Smelting operations and power plants are potential sources of air pollution. Significant impacts on grassland structure and function, however, have not been documented.
Literature which discusses possible effects of air pollution on grassland ecosystems located outside Arizona include Grunhage (1993), Heil and Bobbink (1993) and Hrabe (1990).
Physical Alteration of Grassland Ecosystems
Grazing
In grassland ecosystems, grazing, which is principally by domestic livestock, may produce substantial changes including loss of vegetation cover, accelerated soil erosion, declining watershed condition, and changes in species composition of the plant community (Bahre 1987, Blesky 1992, Blydenstein et al. 1957, Buffington and Herbel 1965, Cook and Stoddart 1963, Costello and Turner 1941, Cottam and Evans 1965, Cottam and Stewart 1940, Daubenmire and Colwell 1942, Duce 1918, Dunford 1954, Ellison 1960, Gardner 1950, Gifford and Hawking 1978, Grover and Musick 1990, Humphrey 1958 and 1968, Johnson 1962, Klemnedson 1956, Knipe 1971, Knoll and Hopkins 1959, Lull 1959, Martin 1975, McClaran and Anable 1992, O'Connor 1991, Sheridan 1981).
Grasslands of southeastern Arizona, unlike grasslands of the great plains, probably did not evolve with a large herbivore (MacDonald 1981).
Early studies identified cattle grazing as a factor in alteration of the herbaceous community with increases in shrubs, cacti and annual grasses at the expense of perennial bunch grasses (Griffiths 1910, Leopold 1924). Cattle grazing in the 1980's was described as an exotic disturbance that modified the native vegetation (Brown 1982, Bock and Bock 1993).
Brady et al. (1989) and the Bocks (Bock and Bock 1991, 1993) found that perennial native grasses responded positively to release from grazing in grassland areas with high rainfall. The herbaceous responses to release from grazing found in Arizona grasslands differed from the Great Plains where the dominant Blue Grama community was not altered (Milchunas et al. 1988).
The response of desert grasslands to grazing is dependent on the mix of grazing- tolerant and grazing-intolerant species and other local conditions (Bock and Bock 1993, Mack and Thompson 1982, Milchunas et al. 1988). Actual changes may also depend on the "dose" of the grazing stressor, that is, the intensity, timing, and season of grazing.
A National Research Council study concluded that most observers agree that overgrazing, drought and other stresses resulted in widespread grassland degradation in the late 1800's and early 1900's (NRC 1994). However, they also concluded that the present health of US rangelands, including grasslands, is a matter of sharp debate because of disagreements over methods of evaluation and a lack of reliable data.
The result is that conclusions about the impacts of grazing on Arizona grasslands, although well documented in the literature, may be uncertain if the NRC report is applicable to desert grasslands.
Assignment of risk to the grazing stressor for Arizona grasslands was based on the following assumptions:
o First, the literature makes it clear that poor grazing practices may drastically impact grasslands. Under the arid to semi-arid climate which characterizes Arizona, resistance to grazing is low to moderate on most Arizona grasslands.
Resilience is high for low impacts. Current theory suggests, however, that high impacts may result in irreversible ecosystem change. Restorability is uncertain following change beyond some thresholds.
o Second, virtually all grasslands have been exposed to domestic livestock grazing, and Montane Meadows are also impacted by elk.
However, careful evaluation must be made of the extensive, yet often limited, rangeland studies such as at the Santa Rita Experimental Range and Jornada Experimental Range, especially for such factors as Arizona rangelands dose of the grazing stressor in terms of intensity, timing, or season of grazing, if actual ecological risk is to be understood with any confidence.
Furthermore, the influence of climatic variation on Arizona grasslands, such as the 1950's drought, makes interpretation of the existing data difficult. The result is that conclusions about the impacts of grazing on Arizona grasslands, and the extent of ecological risk is far from certain.
The Committee concludes that Arizona grasslands are at high risk from the grazing stressor because of the wide-spread exposure of grasslands to grazing and the potential negative impacts of poor grazing practices. However, the Committee also concludes that uncertainty regarding actual grazing practices and impacts of grazing on Arizona grasslands results in a moderate uncertainty for this stressor.
Agriculture
Most Arizona grasslands have been too dry for agriculture, thus lessening the fragmentation problems which have characterized the Great Plains or eastern Washington's Palouse Prairie.
Highways
Grasslands have been fragmented particularly by transportation corridors which destroy habitat on a local scale, allow invasion of exotic species (Tyser and Worley 1992), disrupt animal territories, and encourage urbanization and recreational use of the ecosystem (Wilshire and Nakata 1976).
Transportation corridors also result in loss of grassland habitat. Protection from livestock grazing, however, may result in these corridors serving as "seed banks."
While some species, such as antelope, may be at increased risk because of habitat fragmentation, overall, the risk to Arizona grasslands is low. Presently, the actual extent and ecological effects of fragmentation are uncertain.
Fire Suppression
Fire has played an important role in the development of many of Arizona's grasslands. Many historic grasslands were perpetuated by repeated burning which prevented the invasion of shrubby life forms (Belsky 1992, Branscomb 1958, Brown 1950, Cable 1967, Cove and Patten 1984, Cox et al. 1993, Daubenmire 1968, Humphrey 1953, Humphrey and Mehrhoff 1958, Martin 1983, Thomas and Goodson 1992).
Control of fire in this century, along with the removal of fuel by grazing, has resulted in the loss of many grasslands (see Hastings and Turner 1965).
Other grasslands, like the Montane Meadows, were not fire-dependent, but were fire- tolerant communities.
Consequently, lack of fire, due to suppression and/or lack of fuel as a consequence of grazing, has resulted in a net loss of grasslands in Arizona.
While a continued policy of controlling fire places some remaining Arizona grasslands at moderate to high risk, the actual extent of grasslands at risk is uncertain.
Urbanization
Most Arizona grasslands are not located near population centers, and therefore, urbanization has not affected them.
Notable exceptions include the central Arizona grasslands near Prescott and the Sonoran Savanna Grasslands near Sonoita. These grasslands are at high risk for loss, while most grasslands would be low to moderate risk.
The actual extent of grassland ecosystem loss to urbanization is uncertain.
Biological Alteration of Ecosystems
Species Introduction
Numerous species have been introduced to Arizona's grasslands. Most have not attained pest status.
A notable exception is the introduction of Lehmann's Lovegrass (Eragrostis lehmannia) into southern Arizona (Anable et al. 1992, McClaran and Anable 1992).
This South African grass has produced substantial changes in ecosystem structure with severe short-term impacts (such as on bio-diversity) and unknown long-term impacts. For example, Lehmann's Lovegrass functions as an early successional species in South Africa.
Southern Arizona grassland ecosystems have low to moderate resistance to Lehmann's Lovegrass invasion, which is facilitated by both grazing and fire, and low reversibility and restorability following invasion.
Mesquite (Prosopis spp.), Juniper (Juniperus spp.), and other woody species have also increased on Arizona grasslands (Bahre and Hutchinson 1985, Bahre and Shelton 1993, Glenndenning and Paulson 1955, Hastings and Turner 1965, McPherson et al. 1993) converting them into savannas or woodlands.
There is uncertainty as to the extent to which these invasive species will continue to increase in abundance.
Food and Drinking Water Contamination
This is not significant for grassland ecosystems.
Global Climate Change
Consequences are potentially severe and warrant further careful study. The likelihood of specific consequences, however, is uncertain. Risks are therefore unknown and uncertainty is high.
Land and Soil Contamination
Contamination of land and soil is limited to developed areas and transportation corridors and is generally a problem only on very small spatial scales and thus is not significant for the larger grassland ecosystem.
Natural Hazards
Fire, which is the principal disturbance factor for grasslands, is subject to a high degree of human influence, which decreases its frequency while increasing its intensity.
By altering the fire regime, human intervention also alters the effects of fire. Fires, when they occur, may be hotter and cause more damage than pre-historic fires (Finberg 1994).
Radiation
Not applicable to the natural ecosystem.
Surface Water Contamination
Surface water contamination is not a major factor on grassland ecosystems.
Ground Water Contamination
Little applicability for grassland ecosystems.