Saguaro Cactus and Palo Verde Desert

This ecological system occurs on hillsides, mesas and upper bajadas in southern Arizona and extreme southeastern California. The vegetation is characterized by a sparse emergent tree layer of Carnegiea gigantea (3-16 m tall) and/or a sparse to moderately dense canopy of xeromorphic deciduous and evergreen tall shrubs codominated by Parkinsonia microphylla and Larrea tridentata, with Prosopis sp., Olneya tesota, and Fouquieria splendens less prominent. Other common shrubs and dwarf-shrubs include Acacia greggii, Ambrosia deltoidea, Ambrosia dumosa (in drier sites), Calliandra eriophylla, Jatropha cardiophylla, Krameria erecta, Lycium spp., Menodora scabra, Simmondsia chinensis, and many cacti, including Ferocactus spp., Echinocereus spp., and Opuntia spp. (both cholla and prickly-pear). The sparse herbaceous layer is composed of perennial grasses and forbs with annuals seasonally present and occasionally abundant. Outliers of this succulent-dominated ecological system occur as "Cholla Gardens" in transitional areas in the southern and eastern Mojave Desert ecoregion. In this area, the system is characterized by Cylindropuntia bigelovii, Senna armata, and other succulents, but it lacks the Carnegiea gigantea and Parkinsonia microphylla which are typical farther east. Fouquieria splendens is present in increasingly diminishing amounts in the system where it occurs further west and north.

The vegetation is characterized by a diagnostic sparse, emergent tree layer of Carnegiea gigantea (3-16 m tall) and/or a sparse to moderately dense canopy codominated by xeromorphic deciduous and evergreen tall shrubs Parkinsonia microphylla and Larrea tridentata, with Prosopis sp., Olneya tesota, and Fouquieria splendens less prominent. Other common shrubs and dwarf-shrubs include Acacia greggii, Ambrosia deltoidea, Ambrosia dumosa (in drier sites), Calliandra eriophylla, Jatropha cardiophylla, Krameria erecta, Lycium spp., Menodora scabra, Simmondsia chinensis, and many cacti, including Ferocactus spp., Echinocereus spp., and Opuntia spp. (both cholla and prickly-pear). The sparse herbaceous layer is composed of perennial grasses and forbs with annuals seasonally present and occasionally abundant. On slopes, plants are often distributed in patches around rock outcrops where suitable habitat is present. Outliers of this succulent-dominated ecological system occur as "Cholla Gardens" in transitional areas in the southern and eastern Mojave Desert ecoregion. In this area, the system is characterized by Cylindropuntia bigelovii (= Opuntia bigelovii), Senna armata, and other succulents, but it lacks the Carnegiea gigantea and Parkinsonia microphylla which are typical farther east. Fouquieria splendens is present in increasingly diminishing amounts as the system occurs further west and north.

Climate: Climate is arid to semi-arid, continental with mild winters and hot summers (Niering and Lowe 1984). Precipitation has a bimodal distribution with rain in the winter (December-February) and a summer monsoon (July-September). Extended periods of drought or episodes of extreme cold limit this type. Specifically, establishment of dominant species is constrained by decadal or longer periods of below-average precipitation (Turner et al. 1995). Twenty-four hours of below-freezing temperature causes nearly total mortality of the dominant plants. At the southern end of the system's range, competition from more mesic species may constrain distribution of this system (Turner et al. 1995).

Physiography/landform: This succulent desert scrub ecological system occurs on hillsides, mesas and upper bajadas in southern Arizona and extreme southeastern California. Stands are typically found below 1200 m elevation, with rare occurrences up to 1400 m. Landforms range from steep, rocky slopes of desert mountains to upper and lower bajadas extending out on to alluvial flats. With decreasing elevation, the system typically occurs in xeroriparian habitats (edges of channels and washes) and on rock outcrops.

Soil/substrate/hydrology: At higher elevations of bajadas and on steeper surfaces, the system is found on coarse soils that may be associated with poorly developed geomorphic (aka frequently eroded) surfaces; at lower elevations (bottom of bajadas and alluvial fans far from risk of flooding), it is found on very stable geomorphic surfaces. The soils are often underlain by an impervious caliche layer.

Complex ecological factors determine the occurrence of characteristic species Carnegiea gigantea. Major range-limiting factors are cold winters and dry summers. According to Benson (1982), Carnegiea gigantea is killed by extended frosts and does not occur above 1370 m elevation. Its seeds germinate and seedlings and adults grow mostly during the summer monsoon season, so the lack summer moisture further west restricts it from the Mojave Desert. Seedlings require shade from rocks or shrubs called "nurse" plants for seed germination and seedling establishment. The nurse plant protects seedlings from drying out in the intense desert sun, and possibly from frost and predation (Benson 1982, Brown 1982a). As it grows, Carnegiea gigantea may inhibit the nurse plant and cause dieback in these shrubs or possibly damage itself significantly (Brown 1982a). In Arizona, north slopes are generally too cold for Carnegiea gigantea to germinate; therefore, the best sites are mesic microsites on warm exposures where there is shade and a slight depression to concentrate precipitation. Bats such as lesser long-nosed bat (Leptonycteris yerbabuenae) and Mexican long-tongued bat (Choeronycteris mexicana) pollinate these large night-blooming cacti. Once the fruit ripens in June, lesser long-nosed bat, white-winged dove (Zenaida asiatica), Gila woodpecker (Melanerpes uropygialis), and other birds or mammals consume the fleshy red pulp and disperse the seeds, which pass through their guts intact (Pavek 1993b). Seed dispersal beneath nurse plant shrub canopies such as Parkinsonia microphylla is primarily done by frugivorous birds and is a major factor in saguaro establishment (McAuliffe 1988, 1993). Carnegiea gigantea are vulnerable to fire with smaller individuals (<2-4 m tall) generally killed, especially if large amounts of fuel are present at the plant base, but larger individuals may survive (McLaughlin and Bowers 1982, Pavek 1993b).

This system is not thought to have supported fuel loads to sustain large fires prior to European habitation of the region. Historically, fires in the Sonoran Desert were usually low intensity and uncommon with fire-return interval greater than 250 years because of limited fuel loads (McLaughlin and Bowers 1982, Thomas 1991). Natural fires are associated with dry lightning coincident with monsoonal storms following years when previous winter precipitation was sufficient to create a thick fine-fuel bed of annual plants to carry fire. These fires tend to be patchy due to heavier fuel in microsites, or linear when high winds were associated with convection storms (LANDFIRE 2007a). Replacement fires were very rare or absent (average FRI of 100-1000 years, and perhaps longer) (LANDFIRE 2007a). If they occurred, they did so only during conditions of extreme fire behavior after consecutive years of above-average winter precipitation when necessary fine fuels accumulate. These rare fires - which may or may not have occurred - had tremendous influence on community structure because the dominant overstory plants are extremely susceptible to fires, even those of low intensity (McLaughlin and Bowers 1982, Esque et al. 2004).

LANDFIRE developed a VDDT model for this system which has three classes (LANDFIRE 2007a, BpS 1411090):
A) Early Development 1 Open (5% of type in this stage): Shrub cover is11-50%. Initial post-disturbance community dominated by bursage. Duration 20 years with succession to class B.

B) Mid Development 1 Open (shrub-dominated - 20% of type in this stage): Dominated by bursage and early-seral shrubs such as Encelia farinosa. Perennial warm-season grasses are scattered, and dominant succulents and woody plants have established beneath bursage plants. Duration 50-100 years with succession to class C unless infrequent replacement fire or climatic event (drought, frost) returns vegetation to class A. Lethal freeze and drought are listed as Wind/Weather/Stress in model.

C) Late Development 1 Closed (shrub-dominated - 75% of type in this stage): Succulent- and small tree-dominated community. Persists until infrequent replacement fire or climatic event (drought, frost) returns vegetation to class A. Lethal freeze and drought are listed as Wind/Weather/Stress in model.

Prolonged weather-related stress (drought or frost) thinned dominant overstory plants and, in rare cases, led to stand replacement. It is speculated that these events occurred with similar frequency as stand-replacing fires (LANDFIRE 2007a). Cold stress is more common in stands at the northern extent and at higher elevations on desert mountain ranges. Large (presumably old) saguaro plants are also susceptible to windthrow, particularly after rainstorms saturate the soil (LANDFIRE 2007a). LANDFIRE modelers note there is much uncertainty in model parameters, particularly with respect to the return interval of fire, drought and lethal cold temperatures (LANDFIRE 2007a).

Primary land uses that alter natural processes of this system directly affect vegetation and soil surface through disturbance and fragmentation, and annual non-native species invasion. Recent conversion of this type has commonly come from installation of irrigated agriculture near rivers and forage production sites in northern Sonora, Mexico, and southern Arizona where desert is cleared and Pennisetum ciliare is planted for forage production.

Altered fire regime from encroachment by invasive non-native grasses such as Bromus rubens, Schismus barbatus, and perennial Pennisetum ciliare are serious threats and stressors to this ecosystem. Annual invasive non-native grasses such as Bromus rubens and Schismus barbatus and other annuals can build up enough litter (fine fuels) after a couple wet years to carry fire and cause massive destruction to fire-sensitive desert species. These invasive non-native species have greatly increased the incidence and extent of fires in the Sonoran Desert as these grasses carry fire between shrub interspaces and generally increase fuel loads, fire extent and severity.

Excessive stresses to the system through soil disturbance from off-road vehicle (ORV) use and heavy grazing can alter the composition of perennial species and increase the establishment of native disturbance-increasers and exotic annual grasses. Pennisetum ciliare, a fire-adapted perennial forage grass introduced from the African savanna, has gained a foothold in central and southern Arizona and is expanding its range. It can grow in dense stands that crowd out native plants and can fuel devastating fires in the Sonoran Desert. In addition, competition for water can weaken and kill desert plants, even larger trees and cacti, while dense roots and ground shading prevent germination of native seeds. Additional conversion from urban and exurban development near larger metropolitan areas is also significant (LANDFIRE 2007a). Development, including urbanization, suburban, and energy development, continue to convert or degrade existing stands. Losses around large metropolitan areas such as Phoenix and Tucson are significant, especially in northern Phoenix in this mid-elevation ecosystem. Residential development has significantly impacted locations within commuting distance to urban areas (LANDFIRE 2007a). Impacts may be direct as vegetation is removed for building sites or more indirectly through natural fire regime alteration, and/or the introduction of invasive species. Additionally, massive dust from development likely negatively impacts vegetation and habitat quality for wildlife as this dust is a significant health hazard to humans. Mining operations can drastically impact natural vegetation. Road building and power transmission lines continue to fragment vegetation and provide vectors for invasive species.

Energy development from large-scale solar and, to a lesser extent, wind farms is becoming more common in the desert southwest. These projects span thousands of acres of land. The BLM designated a Solar Energy Zone in California called the "Riverside East Zone" and it contains Sonoran palo verde - mixed cacti scrub. While the BLM and USFWS try to have developers design projects to avoid impacts to the desert dry wash woodland, this results in corridors of washes surrounded by graded and bladed land. Because of these changes in vegetation, landform and soil structure surrounding the corridors, these areas often flood during summer monsoon rains, causing severe erosion and changes their original function in the ecosystem (S. Dashiell pers. comm.). There are some landscape-scale planning processes that attempt to minimize impacts to microphyll woodland: Restoration Design Energy Project in Arizona and BLM's Solar Energy Program and Desert Renewable Energy Conservation Plan (in preparation) (S. Dashiell pers. comm.).

This system is found primarily in southwestern Arizona and western Sonora, Mexico, extending east of the Colorado River in southeastern California where locally there is enough summer precipitation (Whipple Mountains).