Carnegiea gigantea



INTRODUCTORY


AUTHORSHIP AND CITATION : Pavek, Diane S. 1993. Carnegiea gigantea. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/ []. ABBREVIATION : CARGIG SYNONYMS : Cerceus giganteus Engelm. SCS PLANT CODE : CEGI COMMON NAMES : saguaro sahuaro giant cactus pitahaya TAXONOMY : The currently accepted scientific name for saguaro is Carnegiea gigantea (Engelm.) Britt. & Rose [32]. It is a member of the cactus family (Cactaceae). Carnegiea is a monotypic genus. There are no subspecies, varieties, or forms. LIFE FORM : Cactus FEDERAL LEGAL STATUS : No special status OTHER STATUS : NO-ENTRY

DISTRIBUTION AND OCCURRENCE

SPECIES: Carnegiea gigantea
GENERAL DISTRIBUTION : Saguaro occurs throughout most of the Sonoran Desert in Arizona, California, and Mexico [65,68]. The northern limits of its distribution are on the edge of the Hualapait Mountains in Arizona and along the Colorado River in southeastern California [30,32,40,65]. The range of saguaro extends southward; the largest populations of this species occur in Sonora, Mexico [67]. ECOSYSTEMS : FRES30 Desert shrub STATES : AZ CA MEXICO BLM PHYSIOGRAPHIC REGIONS : 7 Lower Basin and Range KUCHLER PLANT ASSOCIATIONS : K027 Mesquite bosque K041 Creosotebush K042 Creosotebush - bursage K043 Paloverde - cactus shrub K044 Creosotebush - tarbush SAF COVER TYPES : 241 Western live oak 242 Mesquite SRM (RANGELAND) COVER TYPES : NO-ENTRY HABITAT TYPES AND PLANT COMMUNITIES : Saguaro is a dominant or codominant member of the paloverde (Cercidium spp.)/saguaro (also known as the spinose suffrutescent community) and the paloverde/bursage (Ambrosia spp.) desertscrub community types on bajadas (i.e., eroded outwash fans) [35,46,74,75,78]. Infrequently, on the southwestern edge of its range, saguaro occurs in creosotebush (Larrea spp.) communities as a xeroriparian species in arroyos and washes [3,5,11,46]. Saguaro is a community type indicator species in the following publication: Vegetation of the Santa Catalina Mountains: Community types and dynamics [46].

MANAGEMENT CONSIDERATIONS

SPECIES: Carnegiea gigantea
IMPORTANCE TO LIVESTOCK AND WILDLIFE : The flowers, fruits, and seeds of the saguaro are important food sources for wildlife, such as collared peccary, long-nosed bats, heteromyid rodents, western white-winged doves, woodpeckers, and insects [2,19,39,50,56]. Saguaro fruits comprised a small amount (e.g., 1 to 5 percent of collected rumen contents) of mule deer summer diets [38,60,63,79,80]. The fruits were also utilized by desert bighorn sheep; they were present in 10 percent of bighorn fecal pellets analyzed [44,83]. Saguaro provides nesting habitat for birds and small mammals [49]. Saguaro mortality can have a high negative impact on bird species that use it for cover and nesting [9]. PALATABILITY : Saguaro is highly palatable to rodents and jackrabbits; however, saguaro tissue contains oxalates that deter some herbivory [47,68]. NUTRITIONAL VALUE : The nutritional value of saguaro fruits is variable throughout the season and among studies. Saguaro fruits are moderately digestible [80]. The amount of protein reported varied from 10 to 26 percent [33,80]. Acid detergent fiber amounts in the fruits varied from 26 to 62 percent, and dry matter was 11 to 35 percent [33,80]. COVER VALUE : Saguaro is considered a softwood snag [12]. Cavity-nesting birds, especially woodpeckers, and sometimes woodrats nest in saguaro [12,31,43,49,50]. VALUE FOR REHABILITATION OF DISTURBED SITES : NO-ENTRY OTHER USES AND VALUES : Saguaro fruits were a major food staple of the Papago and Pima Indians; the fruits were processed (i.e., dried, fermented, canned) and stored for year-round use [18,21,32]. Seeds, rich in fat, were ground for use as flour and in porridges [55]. Fruits were used for trade and in religious practices by the Papago Indians [21,57]. The internal heavy, woody ribs were used for firewood and building houses and fences [1,6,10]. Saguaro is grown in desert gardens as an ornamental [89]. It is the state flower of Arizona and is used in commercial advertisements of Southwestern living [40]. OTHER MANAGEMENT CONSIDERATIONS : Intensive grazing can prevent recruitment of saguaro seedlings [47]. Various areas in the Sonoran Desert have been protected from domestic livestock grazing for 50 years or more. One early study showed that there was no significant (p>0.05) difference between saguaro abundance on protected and unprotected plots [8]. More recent studies, however, have shown that saguaro increased by 33 to 200 percent in protected areas [27,77]. Saguaro seedlings may be impacted negatively when nurse plants are browsed [47]. Models to predict saguaro age from growth parameters (for example, height-age equivalents) have been developed [28,69]. In transplant experiments, mortality was 100 percent for unshaded saguaro seedlings compared to 65 percent for shaded seedlings [22]. Saguaro are susceptible to few diseases [10]. Healthy saguaro often wall off larvae tunnels, woodpecker holes, or other wounds [47]. Some necroses may occur, however, after mechanical damage to saguaro from breakage, frost, or lightning [71]. Insects, saprophytic yeasts, molds, and bacteria readily inhabit and breed in saguaro necroses [24,66,72]. Temperatures below 23 degrees Fahrenheit (-5 deg C) will damage saguaro. Death will occur after exposure to below freezing temperatures for 29 or more consecutive hours [85]. Fluctuations in saguaro numbers often are due to recurring catastrophic freezes [37,46,69]. Saguaro is protected under the Arizona Native Plant Law. Cactus poaching is of concern around Saguaro National Monument and urban centers [90]. Natural environmental extremes in temperature and drought, however, are more of a threat to the survival of this species [37].

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

SPECIES: Carnegiea gigantea
GENERAL BOTANICAL CHARACTERISTICS : Saguaro is a native, arborescent, stem succulent that grows from 9.8 to 52.5 feet (3-16 m) tall and 5.9 to 29.5 inches (5-75 cm) in diameter [6,10,64,65]. It is the largest of the columnar cacti growing in the United States [89]. Saguaro has a deep anchoring taproot (up to 3.2 feet [1 m] deep) and extensive lateral roots (13 to 98 feet [4-30 m] long) [14,65]. The stems are simple with one to five (rarely up to 49) lateral, erect branches [6,10]. Saguaro trunks have many prominent ribs, armed with dense, stout spines that are up to 2.8 inches (7 cm) long [6,10,32]. Flowers are 3.4 to 4.9 inches (8.7-12.5 cm) long and occur at the ends of branches [6,32,45]. Fruits are oblong and contain up to 2,500 seeds [1,10]. RAUNKIAER LIFE FORM : Stem succulent REGENERATION PROCESSES : Saguaro reproduce sexually and are self-incompatible [2]. Reproduction begins when a plant is approximately 7.2 feet (2.2 m) tall [29,68]. Growth is extremely slow [47,68]. At the end of 2 years, seedlings will be about 0.25 inch (0.6 cm) tall [10]. At 3.2 feet (1 m), plants may be 20 to 50 years old [47]. Flowers only develop at branch and main stem tips. An increase in branch number increases the reproductive potential of an individual [37]. Many different animals pollinate the nocturnally blooming saguaro flowers and are detailed elsewhere [2,39,40]. Flowers are open for less than 24 hours [2]. A plant averages four open flowers per day for about 30 days [39]. Plants commonly have more than 100 fruits [47,68]. Seeds germinate readily [10]. Conditions for germination are detailed elsewhere [1,89,91]. Saguaro seeds are short-lived; seed reserves are not maintained in the soil [58,76]. In random soil samples, relative densities of saguaro seeds were between 0.3 and 2.0 percent [56]. Less than 1 percent of the annual seed production germinates due to predation or moisture stress [15,48]. Heavy seedfall can occur during the summer rains [68]. Drought during the 12 to 14 months following germination is the most critical factor threatening seedling survival [48,67]. Perennial shrubs, such as foothills paloverde (Cercidium microphyllum), are important as nurse plants for facilitating saguaro establishment [22,29,36,62]. Seed dispersal by frugivorous birds is a primary factor in saguaro establishment beneath shrub canopies [36]. Saguaro seedling density significantly (p<0.05) decreased from the center to the outer edges of nurse plant canopies [29]. On the margins of its range, recruitment has not kept pace with mortality caused by freezing temperatures and drought [52,76]. Saguaro has lower fruit production at the drier western edge of its range [52]. In some areas where cattle grazing has not occurred, recruitment appeared to occur in pulses correlated with above-normal precipitation [76]. SITE CHARACTERISTICS : Compared to other columnar cacti, saguaro occurs over the broadest range of habitats from gently sloping alluvial flats of the lower bajadas to steep, rocky, high-elevation volcanic slopes [51,53,74]. The greatest densities of saguaro are found in the more mesic eastern parts of its range [37]. Saguaro occurs on shallow soils that are light, coarse-textured, and rocky. The soils are underlain at 3.2 to 9.6 feet (1-3 m) depths by an impervious caliche layer [8,88]. There is usually no differentiation of the soil into horizons [87,88]. Saguaro is found at elevations from 1,640 to 5,000 feet (500-1,525 m) [35,50,56,86]. Saguaro has been reported on all aspects [77,88]. However, most individuals occur on open east- and west-facing slopes, and the fewest occur on north-facing slopes [6,77,86]. The Sonoran Desert is a warm desert with a semiarid to arid continental or an arid subtropical climate [27,67,82]. Precipitation is bimodal, falling December to February and July to August, and amounts vary from year to year [19,42,51]. Average amounts of rain per year are 6 to 15 inches (152-400 mm) [3,14,42,84]. Common arborescent associated species are Organ Pipe Cactus (Lemierocereus thurberi), barrelcactus (Ferocactus wislizeni), cholla (Opuntia spp.), ocotillo (Fouqueria splendens), catclaw acacia (Acacia greggii), ironwood (Olneya tesota), and jojoba (Simmondsia chinensis) [8,13,26,50]. Other associated species are littleleaf krameria (Krameria parvifolia), fluffgrass (Erioneuron pulchellum), tobosa (Hilaria mutica), Indian-wheat (Plantago insularis), and thelypody (Thelypodium lasiophyllum) [13,23,61]. SUCCESSIONAL STATUS : Obligate Climax Series Succession in desert systems is difficult to define [36]. Saguaro may be considered a climax species. Establishing in the shade of perennial shrubs, saguaro usually outlives and surpasses the nurse plants. Regeneration of saguaro continues in suitable areas of its range. SEASONAL DEVELOPMENT : Saguaro flowers from late April to June [1,32,39,45]. Fruits mature during June and July before the summer rains [6,65]. Most growth of saguaro occurs during the summer rainy season [6].

FIRE ECOLOGY

SPECIES: Carnegiea gigantea
FIRE ECOLOGY OR ADAPTATIONS : Saguaro is not truly adapted to fire because there is no postfire stimulation of flowering or seed production [73]. It has several prominent morphological characteristics, however, that may aid its survival when burned. The apex is protected to some degree by tissue folds and spines. Vascular tissue is protected by a thick cortex that has a high thermal capacity close to that of water. As saguaro ages, ribs at the base of a plant may develop a woody bark which is more resistant to burning than young tissue [73]. Although mortally injured by fire, death may be delayed if the saguaro can live off of its reserves, which may enable a plant to flower for 1 to 6 years more [73]. Singed saguaro have been observed to flower from unburned branches. However, scorched plants may die due to reduced vigor [58]. Fire may remove spines from a saguaro, making it vulnerable to herbivory [73]. Nurse plant species influence saguaro fire survival. Although desert fire temperatures are variable, fire temperatures beneath foothills paloverde were lower than under triangleleaf bursage (Ambrosia deltoidea) [16]. Fire-free periods in the Sonoran Desert are greater than 250 years [73]. Saguaro would be eliminated under a fire frequency of less than 30 years [58]. Fires that do occur are usually of low intensity due to small fuel loads [41]. POSTFIRE REGENERATION STRATEGY : Tree without adventitious-bud root crown Secondary colonizer - off-site seed

FIRE EFFECTS

SPECIES: Carnegiea gigantea
IMMEDIATE FIRE EFFECT ON PLANT : Fire top-kills saguaro, and it may kill existing seedlings [73]. Because there is a lag time between time of injury by fire and time of death, postfire mortality may be underestimated [58]. Small saguaro, less than 6.6 to 13 feet (2-4 m) tall, with large amounts of fuel at the plant base do not survive. Larger saguaro may survive with limited damage [16,41]. DISCUSSION AND QUALIFICATION OF FIRE EFFECT : NO-ENTRY PLANT RESPONSE TO FIRE : Postfire recovery is dependent upon individual plant recovery and seed dispersal from unburned plants [58]. Surviving saguaro with little or no injury will resume growth [16]. Absorption of rain following fire-caused injuries can result in fire scars splitting, which opens the plant to insects and infections [73]. Vegetation surveys done the third and fourth years following an early summer fire found no sprouting of saguaro and no seedlings [59]. Two studies that compared burn and adjacent wildfire areas 2 years after prescribed burning found that 100 percent of saguaro present in plots had died. Plots were read before and after fire. There were no sprouts or seedlings [16,17]. DISCUSSION AND QUALIFICATION OF PLANT RESPONSE : NO-ENTRY FIRE MANAGEMENT CONSIDERATIONS : Fire would be an effective tool to control saguaro invasion or density [68,73]. Along major highways in Arizona, motorist-caused fires have completely eliminated the saguaro over large areas [58].

REFERENCES

SPECIES: Carnegiea gigantea
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