SPECIES: Prosopis pubescens

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INTRODUCTORY

SPECIES: Prosopis pubescens

 

Benny J. Simpson, Texas A&M Dallas

 

AUTHORSHIP AND CITATION:
Meyer, Rachelle. 2005. Prosopis pubescens. 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/ [].

FEIS ABBREVIATION:
PROPUB

SYNONYMS:
None

NRCS PLANT CODE [102]:
PRPU

COMMON NAMES:
screwbean mesquite
screwbean
tornillo

TAXONOMY:
The currently accepted scientific name of screwbean mesquite is Prosopis pubescens Benth. (Fabaceae) [9,64,65,102,104,109]

LIFE FORM:
Tree-shrub

FEDERAL LEGAL STATUS:
None

OTHER STATUS:
The Arizona Game and Fish Department lists screwbean mesquite as a harvest restricted species, meaning permits are required to remove plant by-products [6]. In addition, Plants Database lists screwbean mesquite as a salvage assessed species [102]. According to the Arizona Game and Fish Department, permits are required to remove live individuals of species that are salvage assessed [6].


DISTRIBUTION AND OCCURRENCE

SPECIES: Prosopis pubescens

Dick Henderson, Saguaro Juniper Corporation

GENERAL DISTRIBUTION:
The native range of screwbean mesquite extends from southeastern California west into southern Nevada, extreme southwestern Utah, southern and western Arizona, southwestern and south-central New Mexico, western Texas and northern Mexico, including Baja California, Sonora, Chihuahua, and Coahuila [9,65,70,74,89,109]. Screwbean mesquite has been introduced to areas in India, Pakistan, and southern and southwestern Africa [20,84]. However, these introductions have rarely been successful [84].The U.S. Geological Survey provides a distributional map of screwbean mesquite in North America.

ECOSYSTEMS [49]:
FRES30 Desert shrub
FRES33 Southwestern shrubsteppe

STATES/PROVINCES: (key to state/province abbreviations)
UNITED STATES

AZ CA NV NM TX UT

MEXICO
B.C.N. Chih. Coah. Son.

BLM PHYSIOGRAPHIC REGIONS [14]:
7 Lower Basin and Range
13 Rocky Mountain Piedmont

KUCHLER [68] PLANT ASSOCIATIONS:
K027 Mesquite bosques
K040 Saltbush-greasewood
K041 Creosote bush
K042 Creosote bush-bursage
K043 Paloverde-cactus shrub
K044 Creosote bush-tarbush
K059 Trans-Pecos shrub savanna

SAF COVER TYPES [41]:
68 Mesquite
235 Cottonwood-willow
242 Mesquite

SRM (RANGELAND) COVER TYPES [93]:
203 Riparian woodland
211 Creosote bush scrub
501 Saltbush-greasewood
506 Creosotebush-bursage
507 Palo verde-cactus
508 Creosotebush-tarbush
729 Mesquite

HABITAT TYPES AND PLANT COMMUNITIES:
Screwbean mesquite is associated with desert riparian woodland and scrub habitats. Saltbush (Atriplex spp.), arrowweed (Plucea sericea), and introduced tamarisk (Tamarix spp.) co-occur with screwbean mesquite throughout most of its range [25,28,33,58,78].

Screwbean mesquite can be a dominant species in the mesquite bosque habitat type [7,58,86,107]. In southern California, Baccharis (Baccharis spp.) and desert lavender (Hyptis emoryi) are associated understory species in mesquite woodland [86]. Along the Colorado River this habitat type may include honey mesquite (Prosopis glandulosa), velvet mesquite (P. velutina), netleaf hackberry (Celtis reticulata), creosotebush (Larrea tridentata), wolfberries (Lycium spp.), white bursage (Ambrosia dumosa), fourwing saltbush (Atriplex canescens), big saltbrush (A. lentiformis), cattle saltbush (A. polycarpa), blue paloverde (Parkinsonia florida), common elderberry (Sambucus nigra ssp. canadensis), Mojave seablite (Suaeda moquinii) and forbs such as carelessweed (Amaranthus palmeri) and gourds (Cucurbita spp.) [58]. Along the Rio Grande River in southern New Mexico mesquite bosques frequently contain screwbean mesquite with skunkbush sumac (Rhus trilobata), wolfberries, seepwillow (B. salicifolia), arrowweed, and the grasses alkali sacaton (Sporobolus airoides) and saltgrass (Distichlis spicata) [28,33].

Screwbean mesquite occurs in other riparian woodlands, such as the desert microphyll and cottonwood (Populus spp.)- willow (Salix spp.) cover types. For example, in New Mexico the gallery forests of the Rio Grande River contain Rio Grande cottonwood (P. deltoides var. wislizeni), Fremont cottonwood (P. fremontii), sandbar willow (S. exigua), Goodding willow (S. gooddingii), black willow (S. nigra), New Mexico olive (Forestiera pubescens var. pubescens), false indigo (Amorpha fruticosa), seepwillow, and tamarisk, with screwbean mesquite having varied importance [37,38,99,112]. Herbaceous species typical in these areas include common lambsquarters (Chenopodium album) , narrowleaf globemallow (Sphaeralcea angustifolia), white sweetclover (Melilotus alba), and silverleaf nightshade (Solanum elaeagnifolium) [99]. On the Rio Grande River in western Texas, screwbean mesquite occurs with lanceleaf cottonwood (Populus x acuminata), willows, seepwillow, buttonbush (Cephalanthus occidentalis), desert willow (Chilopsis linearis), tamarisk, blackbrush acacia (Acacia rigidula), catclaw acacia (A. greggi), tree tobacco (Nicotiana glauca), and Texas paloverde (Parkinsonia texana) [91,107,108]. Screwbean mesquite is an associate of Chihuahuan Desert riparian forests, which may contain Fremont cottonwood, willows, desert willow, honey mesquite, velvet ash (Fraxinus velutina), seepwillow, common reed (Phragmites australis), tamarisk, and giant reed (Arundo donax) [55]. Similarly, Sonoran streams and rivers which support riparian woodland can contain Fremont cottonwood and Goodding willow with either velvet ash, western honey mesquite (Prosopis glandulosa var. torreyana), velvet mesquite, or screwbean mesquite [98]. The vegetation of Sonoran alkali sinks also contains Fremont Cottonwood, Goodding willow, screwbean mesquite, as well as seepwillow, and big saltbrush, and spiny chloracantha (Chloracantha spinosa), but is frequently dominated by tamarisk and arrowweed in areas with no to moderate salinity [19]. Another mesquite-containing woodland is the desert microphyll woodland described by Thorne [100]. The vegetation may include blue paloverde, catclaw acacia, smoketree (Psorothamnus spinosus), western honey mesquite, desert willow, desert ironwood (Olneya tesota), creosotebush, desert lavender, plus smaller shrubs, such as singlewhorl burrobush (Hymenoclea monogyra), Anderson wolfberry (L. andersonii), broom baccharis (B. sarothroides), fairyduster (Calliandra eriophylla), lotebush (Ziziphus obtusifolia var. obtusifolia), and desertsenna (Senna armata) [100]. Additionally, Sonoran oases contain screwbean mesquite along with California palm (Washingtonia filifera), Olney threesquare (Scirpus americanus), singlewhorl burrobush, arrowweed, saltgrass, alkali goldenbush (Isocoma acradenia), cattle saltbush, wheelscale saltbush (Atriplex elegans), desertholly, (A. hymenelytra), and sweetbush (Bebbia juncea) [88,105].

Screwbean mesquite also occurs in scrub vegetation. For instance, along the lower Colorado and Gila rivers and in the Salton Sea basin, vegetation is typically dominated by tamarisk with some sites also containing arrowweed, big saltbrush, iodinebush (Allenrolfea occidentalis), Mojave seablite, cattle saltbush, honey mesquite, and/or screwbean mesquite [78]. Haigh [54] reported screwbean mesquite, arrowweed, iodinebush, and broom baccharis occurring in a tamarisk-dominated community along an intermittent stream in southern Nevada. The riparian scrub habitats of southern Nevada valleys are comprised of species such as screwbean mesquite, sandbar willow, common reed, tamarisks, arrowweed, seepwillow, and saltbush [30,53]. Mohave wash scrub may consist of screwbean mesquite, catclaw acacia, desert saltbush, desert willow, Mojave rabbitbush (Ericameria paniculata), smoketree, white burrobrush (H. salsola), pygmy cedar (Peucephyllum schottii), western honey mesquite, desert almond (Prunus fasciculata), and skunkbush sumac (Rhus trilobata) [58]. Mesquites (Prosopis spp.) are also common associates of desert saltbush in some areas of the saltbush series of Sonoran desert scrub vegetation [101].

Classifications in which screwbean mesquite is a characteristic or associate species are listed below by location.

California: [3,19,58,78,86,89,100]
Arizona: [78,89]
New Mexico: [28,33,89]
Texas: [28,55,89]
Mexico: [55,78,89]


BOTANICAL AND ECOLOGICAL CHARACTERISTICS

SPECIES: Prosopis pubescens

Lorence Collins, Wildflowers of southern California


GENERAL BOTANICAL CHARACTERISTICS:
This description provides characteristics that may be relevant to fire ecology, and is not meant for identification. Keys for identification are available [9,65,79,104], including a preliminary key for identification of seedlings [113].

Screwbean mesquite is a native, deciduous, small tree or shrub [9,64]. It can reach a maximum height of about 30 feet (10 m) with a 1 foot (30 cm) trunk diameter [36,70,79,104]. In the northernmost portion of its range it commonly grows to between 8 and 13 feet (2.5-4.0 m) tall [9,109]. Growth is typically shrubby and branches are long and thin [36,70,79,104]. The bark is thick and fibrous when mature [70,104]. Spines, 0.2 to 1 inch (5-25 mm) in length, protect the 0.8 to 3 inch (2-7.6 cm) bipinnately compound leaves, which typically have 5 to 9 pairs of leaflets that are 0.1 to 0.375 inch long (3-9.5 mm) [36,65,70,79,104,109]. Flowers are perfect and occur in cylindrical spikes about 1.6 to 3.1 inches long (4-8 cm) [9,36,65,70,79,109]. The tightly coiled, indehiscent pods that form after pollination are 0.8 to 2 inches (2 - 5 cm) in length, 0.16 to 0.25 inch (4-6.4 mm) in diameter, and contain numerous seeds [36,65,70,79,104,109].

RAUNKIAER [87] LIFE FORM:
Phanerophyte

REGENERATION PROCESSES:
Screwbean mesquite reproduces by seed which is typically scarified by the digestive tracts of animals. Some mesquite species have dormant buds on the root crown that sprout when plants are top-killed [5,46,111]. There are no reports of these structures on screwbean mesquite.

Breeding system: Screwbean mesquite is monoecious  [109]. Given that most species of this genus are self-incompatible [52], it is likely true of screwbean mesquite.

Pollination: Mesquites are typically insect pollinated [66,94]. Although pollination information specific to screwbean mesquite is not available, there is no evidence to suggest that is not insect pollinated as well. Screwbean mesquite is one of several mesquite species that make excellent bee forage and is also a source of nectar for butterflies [42,64]. Bees use both the pollen and nectar of screwbean mesquite [50,64].

Seed production: Although there is a lack of information specific to screwbean mesquite, seed production has been investigated for other members of the genus. Ffolliott [46] noted the estimate for seed production of mesquite species in Latin America, obtained in his previous work, as 104 to 105 seeds per tree in a given reproductive season. This is despite a reportedly low proportion of flowers that produce fruit with viable seeds [46]. Glendening and Paulsen [51] reported their largest velvet mesquite pod harvest as 39.5 pounds (17.9 kg), which equated to approximately 142,000 seeds and was obtained from a 10.5-foot (3.2 m) tall tree with a crown diameter of 19 feet (5.8 m). However the 4 year average of pods produced by velvet mesquite trees at elevations between 3,000 and 4,500 feet (914-1372 m) in southern Arizona was 1.38 pounds (0.62 kg). It is probable that seed production and variation between individuals of different sizes is comparable to screwbean mesquite; however the degree of similarity is unknown. 

Seed dispersal: Seeds of screwbean mesquite are transported by water and domestic and wild animals, including coyotes and rodents [11,65,83]. When eaten, seeds get the added advantage of avoiding insects, thus decreasing insect-caused mortality [67]. Digestion also scarifies the seeds, which is generally required for germination [34,67,103]. The sprouting of uneaten velvet mesquite seeds from rodent caches has been reported [51] and, given the use by rodents, could occur in screwbean mesquite as well.

Seed banking: After 44 years, 60% of scarified velvet mesquite seeds from herbarium specimens germinated on damp filter paper in a dark oven set to 80 F (26.7 C) [51]. Although no similar information is available for screwbean mesquite, it is probable that their seeds also can remain dormant for long periods.

Germination: Scarification is necessary for screwbean mesquite seeds to germinate [34,67,103]. This typically occurs naturally by passing through the digestive tract of animals, but also occurs due to weathering [65,67,83,103]. Rates of germination in the wild are unknown; however, several laboratory studies have been performed. Typical rates of germination were between 60% and 70%, for treatments which included chemical or mechanical scarification [34,90,103]. Jackson and others [61] obtained the highest germination rates, with 98%-100% of seeds germinating in treatments with salt concentrations at or below 6,000 mg/l. They used a chemical scarification method in which seeds were soaked in 98% sulfuric acid for 25 minutes. Thermal scarification may increase germination rates compared to nonscarified controls, but have been substantially less effective than mechanical and chemical treatments. Dreesen and Harrington [34] demonstrated an increase in germination rates compared to nonscarified seeds when using a thermal scarification method where water heated to 194 F (90 C) was poured over seeds and then steeped for 4 hours. The mean increase was 7% for one seed source and 18% for another. However, mechanical scarification using a commercial scarifier, which uses a spinning paddle to throw seeds against an drum lined with 100 grit sand paper, showed a much larger increase to a mean of 62% germination from one seed source and 92% from the other [34]. Vilela and Ravetta [103] reported germination rates of just under 60% for a mechanical scarification method in which seeds were nicked with a razor blade; this was much higher than the thermal method which had germination rates of less than 5%. Chemical scarification showed an even larger improvement, with a germination rate of approximately 64%. In this case, thermal scarification involved dropping seeds in boiling water until it reached room temperature and chemical scarification was performed by soaking seeds in sulfuric acid 1 N for 15 minutes, rinsing them in running water 3 times for 2 minutes, and then soaking them in water for 15 to 30 minutes [103]. Tumble scarification, in which seeds were placed in a rock tumbler with pea gravel and coarse grit for 2 to 3 hours, resulted in slightly less germination than nonscarified seeds [34].

Soil also has an effect on germination. In a laboratory study, the soil with the highest germination rate (65%) was nursery mix (ground sphagnum (Sphagnum spp.), peat moss, and vermiculite in 1:1:1 ratio). Soil from a site with velvet mesquite resulted in 35% germination of screwbean mesquite, while only 25% of seeds germinated when fertilizer was added to the soil from the site with velvet mesquite. These figures are from seeds scarified by chemical, mechanical, or thermal methods randomly planted in 1 of the 3 mixtures [103].

Seedling establishment/growth: Although mesquite seeds can germinate on the soil surface, seedlings require a covering of 0.4 to 0.8 inches (1-2 cm) to provide anchorage and moisture [46].

Survival of screwbean mesquite seedlings in laboratory studies has been varied. Jackson and others [61] observed 100% survival for 120 days in all treatments with salt concentrations of 36,000 mg/l or less. Vilela and Ravetta [103] also obtained high seedling survival, with seedlings in the nursery potting mix having nearly 100% survival to 60 days after germination. Those in soil from a site with velvet mesquite had about 87% survival, while approximately 84% of those growing in the fertilized velvet mesquite soil survived 60 days after germination. In contrast, Rhodes and Felker [90] found screwbean mesquite a difficult species to grow in a greenhouse. Over the first few weeks of their trial screwbean mesquite seedlings exhibited a 68% survival rate. All seedlings subsequently died, but it is unknown whether the mortality was due to disease or the experimental increase in salt concentrations to 1.2%. Again, field data are not available.

Asexual regeneration: There are no records in the literature of screwbean mesquite propagating vegetatively in the wild. However, Goel and Behl [52] classified screwbean mesquite as "easy to root" in an investigation of techniques for increasing output of genetically desirable individuals for use in restoration planting. Between 80% and 90% of attempts resulted in rooting of screwbean mesquite, depending on the method used. Eighty percent of cuttings rooted. Air layering, a rooting method in which a ring of bark is removed and covered in sphagnum, then covered in plastic, was slightly more successful, and air layering with the application of the root promoting indole acetic acid resulted in 90% of samples rooting [52].

SITE CHARACTERISTICS:
Screwbean mesquite is found along streams, washes, floodplains, gullies, and in alkali sinks, oases, arroyos, and bajadas in the desert southwest [9,36,58,105]. It reaches dominance on floodplains in a zone near the edge of the 1st terrace and on higher alluvial terraces of large perennial waterways, approximately 5 to 20 feet (1.5-6 m) above the river channel [58,72,78,83].

Elevation: Screwbean mesquite has been reported at elevations from near sea level to 5,500 feet (1,676 m) [70]. Elevational ranges by state are shown in the table below.

State Elevation
Utah 2,400 to 3,300 feet (730 - 1,000 m) [9,109]
Nevada below 3,300 feet (<1,000 m) [9]
Arizona just above sea level to 5,500 feet (<1,675 m)
New Mexico just above sea level to 5,500 feet (<1,675 m) [70]
California below 5,000 feet (1,525 m) [58]
Texas 1,800 to 4,000 feet* (550 - 1,220 m) [107]
* screwbean mesquite is a dominant species in a habitat type that occurs in this elevational range

Soils: Screwbean mesquite occurs on a wide range of soil textures [28,72]. Campbell and Dick-Peddie [28] reported screwbean mesquite on sites with light and sandy to heavy, clay soil textures. Individual articles have reported screwbean mesquite growing in silty clay loam [52], rocky clay silt [40], and clay to loam soil textures [99]. Several authors note the presence of screwbean mesquite in sandy areas [9,58,71,100]. Anderson [1] reports that in general, screwbean mesquite does not flourish on clay soils. Vilela and Ravetta [103] observed higher germination rates and seedling survival in nursery mix than in soil from a site containing velvet mesquite, which performed better than adding fertilizer to the same soil. See the Germination and Seedling establishment/growth sections for more detailed description of these results.

Screwbean mesquite can often be found in slightly to moderately saline soils [65,71]. Screwbean mesquite was found to be more salt tolerant than Fremont cottonwood and Goodding willow at shallow [61] and deep [1] soil levels. In a laboratory study Jackson and others [61] observed 100% survival over 120 days when salt concentrations were 36,000 mg/l or less. After 120 days of exposure to 60,000 mg/l salt concentrations 70% of screwbean mesquite survived. There was a trend for less stem growth in the 36,000 and 60,000 mg/l treatments. It is possible that screwbean mesquite responses were lower than those of other species due to the slow growth of screwbean mesquite. Percent germination showed a dramatic change, with 98%-100% germination at or below 6,000 mg/L and 0% germination at salt concentrations of 18,000 mg/L and higher [61]. When revegetating Bosque del Apache National Wildlife Refuge, NM, screwbean mesquite was planted on sites with salinities between 3 and 8 dS/m [99].

Screwbean mesquite occurs on alkaline sites, as well. For instance, Goel and Behl [52] performed experiments in which screwbean mesquite was grown in an area with pH ranging between 8.5 and 10.6 and the soil from a site with velvet mesquite used in Viela and Ravetta's [103] experiments had a pH of 8.2.

In addition, many mesquite species have been shown to fix nitrogen [45]. Although nitrogen fixation in screwbean mesquite has been reported [84], data demonstrating this is lacking.

Water Although a drought-tolerant species, depth to subsurface water is important to screwbean mesquite [28,56,83,100]. It is considered an obligate riparian species in most of its range [33]. Depths of subsurface water greater than 13 feet (4 m) are likely to be inhospitable to screwbean [1] and in revegetation efforts on the Bosque del Apache National Wildlife Refuge it was not planted in areas where depth to the water table was greater than approximately 4 feet (1.2 m) [99]. Reports of screwbean mesquite on upland areas have not included information, such as water table depths, that would assist in determining what is allowing for screwbean mesquite occurrence outside the riparian zone [60,101]. Annual precipitation in the mesquite habitat type, in which screwbean mesquite can be a dominant species, is between 3 to 20 inches (7.5-50 cm) [72].

Historically, flooding was a typical disturbance in habitat types in which screwbean mesquite occurs [1,58,83]. It can survive 1 to 3 months of flooding during the growing season [106], and establishes on recently flooded areas [43,99].

SUCCESSIONAL STATUS:
Shade tolerance: Screwbean mesquite is moderately shade tolerant, with a layer occasionally forming under cottonwood and willow canopies [28]. Vilela and Ravetta [103] found high nitrogen concentrations in green screwbean mesquite stems and pose the possibility that they could be photosynthetically active.

Although typical succession, from one community type to another, has not been demonstrated in desert riparian habitats of the American southwest [22,62], this could be changing. Historically species establishing on disturbed sites would be the same species present before the disturbance and were likely to persist until another disturbance occurred [62]. However, Cleverly and others [30] found tamarisk dominating older sites. Over time, changes to water regimes including less frequent flooding, lowering water tables, and increasing salinity will favor tamarisk [25,30]. Dick-Peddie [33] classifies communities dominated by tamarisks as a successional-disturbance vegetation type. In addition, tamarisk dominance seems to result in more frequent fires, likely due to deeper water tables and decreased flooding producing larger and drier fuel loads [4,21,22]. Due to better regeneration than other burned riparian species [24], tamarisk and arrowweed replace more original vegetation with each successive fire [22]. Smith and others [96] provide a review of these studies.

SEASONAL DEVELOPMENT:
Flowering typically starts in May [9,65,74,79] and pods begin ripening in July [9]. Campbell and Dick-Peddie [28] note an approximate 20-day delay in the development of screwbean mesquite floral and vegetative parts in Albuquerque, NM, compared to El Paso, TX.


FIRE ECOLOGY

SPECIES: Prosopis pubescens
FIRE ECOLOGY OR ADAPTATIONS:
Fire adaptations: Screwbean mesquite can survive fire [22,58,78], but little is known of the adaptations that allow for this. Weak resprouting after fire has been reported [105], but whether this was from surviving apical buds or adventitious buds on the root crown, as in other southwestern mesquites [5,110], is not discussed. Screwbean mesquite colonization of areas recently disturbed by flood has been reported [43,99]. However, the approximately 15-year lag time before screwbean mesquite established on an area burned in a 1930 fire in Thousand Palm Oasis, CA, illustrates that it does take longer for screwbean mesquite to establish on at least some burned areas [105].

Fire regimes: Historically fire was probably infrequent in many screwbean mesquite-containing habitats. In dry areas with low vegetation cover, fire is usually able to spread only after prolonged wet periods result in increased fuel loads [8,75]. Little is known of the historic fire frequencies in riparian habitats containing screwbean mesquite. Northwestern coniferous riparian areas have been shown to have longer and more variable fire intervals than surrounding uplands [95]. In northern riparian deciduous communities, fires are infrequent due to typically high moisture content and high leaf litter decomposition rates [16]. Although site characteristics, such as moisture conditions and the fire frequency of neighboring habitats, are likely to result in variation between riparian areas, a relatively infrequent fire interval is probably applicable to southwestern cottonwood-willow communities as well [24,92]. In mesquite bosque habitats on upper river terraces it is possible that fires were comparatively more frequent and may have been a factor, along with flooding, in maintaining the open understory of these forests [58,78]. However, Busch [22] found significantly smaller area of burned mesquite-dominated or codominated communities than was expected based on the proportion this type in the riparian areas investigated. In addition, Busch [22] found that screwbean mesquite communities with less vertical canopy development burned more often than average stands. Some riparian areas, such as desert oases, may have experienced more frequent fires [105]. There is evidence that some wetland areas, at least in the Sonoran region, experienced frequent fires before European settlement [31]. Although the historic role fire played in southwestern riparian habitats is not well understood, there is strong evidence that currently fire occurs in riparian habitats invaded by tamarisk at higher frequencies than other riparian communities [22]. One study of tamarisk stands along the Colorado River found that of 25 sites 21 had burned over 15 years [4]. Although screwbean mesquite may survive these fires, its recovery is likely to be much slower than that of tamarisk [1,24,59]. With an increasing frequency of fire screwbean mesquite will eventually be replaced [22,30,78].

The following table provides fire return intervals for plant communities and ecosystems where screwbean mesquite can occur. For further information, see the FEIS review of the dominant species listed below.

Community or Ecosystem Dominant Species Fire Return Interval Range (years)
saltbush-greasewood Atriplex confertifolia-Sarcobatus vermiculatus < 35 to < 100
paloverde-cactus shrub Cercidium microphyllum/Opuntia spp. < 35 to < 100
creosotebush Larrea tridentata < 35 to < 100 [85]
mesquite Prosopis glandulosa < 35 to < 100 [76,85]

POSTFIRE REGENERATION STRATEGY [97]:
Reports of screwbean mesquite's occurrence in burned areas do not explain its postfire regeneration strategy.


FIRE EFFECTS

SPECIES: Prosopis pubescens
IMMEDIATE FIRE EFFECT ON PLANT:
Screwbean mesquite can survive some fires [22,105], but the mechanisms involved or conditions required have not been investigated.

DISCUSSION AND QUALIFICATION OF FIRE EFFECT:
Survival of other mesquite species and riparian vegetation in general depends on a number of factors including fire severity and site conditions such as moisture content before the fire and presence and extent of flooding after fire [5,26,32,111]. In an investigation of both honey and screwbean mesquite, Anderson [1] states that mesquite recovery from fire, if it occurs, is very slow.

PLANT RESPONSE TO FIRE:
Current available evidence suggests that screwbean mesquite will most likely decline due to fire, although the extent can vary. Busch and Smith [23] note screwbean mesquite's absence from a site severely burned 3 years previously, despite unburned areas having scattered individuals. Egan [35] suggests boron or salt accumulations may explain the lack of growth on sites 5 growing seasons after a severe fire. In at least some situations, it can take about 15 years for screwbean mesquite to establish after fire [105]. Busch [21] found that although honey and screwbean mesquite occurred on recently burned sites, mesquite did not dominate any sites after fire, not even those areas that had been classified as mesquite-dominated before fire. However, screwbean mesquite did occur on burned sites of every vegetation type, and Busch and Smith [25] found only slight effects of fire on screwbean mesquite cover and frequency in cottonwood-willow communities.

DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:
Screwbean mesquite is likely to be similar to other southwestern mesquites, in that response to fire depends on several factors including age of individuals, moisture availability, and fire frequency and intensity [5,26,27,73,111]. As they mature honey and velvet mesquites become more resistant to fire [5,26,27,73,111]. Amount of fuel may also influence mortality. Cable [26] observed higher mortality of velvet mesquite in a site with fuel levels of 4,480 lb/acre than an area with fuels of 2,200 lbs/acre. However, Wright and others [111] found honey mesquite mortality to be independent of amount of fuel between 4,000 and 7,000 lbs/acre. They did observe higher mortality of honey mesquite in dry years, due to fires burning the root crown [111]. When the root crown is not damaged, mesquites exhibit basal sprouting after being top-killed [5,26,111]. Mesquites can also sprout from axillary buds on branches within the crown [5,26]. If screwbean mesquite has the ability to sprout from the root crown after fire, its response could be quite similar to that of other mesquites. If not, it is likely to be much more susceptible to fires.

Seedling establishment of velvet mesquite on burned sites was less than in unburned plots [27]. There may be multiple causes for this including high seedling mortality [5,73,111], changes to site conditions which make establishment difficult [24], partially top-killed mesquites producing fewer seeds [110], and mortality of seeds when exposed to high temperatures. Slightly increased germination rate of thermally scarified screwbean mesquite seeds compared to nonscarified seeds was demonstrated in greenhouse studies [34], but has not been reported in field studies (see the Germination section).

In the riparian habitats in which screwbean mesquite occurs, competition from tamarisk also affects postfire communities. As fire frequency increases, flooding decreases, and water tables drop, the competitive advantage of tamarisk over other riparian species increases. In addition, fire can increase alluvium salinity [24], further increasing the competitive advantage of tamarisk in the postfire environment. A more in depth discussion of the interaction between tamarisk and fire can be found in the Fire regimes and Successional Status sections.

FIRE MANAGEMENT CONSIDERATIONS:
Burning at low intensities on sites with relatively unaltered water regimes and little or no tamarisk is likely to have the least detrimental effects on screwbean mesquite. Fire has been used in programs to eradicate tamarisk but has generally been unsuccessful [83]. For information on the management of tamarisk, see the tamarisk summary.


MANAGEMENT CONSIDERATIONS

SPECIES: Prosopis pubescens
IMPORTANCE TO LIVESTOCK AND WILDLIFE:
Screwbean mesquite is important as cover and food to wildlife and is eaten by livestock.

Palatability/nutritional value: Many animals find the pods of screwbean mesquite palatable. Coyotes, rodents, roadrunners, Gambel's quail and Mearns quail all eat screwbean mesquite pods [53,83,104]. The species of rodents eating screwbean mesquite pods have not been reported, but in a review Bogusch [15] lists many rodents which feed on southwestern mesquites, including Merriam's kangaroo rat, desert pocket mouse, and the white-throated woodrat. Other species noted to eat mesquite pods or leaves were white-winged doves, ravens, hooded skunks, and deer. Cattle are also known to eat screwbean mesquite pods [10,65]. However it is possible that the spines of screwbean mesquite deter some browsers [83].

Although screwbean mesquite is an important part of the diet for many species, it is not recommended as animal feed. Like other mesquites, screwbean mesquite is high in seed protein and pericarp sugar [11]. However, in a feeding trial on young chickens screwbean mesquite was shown to result in a negative protein efficiency ratio, which means individuals fed on this diet exhibited a net nitrogen loss [11]. The following table shows the composition of screwbean mesquite pods [11].

% H2O % Protein (N x 6.25)* % Fiber % Ash % Sugar
Whole pod 5.9 11 17 3.8 25
Seeds 7.4 26 - - -
*not corrected for nonprotein nitrogen

Cover value: Screwbean mesquite-containing habitats provide good cover for birds, and several bird species nest in screwbean mesquite communities [7]. Johnson and others [63] state "some of the cottonwood-willow and associated mesquite riparian vegetation types are the most productive avian habitats in the western U.S." Gullion [53] notes the high quality of cover for Gambel's quail provided by vegetation comprised of screwbean mesquite and other shrubs. Abert's towhee, mourning dove and Gambel's quail nest in screwbean mesquite communities year round [77]. Verdin and black-tailed gnatcatchers are resident summer species in mesquite communities [77,82]. Some summer nonresidents include ash-throated flycatcher, Lucy warbler, northern oriole, and the white-winged dove [77]. In winter, ruby-crowned kinglets and yellow-rumped warblers occur [77,82]. Engel-Wilson and Ohmart [39] cite their previous finding that during winter more avian habitat specialists were found in cottonwood-willow and screwbean mesquite communities, even though only 99 acres (40 ha) within the 49,420-acre (20,000 ha) study area were comprised of these types. Other bird species that inhabit screwbean mesquite-containing habitats include black-chinned hummingbirds, cactus wrens, ladder-backed woodpeckers, pyrrhuloxias and Crissal thrashers [18,82,107].

Screwbean mesquite communities also provide cover to small mammals. Thickets of Sonoran desertscrub support desert cottontail as well as several rodents [101]. Screwbean mesquite habitat types have relatively high diversity of nocturnal rodents including cactus mouse, desert pocket mouse, Merriam's kangaroo rat, and the white-throated woodrat [2].

VALUE FOR REHABILITATION OF DISTURBED SITES:
Screwbean mesquite has been used in revegetation efforts in areas dominated by tamarisks [17,35,99]. It can be planted on sites with higher salt concentrations than many other native riparian species can tolerate [61,99]. However results of screwbean mesquite plantings in revegetation efforts have been mixed. Successes are mainly attributed to planting on sites within the requirements of screwbean mesquite [17,99]. See the Asexual regeneration section for success rates of different propagation techniques.

OTHER USES:
According to literature reviews and some ethnographic investigations [10,38], screwbean mesquite was a food source for many southwestern tribes, including the Chauilla, Pima, Apache, Cocopa, and Quechan [10,29,44]. The Chauilla Indians prepared and used the sweet, dried pods in the much same way as other mesquite pods, by grinding them into meal for bread or gruel [10,12,15,104]. Other tribes, such as the Pima, preferred to pit-cure the pods before eating or further processing [10,29]. In addition, the pods or meal could be used to make beverages [12,15,29,104] and a syrup was made by boiling the pods [15,36].

Screwbean mesquite was used by many southwestern tribes in a variety of other ways. The bark and roots had medicinal value and were used to treat wounds [10,15,65,104]. The use of gum, bark, and blossoms as food, medicine, dyes, and in making pottery have been reported for other southwestern mesquites [10,12,88], but it is uncertain if screwbean mesquite was also used for these purposes.

Currently use of screwbean mesquite is limited. Besides a small market for timber products, the current main use of mesquite is in the production of honey [42]. However, honey mesquite is typically used and the extent of screwbean mesquite use has not be reported.

Wood Products:
The wood of mesquites is hard, durable, and attractive [69,80]. Due to the slender nature of the branches and often shrubby growth form screwbean mesquite is most typically used for fuel and fence posts [13,29,65,70,80,104]. Native Americans used the wood for fuel as well as for weapons, tools, and construction [10,29]. The wood is also used locally for small items, such as tool handles and trinkets [13,65,104]. Custom woodworkers use mesquite wood, including that of screwbean mesquite, for making small pieces of furniture and specialty items [69].

OTHER MANAGEMENT CONSIDERATIONS:
As an invasive, screwbean mesquite is much less of a concern than other southwestern mesquites [47]. It does have some weedy tendencies [84], and Northington and Burgess [81] report a probable introduction of 3 screwbean mesquite trees in Guadalupe Mountains National Park, TX. The source of a small population in the San Joaquin Valley, CA, is uncertain, but it may have been introduced as well [57]. However, widespread invasion of screwbean has not been reported. Being largely restricted to riparian habitats is a likely factor limiting the potential spread of screwbean mesquite.


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