SPECIES: Acacia constricta
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|Photos © 2002, Steve Backauf|
For the purposes of this review, "whitethorn acacia" refers to both varieties. When distinguishing between the 2 varieties, the variety will be indicated by scientific name. When citing literature that refers only to genus, this will be indicated as "Acacia spp."
Whitethorn acacia may hybridize with viscid acacia (A. neovernicosa), although the likelihood of this occurring is limited. Development timing of the 2 species is distinct [29,58]. Ishaque and others  suggest that viscid acacia starts growing approximately 1 month prior to whitethorn acacia, and Powell  reports that whitethorn acacia flowers 1 to 2 weeks earlier than viscid acacia when the 2 species occur together.LIFE FORM:
Whitethorn acacia is a drought, cold, and heat tolerant deciduous shrub [21,30]. It may terminate leaf and/or flower production during severe drought . Whitethorn acacia can reach 19.7 feet (6 m) tall . It is armed with straight spines that are 3 to 6 mm long and occur in pairs at the stem nodes. The compound leaves are thick, 1 to 2 inches (2.5-5 cm) long, and have 3 to 6 paired leaflets per leaf [7,80]. Extrafloral nectaries occur along leaf rachises . Ants feed on these nectaries. The fruit is a legume. Seeds average about 3 mm wide, 5-6 mm long, and weigh an average of 0.67 ounces (19.1 g) [7,62]. The fruit pod has a woody or papery texture and is compressed tightly between the seeds. Seeds are oblong shaped and flattened .
Acacia constricta var. paucispina is most common at high elevations and can be twice as large as A. constricta var. constricta [80,92]. Commonly the spines of A. constricta var. constricta are fewer and reduced, while the leaves are typically larger on A. constricta var. paucispina. It is rare to see A. constricta var. paucispina growing in dense stands, while A. constricta var. constricta does occur in dense stands .
Whitethorn acacia plants are long lived. Plants in the Sonoran Desert were more than 72 years old. While long-term and short-term survival probabilities are high for this species, recruitment years are few, and the time required to see a 50% population turnover is likely much longer than 72 years .RAUNKIAER  LIFE FORM:
Breeding system: Cross pollination is predominant in whitethorn acacia, although self pollination can occur .
Pollination: Honey bees are thought to be the most common pollinator for whitethorn acacia . Flowers are short lived, and bees only pollinate flowers on the day flower buds open. Two- and three-day-old inflorescences are wilted and uninviting. In a controlled study, Wagner  found that significantly (p=0.004) fewer seed pods per flower were produced in self-pollinated flowers than in outcrossed flowers.
Seed production: Whitethorn acacia seeds are large, and production may vary with environmental stress. Wagner  found that when ant (Formica perpilosa) colonies were located at the base of whitethorn acacia shrubs, seed production averaged double that of shrubs without ant nests. During periods of severe drought, whitethorn acacia may not produce flowers .
Seed dispersal: It is likely that whitethorn acacia seed is dispersed by a variety of birds and mammals. In a southeastern New Mexico study, whitethorn acacia seed was identified in 41.4% of scaled quail crops that were harvested in the fall and winter . Whitethorn acacia seed was also recovered from the stomachs and cheek pouches of Merriam's kangaroo rats and Arizona, Bailey's, and rock pocket mice in the Sonoran Desert . While large mammals do not prefer whitethorn acacia, deer occasionally feed on leaves and pods . When 1,000 and 2,500 seeds were fed to domestic sheep and cattle, 5.6 % and 1.9% respectively, of whitethorn acacia seeds germinated, suggesting that livestock dispersal of whitethorn acacia seed may be limited. There was no comparison between digested and undigested seed germination in the aforementioned studies, but whitethorn acacia seed does require scarification and increased germination occurs with increased seed coat removal .
Seed banking: Whitethorn acacia produces a seed bank, a portion of which may be the result of small mammal caches. Cox and others  found that kangaroo rats buried whitethorn acacia seed 0.8 to 1.6 inches (2-4 cm) below the soil surface after partially or fully removing seed coats. It is noteworthy that all whitethorn acacia seed on the soil surface was killed following a prescribed grassland fire, while seed buried 0.8 inch (2 cm) under the soil surface suffered no damage from the same fire .
Germination: Whitethorn acacia germinates best at warm temperatures. In the laboratory, optimal germination temperatures of whitethorn acacia seed were between 78.8 °F and 87.8 °F (27 °C and 30 °C). However, some germination (≥26%) occurred at all tested temperatures ( 60.8 °F to 100.4 °F (16 °C-38 °C)). Germination improved with increased seed coat removal .
Seedling establishment/growth: Seedling establishment success is likely increased when seed is buried or protected. On a grassland site in southeastern Arizona, whitethorn acacia seed on the soil surface germinated, but seedlings did not establish. In the same area, the largest quantity of seedlings emerged when seed was planted 0.4 to 0.8 inch (1-2 cm) below the surface of sandy loam soils . Whitethorn acacia seedling growth rates increased when seedlings were grown in soil inoculated with this species' associated rhizobia .
Several distinguishing seedling characteristics have been noted by Zisner . Upon uprooting, whitethorn acacia seedlings emit a strong nitrogenous odor. A similar odor was found in other Acacia species but not in all Fabaceae species. Using this information along with other botanical characteristics, whitethorn acacia seedlings about 45 days old can be distinguished from other common Sonoran Desert species. An early identification key is available .
The growth of this desert species is likely regulated by moisture availability. During a normal precipitation year on Chihuahuan Desert rangeland, whitethorn acacia shrubs increased twig diameter by 47%. Shrubs produced fewer flowers, seeds, and twigs during a drought year .
Asexual regeneration: Following top-kill, this species sprouts from the root crown [44,78,91].SITE CHARACTERISTICS:
Climate: The climatic conditions that characterize whitethorn acacia's range can be extreme. In the Rio Grande Valley of New Mexico, summer maximum temperatures can exceed 109 °F (42.8 °C) and winter lows can drop to 10 °F (-12.2 °C) . Precipitation is commonly distributed in a bimodal pattern with most rain falling in intense spring and summer storms and less rain falling in the winter months in more moderate storms. In southeastern Arizona, the precipitation is still bimodal, but summer precipitation is more tightly restricted to July and August with often no precipitation in April, May, and June . Campbell and others  describe droughts for the winters in New Mexico and Texas. Average yearly precipitation for these arid environments ranges from 8.5 to 10 inches (216-254 mm) with extremes of 3.5 to 19.6 inches (89-498 mm) reported [6,19]. Drying winds are also common, and in the summer months evaporation can exceed the annual precipitation by 50% .
Soils: Sandy to loamy soils are most often described in association with whitethorn acacia [6,18]. An impervious caliche or lime layer is also commonly associated with most of these soils [6,21,86]. Whitethorn acacia was found on soils with 6.3% to 10.9% moisture content during the period of most intense drought conditions, and dominated where the moisture content was 6.3% . Wagner  found soil moisture, nitrate, ammonium, and phosphorus concentrations were higher when shrubs had ant nests beneath their canopies.
Elevation: A range of elevations is tolerated by whitethorn acacia:
|AZ||2,500 to 5,000 feet (762-1524 m) 
In Huachuca Mts., predominantly below 4,500 feet (1,372 m) 
|NM||4,500 to 5,500 feet (1,372-1,676 m) in southern New Mexico |
|TX||1,500 to 6,500 feet (457-1,981 m) |
|Guadalupe Escarpment, NM & TX||3,600 to 5,000 feet (1,097-1,524 m) |
In a study designed to address the effect of disturbance on desert scrub vegetation in southeastern Arizona, whitethorn acacia density was greatest on those sites disturbed by grazing alone. Whitethorn acacia density was lowest on sites that were plowed at depths of 17.7 to 21.7 inches (45-55 cm) for 5 years. Completely undisturbed sites supported a density of whitethorn acacia intermediate between the undisturbed and plowed sites .
Whitethorn acacia commonly flowers once in spring (April-June) and again in the summer or fall (July-October) [5,30]. Flower buds may form in November or December, but full flowers rarely develop. In a study in the Tucson Mountains of the Sonoran Desert, researchers found that whitethorn acacia flowering began when rainfall exceeded 0.43 inch (11 mm) and was followed by a heat sum of approximately 522 degree-days above 15 °C .
Fire regimes: The historic fire regimes for areas where whitethorn acacia occurs are likely different over the range of this species. They likely differ as the vegetation types change across the arid and semiarid desert ecosystems. Humphrey  separates the Sonoran Desert, Chihuahuan Desert, tobosa grasslands, and desert grasslands when attempting to describe the fire regimes of the Southwest. As a rule, he suggests that "the more arid the desert, the less fuel is produced, and the less frequent and severe are any fires that may occur." Fires were rare in the Sonoran Desert due to the noncontinuous fuels that result from dominance by trees and shrubs that are not well suited to burning. Fires were more frequent in the Chihuahuan Desert than in the Sonoran Desert. The greater abundance of perennial grasses and low-growing shrubs in the Chihuahuan Desert makes the landscape better able to carry fire. Where these desert systems meet what Humphrey  calls the desert grasslands, fires were more common, and the reduction of fires at these ecotones has likely facilitated the encroachment of woody vegetation and the conversion of grasslands to shrublands.
Reliance on fire-scarred trees to determine fire history is impossible in grass and shrubland systems; the use of historical records, plant adaptations, and other means is necessary to estimate fire regimes in these ecosystems. Miksicek  recovered whitethorn acacia and other woody desert vegetation charcoal from central Arizona. He suggested that vegetation charcoal and other agricultural features is evidence that the Hohokam people (2,205 B.P.-555 B.P.) used fire in central Arizona to clear areas for cultivation.
Cactus-shrub dominated ecosystems: Areas of the Sonoran Desert in Arizona that are dominated by saguaro (Carnegiea gigantea) and paloverde (Cercidium spp.) are thought to have burned less frequently than the desert grassland areas, and have historically burned less frequently than they do today [2,12,44]. This assertion is based on the postfire vegetation response in these areas: most of the vegetation in these communities does not sprout following fire, which suggests that fire was not evolutionarily important to these communities . However, anthropogenic changes that include the increase of red brome, an exotic species that readily carries fire, and an increase in the number of fire ignitions is thought to have increased the probability of fires in saguaro- and paloverde-dominated communities [2,12,44]. These anthropogenic influences are especially evident following above-average winter precipitation or El Niño events that favor the growth of red brome . Esque and Schwalbe  suggest red brome may favor a fire cycle similar to that of cheatgrass (Bromus tectorum) in sagebrush communities and may forever alter fire regimes in these communities. The return of Sonoran Desert communities to prefire structure and species composition is likely to take many decades, as recovery time for saguaro and paloverde is slow [12,63].
Grassland-dominated ecosystems: The grassland ecosystems of southwestern deserts now dominated by shrubs are thought to have burned regularly enough to restrict woody vegetation to riparian and drainage areas [15,27]. Cox and others  partially attribute the increasing abundance of whitethorn acacia in once grass-dominated desert areas to fire exclusion. Humphrey  suggests that periodic fires likely maintained the grasslands of Yavapai County, Arizona. However, since the 1800s woody vegetation has occupied several million acres that were historically grasslands with a few scattered shrubs. Livestock grazing is often the reason given for this fire frequency change. Grazing animals may have played a role in dispersing shrub seed, especially mesquite, from riparian to upland areas. The selective removal of grasses decreased the "competitive" ability of grasses as shrubs were establishing, and the resulting decrease in grass coverage and fuels removed the ability of the ecosystem to carry fire . Dick-Peddie and Alberico  likewise attribute decreased fire frequency within the Chisos Mountains of Texas to grazing influences on fuels and the exclusion of natural fires.
While research has not addressed differences in the fire regimes for Acacia constricta var. paucispina and A. constricta var. constricta, the latter variety can grow in dense stands  and likely fire behaves differently when densely and sparsely vegetated shrublands are compared.
The following list provides fire return intervals for plant communities and ecosystems where whitethorn acacia is important. For further information, see the FEIS reviews on the dominant species listed below.
|Community or Ecosystem||Dominant Species||Fire Return Interval Range (years)|
|desert grasslands||Bouteloua eriopoda and/or Pleuraphis mutica||5-100 |
|plains grasslands||Bouteloua spp.||< 35|
|blue grama-buffalo grass||B. gracilis-Buchloe dactyloides||< 35 [56,94]|
|grama-galleta steppe||Bouteloua gracilis-Pleuraphis jamesii||< 35 to < 100|
|blue grama-tobosa prairie||B. gracilis-P. mutica||< 35 to < 100|
|paloverde-cactus shrub||Cercidium microphyllum-Opuntia spp.||< 35 to < 100|
|blackbrush||Coleogyne ramosissima||< 35 to < 100|
|creosotebush||Larrea tridentata||< 35 to < 100|
|Ceniza shrub||L. tridentata-Leucophyllum frutescens-Prosopis glandulosa||< 35|
|galleta-threeawn shrubsteppe||Pleuraphis jamesii-Aristida purpurea||< 35 to < 100 |
|mesquite||Prosopis glandulosa||< 35 to < 100 [46,56]|
|mesquite-buffalo grass||P. glandulosa-Buchloe dactyloides||< 35|
|Texas savanna||P. glandulosa var. glandulosa||< 10 |
Likely the response of whitethorn acacia following fire depends on fire severity and damage to the root crown. One year following an arson fire near Phoenix, Arizona, the density of whitethorn acacia was 76 shrubs/0.5 ha on burned plots and 72 shrubs/0.5 ha on nearby unburned plots. All 72 whitethorn acacias on the burned sites sprouted. No data were given on the severity of this fire . After being top-killed by a propane torch, which may have produced heating greater than that of the arson fire, whitethorn density had declined 80%, 70%, and 33% when plots were revisited 3 months, 15 months, and 2 years later, respectively . In burned areas of Carlsbad Caverns and Guadalupe Mountains National Parks, Ahlstrand  suggests that prefire and postfire communities are compositionally the same, just changed in relative coverage and "competitive" ability.
The Research Project Summary Ibarra-F and others 1996 provides information on mortality of whitethorn acacia after prescribed fires in buffelgrass (Pennisetum ciliare) pastures in Sonora, Mexico.FIRE MANAGEMENT CONSIDERATIONS:
Large mammals: Desert mule deer do not feed extensively on whitethorn acacia, but whitethorn acacia often makes up a small amount of their diets. Ishaque and others  report that deer browse whitethorn acacia leaves and pods. A study conducted in the Belmont Mountains of Arizona found the diets of desert mule deer were 0.4% whitethorn acacia in the winter months. Likewise, when the researchers reviewed and compiled results from other desert mule deer diet studies, they found the use of whitethorn acacia was low (1%-5%) in summer, fall, and winter months and 0% in spring months . Short  also found mule deer diets contained low amounts of whitethorn acacia (0.1-3%), with mule deer consuming the highest percentage of whitethorn acacia in summer months. In Carlsbad Caverns National Park, mule deer browsed whitethorn acacia from August to November, with most usage in October and November . This low yet common usage of whitethorn acacia by mule deer suggests that whitethorn acacia may offer some needed trace nutrient and/or that whitethorn acacia may be utilized when more palatable plants are scarce.
Other large desert mammals may also utilize whitethorn acacia as a food source. Fecal samples of bighorn sheep collected from the Kofa Wildlife Refuge in Yuma County, Arizona, contained Acacia spp. .
Small mammals: Many small desert mammals feed on whitethorn acacia. Rodents prefer whitethorn acacia seed. Rabbit species likely browse whitethorn acacia when more preferred food sources are unavailable. In the lower Sonoran Desert, whitethorn acacia is a key food source of the southwestern pack rat . Whitethorn acacia seed was recovered from the stomachs and cheek pouches of 3 pocket mouse species and 1 kangaroo rat species in the Sonoran Desert, and the researcher considered whitethorn acacia a preferred food for these rodents . In desert shrub areas near Phoenix, Arizona, Stamp and Ohmart  found that whitethorn acacia seed made up 3.6% of the cheek pouch contents of desert pocket mice. Black-tailed and antelope jackrabbits, cottontail rabbits, and rodents utilize whitethorn acacia bark as a food source when other food is unavailable [30,81]. Arizona desert cottontails showed high preference for Acacia spp. in early spring. There had been no precipitation in the preceding 5 months and more palatable vegetation was not available .
Game birds: Whitethorn acacia is important to the survival of several southwestern bird species. Scaled quail feed extensively on whitethorn acacia seed [6,30]. In a southeastern New Mexico study, whitethorn acacia seed was identified in 41.4% of the crops of scaled quail that were harvested in the fall and winter, but whitethorn acacia seed was just 8.3% of crop content in summer harvested birds . However, for this study the fall and winter sample size was 277, while just 12 birds were sampled in the summer. Gullion  notes that Acacia spp. are an important seed source for Gambel's quail.
Other avifauna: Other bird species that utilize desert shrub habitats where whitethorn acacia is commonly present include black-chinned hummingbirds, ladder-backed woodpeckers, ash-throated flycatchers, verdins, cactus wrens, mockingbirds, black-tailed gnatcatchers, brown-headed cowbirds, pyrrhuloxias, and house finches .
In Organ Pipe Cactus National Monument, verdins used Acacia spp. most when foraging . Whitethorn acacia provides habitat and forage for phainopeplas, a bird species that disperses desert mistletoe (Phoradendron californicum) seed . Desert mistletoe is parasitic on whitethorn acacia. Phainopeplas used Acacia spp. second only to honey mesquite (Prosopis glandulosa) for foraging in Organ Pipe Cactus National Monument .
Research by Stamp  suggests that whitethorn acacia may provide important avian breeding habitat. In the Lower Verde River region of Arizona where whitethorn acacia occurs, Abert's towhees, Lucy's warblers, mourning doves, and white-winged doves were found in densities greater than 20 breeding pairs per 40 ha . In another study, whitethorn acacia shrubs taller than 6.6 feet (2 m) were selected by southwestern breeding birds for nesting sites in desert scrub communities .
Amphibians/Reptiles: It is possible that whitethorn acacia provides habitat for some southwestern herptiles. The narrow-mouthed toad and yellow mud turtle are commonly found in spring or man-made water areas where whitethorn acacia is common .
Palatability/nutritional value: Since the utilization of whitethorn acacia is low for most herbivores, it is reasonable to consider its palatability to be low. Ishaque  reports that the crude protein content of whitethorn acacia leaves is 25% in mid-summer and decreases to 18% to 22% by the end of the growing season. Whitethorn acacia is high in cyanide-forming compounds, and death may result when it is eaten in high concentrations .
Whitethorn acacia seeds contain on average 93.57 calories/seed or 4,912 calories/g .
Cover value: Southwestern bird and small mammal species use whitethorn acacia for cover. Campbell and others  consider whitethorn acacia an important cover species for scaled quail. Acacia spp. provide important habitat for Gambel's quail [23,24]. In southern Arizona, Goodwin and Hungerford  report that 11 of 87 Gambel's quail roosting sites were in Acacia spp. Whitethorn acacia provides den cover for the southwestern packrat .VALUE FOR REHABILITATION OF DISTURBED SITES:
Whitethorn acacia's success in revegetation of mine reclamation sites depended on the protection of the revegetated sites. Whitethorn acacia seedlings suffered 100% mortality on the revegetated exposed east slopes of copper mine waste areas of Tucson, Arizona. On the more protected north slopes, some seedlings survived for at least 7 years following the initial seeding . However, actual survival rates were unclear.OTHER USES:
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