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Provisional. A provisional ecological site description has undergone quality control and quality assurance review. It contains a working state and transition model and enough information to identify the ecological site.
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Figure 1. Mapped extent
Areas shown in blue indicate the maximum mapped extent of this ecological site. Other ecological sites likely occur within the highlighted areas. It is also possible for this ecological site to occur outside of highlighted areas if detailed soil survey has not been completed or recently updated.
MLRA notes
Major Land Resource Area (MLRA): 040X–Sonoran Basin and Range
AZ 40.3 – Colorado Sonoran Desert
Elevations range from 300 to 1200 feet and precipitation averages 3 to 7 inches per year. Vegetation includes creosotebush, white bursage, brittlebush, Mormon tea, teddybear cholla, elephant tree, smoke tree, ocotillo, and big galleta. The soil temperature regime is hyperthermic and the soil moisture regime is typic aridic. This unit occurs within the Basin and Range Physiographic Province and is characterized by numerous mountain ranges that rise abruptly from broad, plain-like valleys and basins. Igneous and metamorphic rock classes dominate the mountain ranges and sediments filling the basins represent combinations of fluvial, lacustrine, colluvial and alluvial deposits.Table 1. Dominant plant species
Tree (1) Parkinsonia microphylla
Shrub (1) Larrea tridentata var. tridentata
(2) Ambrosia dumosaHerbaceous (1) Pleuraphis rigida
(2) Muhlenbergia porteriPhysiographic features
This site occurs as nearly level to gently sloping fan terraces and old stream terraces no longer flooded.
Table 2. Representative physiographic features
Landforms (1) Terrace
(2) Fan
(3) Stream terrace
Elevation 75 – 1000 ft Slope 1 – 3 % Climatic features
Precipitation in this common resource area ranges from 3-7 inches yearly. Despite historical averages in rainfall amounts, as one moves from east to west in this resource area rains become more unpredictable and variable with Coefficients of Variation of annual rainfall equal to 44% at Gila Bend and 65% at Mohawk. Winter-Summer rainfall ratios are 40-60%. Summer rains fall July-September, originate in the Gulf of Mexico and are convective, usually brief intense thunderstorms. Summer thunderstorms usually form over the mountains in the afternoon and spread to the valleys and plains in the evening. The intensity of this precipitation is moderate to heavy, but rarely lasts more than half an hour. Many times these storms produce little more than gusty winds and light showers. Cool season moisture tends to be frontal, originate in the Pacific and Gulf of California and falls in widespread storms with long duration and low intensity. Snow is very rare and falls normally only in the higher mountains.
Mean temperatures for the hottest month (Jul) is 93 F; the coldest month (Jan) is 53 F. Extreme temperatures of 125 F and 10 F have been recorded. Long periods with little or no effective moisture occur frequently.
The winter-spring precipitation is the most dependable on the site. Perennial grasses, though classed as warm season growers, grow actively year-round when moisture is available. Shrubs and trees generally respond to seasonal moisture. The two rainy periods bring about their respective production of either winter or summer annual grasses and forbs.Table 3 Representative climatic features
Frost-free period (average) 360 days Freeze-free period (average) 0 days Precipitation total (average) 10 in BarLineFigure 2. Monthly precipitation range
BarLineFigure 3. Monthly average minimum and maximum temperature
">Influencing water features
Soil features
Soils deep loamy, calcareous to the surface and, droughty with only fair plant-soil moisture relationships.
Soils mapped on this site include: in SSA-645 Aguila-Carefree area MU's Gilman (unflooded)-56, 59, Denure-59, Rillito-71, 102 & 117, Tremant-114 & 119; SSA-649 Yuma-Wellton area MU's Antho (unflooded)-1 & 2, Dateland-5 & 6, India-13, Ripley-24, Superstition-29 & Tremant-11 & 31; SSA-653 Gila Bend-Ajo area MU's Cavelt-19, Coolidge-8, Coolidge GrVFSL-11, Denure GrFSL-17 & 18, Denure-19, Mohall FSL-44, Mohall L-45, Mohall CL-47, Tremant GrFSL-59, Tucson L-50, Wellton-32, Wellton L-64, Wellton LFS-65 & Wellton GrVFSL-65; SSA-656 Colorado River Indian Reservation MU Antho-3.Table 4. Representative soil features
Surface texture (1) Gravelly loam
(2) Sandy loam
(3) Fine sandy loam
Family particle size (1) Loamy
Drainage class Well drained Permeability class Moderately slow to moderately rapid Soil depth 60 in Surface fragment cover <=3" 0 – 15 % Surface fragment cover >3" Not specified Available water capacity
(0-40in)8.4 – 10.8 in Calcium carbonate equivalent
(0-40in)3 – 25 % Electrical conductivity
(0-40in)0 – 2 mmhos/cm Sodium adsorption ratio
(0-40in)Not specified Soil reaction (1:1 water)
(0-40in)7.9 – 8.4 Subsurface fragment volume <=3"
(Depth not specified)0 – 15 % Subsurface fragment volume >3"
(Depth not specified)Not specified Ecological dynamics
The plant communities found on an ecological site are naturally variable. Composition and production will vary with yearly conditions, location, aspect, and the natural variability of the soils. The Historical Climax Plant Community represents the natural potential plant communities found on relict or relatively undisturbed sites. Other plant communities described here represent plant communities that are known to occur when the site is disturbed by factors such as fire, grazing, or drought.
Production data provided in this site description is standardized to air dry weight at the end of the summer growing season. The plant communities described in this site description are based on near normal rainfall years.
NRCS uses a Similarity Index to compare existing plant communities to the plant communities described here. Similarity index is determined by comparing the production and composition of a plant community to the production and composition of a plant community described in this site description. To determine Similarity index, compare the production (air dry weight) of each species to that shown in the plant community description. For each species, count no more than the maximum amount shown for the species, and for each group, count no more than the maximum amount shown for each group. Divide the resulting total by the total normal year production shown in the plant community description. If the rainfall has been significantly above or below normal, use the total production shown for above or below normal years. If field data is not collected at the end of the summer growing season, then the field data must be corrected to the end of the year production before comparing it to the site description. The growth curve can be used as a guide for estimating production at the end of the summer growing season.
State and transition model
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Click on state and transition labels to scroll to the respective textEcosystem states
State 1 submodel, plant communities
State 1
Historic Climax Plant CommunityCommunity 1.1
Historic Climax Plant CommunityThe potential plant community is a mixture of desert shrubs dominated by creosotebush. Winter and summer annual grasses and forbs are abundant in years with above-average moisture in their respective seasons.
Continued overgrazing results in creosotebush becoming dominant. When the understory cover is depleted, sheet and rill erosion increases on the site. In advanced stages, gullies are common.Figure 4. Annual production by plant type (representative values) or group (midpoint values)
Table 5. Annual production by plant type
Plant type Low
(lb/acre)Representative value
(lb/acre)High
(lb/acre)Shrub/Vine 150 180 210 Forb 45 60 75 Grass/Grasslike 45 60 75 Tree 5 10 15 Total 245 310 375 Figure 5. Plant community growth curve (percent production by month). AZ4041 , 40.3 3-7" p.z. all sites. Most growth occurs in the winter to early spring, plants are dormant May through October..
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec J F M A M J J A S O N D 5 20 40 25 0 0 0 0 0 0 5 5 Additional community tables
Table 6. Community 1.1 plant community composition
Group Common name Symbol Scientific name Annual production () Foliar cover (%) Grass/Grasslike1 Perennial Grasses 15–30 threeawn ARIST Aristida 3–6 – bush muhly MUPO2 Muhlenbergia porteri 3–6 – big galleta PLRI3 Pleuraphis rigida 3–6 – 2 Annual Grasses 30–45 sixweeks threeawn ARAD Aristida adscensionis 6–9 – needle grama BOAR Bouteloua aristidoides 6–9 – sixweeks grama BOBA2 Bouteloua barbata 6–9 – littleseed muhly MUMI Muhlenbergia microsperma 6–9 – Bigelow's bluegrass POBI Poa bigelovii 6–9 – Texas fluffgrass TRTE2 Tridens texanus 6–9 – sixweeks fescue VUOC Vulpia octoflora 6–9 – Forb3 Misc. Forbs 45–60 bristly fiddleneck AMTE3 Amsinckia tessellata 0–1 – brittle spineflower CHBR Chorizanthe brevicornu 0–1 – devil's spineflower CHRI Chorizanthe rigida 0–1 – cryptantha CRYPT Cryptantha 0–1 – hairy prairie clover DAMO Dalea mollis 0–1 – flatcrown buckwheat ERDE6 Eriogonum deflexum 0–1 – woollystar ERIAS Eriastrum 0–1 – spurge EUPHO Euphorbia 0–1 – Gordon's bladderpod LEGO Lesquerella gordonii 0–1 – pepperweed LEPID Lepidium 0–1 – trefoil LOTUS Lotus 0–1 – bristly nama NAHI Nama hispidum 0–1 – glandular threadplant NEGL Nemacladus glanduliferus 0–1 – combseed PECTO Pectocarya 0–1 – phacelia PHACE Phacelia 0–1 – desert Indianwheat PLOV Plantago ovata 0–1 – sleepy silene SIAN2 Silene antirrhina 0–1 – desert globemallow SPAM2 Sphaeralcea ambigua 0–1 – Coulter's globemallow SPCO2 Sphaeralcea coulteri 0–1 – 4 Other Forbs 3–15 brownfoot ACWR5 Acourtia wrightii 0–1 – carelessweed AMPA Amaranthus palmeri 0–1 – milkvetch ASTRA Astragalus 0–1 – desert marigold BAMU Baileya multiradiata 0–1 – spiderling BOERH2 Boerhavia 0–1 – yellow tackstem CAPA7 Calycoseris parryi 0–1 – western tansymustard DEPI Descurainia pinnata 0–1 – desert trumpet ERIN4 Eriogonum inflatum 0–1 – buckwheat ERIOG Eriogonum 0–1 – little deserttrumpet ERTR8 Eriogonum trichopes 0–1 – desert lily HEUN2 Hesperocallis undulata 0–1 – Arizona poppy KAGR Kallstroemia grandiflora 0–1 – strigose bird's-foot trefoil LOSTT Lotus strigosus var. tomentellus 0–1 – Coulter's lupine LUSP2 Lupinus sparsiflorus 0–1 – glandular threadplant NEGL Nemacladus glanduliferus 0–1 – desert tobacco NIOB Nicotiana obtusifolia 0–1 – evening primrose OENOT Oenothera 0–1 – Shrub/Vine5 Dominant Shrubs 120–180 creosote bush LATR2 Larrea tridentata 90–120 – burrobush AMDU2 Ambrosia dumosa 45–60 – 6 Misc. Shrubs 15–30 littleleaf ratany KRER Krameria erecta 9–18 – white ratany KRGR Krameria grayi 9–18 – desert-thorn LYCIU Lycium 9–18 – desert ironwood OLTE Olneya tesota 9–18 – triangle bur ragweed AMDE4 Ambrosia deltoidea 5–10 – 7 Succulents 3–15 saguaro CAGI10 Carnegiea gigantea 0–1 – Wiggins' cholla CYEC3 Cylindropuntia echinocarpa 0–1 – jumping cholla CYFUF Cylindropuntia fulgida var. fulgida 0–1 – Christmas cactus CYLE8 Cylindropuntia leptocaulis 0–1 – branched pencil cholla CYRA9 Cylindropuntia ramosissima 0–1 – Engelmann's hedgehog cactus ECEN Echinocereus engelmannii 0–1 – candy barrelcactus FEWI Ferocactus wislizeni 0–1 – nightblooming cereus PEGRG Peniocereus greggii var. greggii 0–1 – Tree8 Tree 5–15 yellow paloverde PAMI5 Parkinsonia microphylla 5–15 – Interpretations
Animal community
The only factor limiting use of this site is distance to water. The site is more suited for use by stocker cattle in the spring as this is the time forage is available. Water developments are important to permit grazing management.
Lack of water and poor cover limit the value of this site as habitat. It is home to a variety of burrowing mammals, and mainly a forage area for larger mammals and bird species.Recreational uses
This site is located on fan terraces and old stream terraces. The site is usually a monotonous landscape of creosotebush, but has an abundance of desert wildflowers following good winter rainy seasons. Very few days in the fall, winter or spring are too uncomfortable to enjoy outdoor activities. Jun-Aug, however, are restricted by afternoon heat. Horseback riding, wildlife observation, hunting, hiking, photography, camping and picnicking are activities suited to this site.
Supporting information
Type locality
Location 1: Maricopa County, AZ Township/Range/Section T3N R7E S19 General legal description Typical pedon of Rillito GrL is about 1,000 feet East and 1,450 feet South of the North West corner of Section 19, T 3 N, R 7 E. Contributors
Larry D. Ellicott
Steve Barker
UnknownRangeland health reference sheet
Interpreting Indicators of Rangeland Health is a qualitative assessment protocol used to determine ecosystem condition based on benchmark characteristics described in the Reference Sheet. A suite of 17 (or more) indicators are typically considered in an assessment. The ecological site(s) representative of an assessment location must be known prior to applying the protocol and must be verified based on soils and climate. Current plant community cannot be used to identify the ecological site.
Author(s)/participant(s) Contact for lead author Date Approved by Approval date Composition (Indicators 10 and 12) based on Annual Production Indicators
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Number and extent of rills:
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Presence of water flow patterns:
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Number and height of erosional pedestals or terracettes:
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Bare ground from Ecological Site Description or other studies (rock, litter, lichen, moss, plant canopy are not bare ground):
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Number of gullies and erosion associated with gullies:
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Extent of wind scoured, blowouts and/or depositional areas:
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Amount of litter movement (describe size and distance expected to travel):
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Soil surface (top few mm) resistance to erosion (stability values are averages - most sites will show a range of values):
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Soil surface structure and SOM content (include type of structure and A-horizon color and thickness):
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Effect of community phase composition (relative proportion of different functional groups) and spatial distribution on infiltration and runoff:
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Presence and thickness of compaction layer (usually none; describe soil profile features which may be mistaken for compaction on this site):
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Functional/Structural Groups (list in order of descending dominance by above-ground annual-production or live foliar cover using symbols: >>, >, = to indicate much greater than, greater than, and equal to):
Dominant:
Sub-dominant:
Other:
Additional:
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Amount of plant mortality and decadence (include which functional groups are expected to show mortality or decadence):
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Average percent litter cover (%) and depth ( in):
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Expected annual annual-production (this is TOTAL above-ground annual-production, not just forage annual-production):
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Potential invasive (including noxious) species (native and non-native). List species which BOTH characterize degraded states and have the potential to become a dominant or co-dominant species on the ecological site if their future establishment and growth is not actively controlled by management interventions. Species that become dominant for only one to several years (e.g., short-term response to drought or wildfire) are not invasive plants. Note that unlike other indicators, we are describing what is NOT expected in the reference state for the ecological site:
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Perennial plant reproductive capability:
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