Edaphic factors affecting the distribution of Creosotebush, Larrea tridentata (DC.) Cov. in desert grassland sites of southeastern arizona
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Authors Johnson, Donald Edward, 1932-
Publisher The University of Arizona.
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Link to Item http://hdl.handle.net/10150/551523 EDAPHIC FACTORS AFFECTING THE DISTRIBUTION OF CREOSOTEBUSH
LARREA TR3PENTATA (DC.) COV. HI DESERT GRASSLAND SITES OF
SOUTHEASTERN ARIZONA by
Donald E. Johnson
A Thesis Subnitted to the Faculty of the
DEPARTMENT OF WATERSHED MANAGEMENT
In Partial Fulfillment of the Requirements for the Degree of
MASTER OF SCIENCE
In the Graduate College
THE UNIVERSITY OF ARIZONA
19 6 1 STATEMENT BY AUTHOR
This thesis has been submitted in partial fulfillment of requirements for an advanced degree at the University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the library*
Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in their judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author.
APPROVAL BY THESIS DIRECTOR
This thesis has been approved on the date shown below:
Date
ii ACKNC3WLED05EMTS
Gratitude is expressed to Dr. R. R. Humphrey for his
guidance and help in editing the manuscript, to Dr. T. C. Tucker
for his assistance in soil analyses, and to Dr. C. T. Mason for his aid in the identification of the plant species involved in the
study.
I "wish also to express my sincere thanks to Dr. D. G.
Wilson and Professor E. M. Schmutz for their critical reading of
the manuscript, to P. W. Riggs for the use of the land used in the
study, and to the Regional W-25 Technical Committee of the Western
Agricultural Experiment Stations for the funds under which this
study was carried out. Thanks are also due my personal friends for
their assistance in the field work.
iii TABLE OF CONTENTS
Page
List of Tables...... v
List of Figures...... vi
INTRODUCTION...... 1
Location and Description of the Study Area IO Precipitation...... History of the Area......
LITERATURE R E V I E W ...... 8
VEGETATION SAMPLING ...... 11
Percent Frequency Analysis...... 13 Cover A n a l y s e s ...... 15
RESULTS OF PLANT ANALYSES...... 17
SOIL SAMPLING...... 33
Physical Soil Analyses...... 33 Chemical Soil Analyses...... 34
RESULTS OF SOIL ANALYSES ...... 35
Physical Soil Analyses...... 35 Mechanical Analyses...... 35 Moisture-Holding Capacity...... 39 Chemical Soil Analyses...... 42 Calcium Carbonate...... 44
DISCUSSION...... 47
SUMMARY ...... 51
REFERENCES CITED...... 53
APPENDIX...... 55
iv LIST OF TABLES
Table Page
1. Record of Precipitation (1956-1960, inclusive). West Well Ranch, Cochise County, Arizona ...... 5
2. Length of Transect Occupied and Percent Slope of Ten Plant Communities Along a 3000-Foot Transect on the North-Facing Slope of Turkey Creek Ridge. . . . 12
3* Percent Frequency of Perennial Plant Species in Ten Communities, Turkey Creek Ridge, Cochise County, Arizona...... 18
4* Total Cover of Five Plant Communities, Turkey Creek Ridge, Cochise County, Arizona...... 21
5* Mechanical Soil Analysis in Five Cochise County, Arizona Plant Communities. (Average of Three Samples at Each Depth) ...... 36
6. The Range in Percent of Each Soil Fraction Through out the Sampling Depth of Five Plant Communities, Cochise County, Arizona ...... 37
7* Available Moisture in Two Shrub Communities. (Aver age of Three Samples at Each Sampling D e p t h ) ...... 40
8. Available Moisture in Three Grass Communities. (Average of three Samples at Each Sampling Depth). . . 41
9. Calcium Carbonate as Percent of Total Soil Sample in Four Communities. (Only One Sample Taken at Each Sampling Depth) ...... 45
v LIST OF FIGURES
Figure Page
1. General view of the study area: North-facing slope of Turkey Creek Ridge, Sulphur Springs Valley, Cochise County, Arizona ...... 3
2. Positions of ten plant communities along a per manent transect, Turkey Creek Ridge, 19 6 1 ...... 14
3« Hilaria mutica community, Turkey Creek Ridge study plot...... 23.
4* Flourensia community, Turkey Creek Ridge study p l o t ...... 25
5 * Larrea community, Turkey Creek Ridge study plot .... 27
6. Bouteloua community, Turkey Creek Ridge study plot ...... 30
7. Bouteloua-Andropogon community, Turkey Creek Ridge study plot ...... 31
8. Changes in soil pH values with sampling d e p t h ...... 43
vi ABSTRACT OF THESIS
Johnson, D. E. 1961. Edaphic factors affecting the distri bution of Creosotebush, Larrea tridentata (DC.) Cov., in desert grassland site's of southeastern Arizona. Department of Watershed
Management, University of Arizona.
Edaphic factors that might explain the distribution of Larrea in the desert grassland of southeastern Arizona were investigated.
The composition, distribution and total cover of the vegetation in several adjoining desert grassland and Chihuahuan desert shrub associations were analyzed, and a permanent line transect established to record any future vegetational changes.
Soils of two shrub and three grass communities were mechani-c
cally analyzed for textural class, water-holding percent and available moisture; and chemically for calcium carbonate content and pH.
Results can be summarized as follows:
1. Soils that supported grasses usually contained a larger
percentage of clay and had a higher percent available moisture than
soils that grew largely shrubs.
2. Larrea was most abundant on sites where shallow, sandy
soils with low available moisture, caliche hardpan, or erosion pave ment resulted in relatively unfavorable moisture relationships for
the growth of other species. 3* The fora of calcium carbonate, as affecting permeability and root penetration, was more a factor in determining Larrea distri bution than was percent of occurrence.
4. Factors that seem most likely to prevent the establishment of Larrea in the grass communities studied are: poorly drained heavy
soils, competition from better adapted species, or. low pH values that prevent seed germination.
5. Soil moisture relationships in the Bouteloua eriopoda
community, the only community that Larrea was invading, resembled those in the shrub associations more closely than in the other grass
communities BITRODUCTICN
Larrea tridentata (DC.) Cov., commonly called creosotebush or, occasionally, greasewood, is a shrubby member of the Zygophyllaceae.
Larreats pungent odor, especially after a rain, and the characteristic olive-green aspect of its foliage are well known throughout much of southwestern United States. Alone, or with one or more co-dominants,
Larrea occupies more than 30 million acres in the Southwest (Duisberg,
1952). Most of this extensive area produces little palatable forage and Larrea itself is grazed neither by domestic livestock nor by wild life. The characteristic uniform, wide spacing of the mature shrubs offers little protection to the soil from erosion by wind or water.
Larrea occurs over a wide range of climatic conditions on most
southwestern desert soils. Many sites where it grows are incapable of producing more valuable species; other sites could grow perennial grasses or other plants. Of special interest in recent years has been the invasion of desert grasslands by a host of shrubby plants, includ ing Larrea. When such an invasion occurs, perennial grasses, which produce the bulk of range forage, are crowded out and the grazing value
of the range is considerably reduced.
The invasion of Larrea into the desert grasslands has been
noted by several observers. Humphrey and Mehrhoff (1958) reported an increase in density, and the extension of Larrea range from 950 acres
in 1904 to 13,000 acres in 1954 on the Santa Rita Experimental Range
1 2 in southern Arizona. Gardner (1951) reported Larrea invasion on
heavily grazed grasslands of the Rio Grande Valley in New Mexico.
Hydrological investigations near Tombstone, Arizona revealed that a
large area supporting Larrea and other Chihuahuan Desert shrubs was
almost certainly grassland prior to white occupation (USDA, 1959).
Interest in the ability of Larrea to thrive on a multitude of
desert soil types under an extreme range of climatic conditions has led
to many investigations of the species, as has concern over the invasion
of this shrub into the desert grassland. Although more than 300 refer
ences to Larrea occur in the literature, information on the species is
far from complete. Certainly, further information regarding distribu
tion might prove useful in explaining relationships between Larrea and
more desirable plants and in managing desert grassland ranges to control
larrea invasion. The immediate purpose of this study is to investigate
edaphic factors that might affect the distribution of larrea in the
desert grassland.
Location and Description of the Study Area
The study area was located in Cochise County on Turkey Creek
Ridge, one of several ridges that rise abruptly from Sulphur Springs
Valley in southeastern Arizona (Fig. 1). The study was conducted on
the north-facing slope of the ridge in Section 33, Township 17 South,
Range 26 East of the Gila and Salt River Base and Meridian, Cochise
County. The ridge rises 378 feet from the 4305-feet-above-sea-level
elevation of the valley floor (U. S. Geological Survey, 1958). Fig. 1. General view of the study area: North-facing slope of Turkey Creek Ridge, Sulphur Springs Valley, Cochise County, Arizona V) 4
The study area is typified in part by desert grassland and in part by Ch:ihuahuan Desert vegetation. The steeper slopes, dominated by Larrea and Flourensia. are representative of much of the Chihuahuan
Desert vegetation which finds its best expression to the south in Mexico.
Larrea tridentata. which dominates the lower slopes of Turkey
Creek Ridge, is bordered on the east by a shrub community of Flourensia c e m u a . The steeper slopes above the Larrea support communities of short-grasses and mid-grasses with a scattering of shrub species. A community of Hilaria rnntica lies below the larrea on the valley floor.
The plant cover on the south-facing slope of the ridge consists largely of perennial grasses with some Yucca elata but with none of the exten sive shrub communities that characterize much of the opposite slope.
As the initial objective of the study was to investigate edaphic factors that might affect the distribution of Larrea, the study was restricted to the north-facing slope which supported in part an extensive Larrea community.
Precipitation
A summer-winter precipitation pattern prevails in the study area as in much of Arizona. Little, if any, precipitation falls as snow.
Winter rains begin in January and continue through March; April, May and
June constitute the usual spring drought period. Summer rainfall occurs from July through October, followed by the dry months of November and
December. Precipitation records for the last five years were obtained from a rain gage located three-quarters of a mile southeast of the study area (Table l). The record indicates an extremely variable yearly 5
Table 1. Record of Precipitation (1956-1960, inclusive). West Well Ranch, Cochise County, Arizona
Precipitation by Tears (inches)
Month Year Mean
1956 1957 1958 1959 1960
January - 2.00 .25 1.53 0.75
February - .20 1.31 .65 •70 0.57
March - 1.90 2.49 - .16 0.91
April - - .35 - - 0.07
May - - - - - 0.00
June •30 - - .89 - 0.24
July 2.85 3.61 2.17 1.51 2.32 2.49
August 1.17 4.35 3.40 8.06 1.06 3.61
September 1.25 - 5.88 - 1.25 1.67
October •09 2.24 - 2.04 2.27 1.33
November -- - - - 0.00
December -— - 1.60 .40 0.40
Total 5.66 14.30 15.85 14.75 9.69 12.04 6
precipitation, with the rainfall for any one year rarely approaching
the mean. However, the five-year average (1956-1960) of 12.04 inches
closely approaches the 16-year average (1895-1910) of 11.59 inches,
recorded for the U. S. Weather Bureau at Allaire1s Ranch located 16 miles to the northwest. This 16-year average included the wettest
year recorded for the area, 22.14 inches in 1905 (Keinzer and Kelton,
1913).
History of the Area
Sulphur Springs Valley is the result of a prehistoric geologic
deformation (Meinzer and Kelton, 1913). Upheavals raised the mountain
ranges that border the valley on the east and west and depressed the
intervening valley. Increased rainfall, resulting from the elevation,
dissected the mountains with deep canyons and deposited vast amounts of
eroded soil and minerals onto the valley flood plain. Soils were seg
regated into zones ranging from dense clays to fine sands. Soluble
salts were leached from some areas and concentrated in others; the
water-table lay at the soil surface in some places and at depths of
hundreds of feet in others (Meinzer and Kelton, 1913). Smaller moun
tains, standing between the high ranges, were all but covered with
water-borne sediments and exist today as buttes or ridges rising ab
ruptly from the valley floor.
Much of southern Arizona, including Sulphur Springs Valley, was
obtained from Mexico in the Gadsden Purchase of 1853 • Raids by Apache
Indians, 1859-1872, disrupted attempts to settle the valley by both
Mexicans and American cattlemen, who arrived with large heads of cattle 7 in 186?♦ In 1872, most of the valley was set aside as a reservation for the Apache Indians. The resumption of Apache raids against the settlers in 1876 necessitated moving of the Indians to the San Carlos
Reservation, which opened the entire valley for settlement. Today, the area is still largely rangeland, although cultivation, started in 1905, has become important locally.
The West Well Ranch, upon which the study area was located, is owned and operated by Mr. Paul W. Riggs, who obtained the ranch from the Chiricahua Cattle Company in 1935• The study site was located in a pasture of about 640 acres which has been grazed by 125-130 head of calves during the months of December through February for the past several years. LITERATURE REVIEW
Larrea is distributed over arid and semi-arid regions from west Texas to California and from Utah and Nevada to north-central
Mexico and much of Baja California. It grows at elevations ranging from sea level in Baja California to 8000 feet near Mexico City (Evan,
1938). Shreve (1940) observed that Larrea distribution is not limited by climatic factors such as extreme aridity and high temperatures and
Evan (1938) reported the wide range of precipitation patterns under which the shrub grows. Although Larrea thrives under extremely arid conditions, it has been said to resemble a mesophyte rather than a zerophyte as long as water is available (Ashby, 1932). Experiments on test plots by Spalding (1904) revealed that Larrea transpired 8.9 times as fast in well-watered soils as in dryer soils.
According to Rzedowski and Leal (1958) climate, particularly degree of aridity, is the most important factor in determining the southern limit of Larrea. although this influence seems to be indirect, affecting competition as suggested by Shreve (1940). Although edaphic factors are usually less important their influence is often highly cri tical in delimiting Larrea associations. The factors affecting the eastern and western distribution limits of Larrea are not as readily apparent as those governing the northern and southern limits. Evan
(1938) reports that no known limiting factors were apparent when small fringe areas were studied and that factors affecting the distribution
8 9 at the eastern limits were not the same as those affecting distribution in California.
Although soil as affecting the distribution of vegetation has been studied for many years, comparatively little is known concerning desert soils as related to the distribution of specific species. As an example, there is little information on the edaphic factors that restrict Larrea distribution on desert soils. Shreve (1940) reports that highly alkaline or saline soils are unfavorable to Larrea growth.
Fosberg (1940) indicates that good drainage may be required for the shrub*s establishment, observing that playas and flats that serve as catchment basins restrict Larrea growth. Dalton (1961) notes that deep, fine-textured soils with moderately high water-holding capacities usual ly support a great variety of species but often may contain no Larrea.
Factors other than edaphic that may affect Larrea distribution include slope exposure, grazing use, and reduction in fires. Keppel, et al, (i960), studying the effect of slope exposure on plant distribu tion near Tombstone, Arizona, found significant differences in only two of 12 species. Bouteloua curtinendula was more abundant on north-facing
slopes and Larrea on southwest-facing slopes. Another Tombstone study
(USDA, i960) dealing with the vegetative cover of 14 soils on 10 parent geologic materials reported that although the cover of grasses and
shrubs, as well as that of individual species, is correlated in varying degree with physical and chemical soil characteristics, only one major
species, Mortonia scabrella, was sharply limited by soils. An interest
ing correlation between plant cover and distance from Tombstone was re
ported, however. As the distance increased total grass cover increased 10 and total shrub cover decreased, suggesting that grass cover may have decreased and shrub cover increased since settlement of Tombstone about
1880. Overgrazing by cattle near the town would have favored establish ment of the low-palatability shrubs over grass, particularly on highly
calcareous soils where the shrub-grass balance was most delicate. The probability of an increase of shrubby species in grasslands due to a reduction in fire frequency since white occupation has been expressed by Humphrey (1958) and others. VEGETATION SAMPLING
An attempt was made to list all species of plants in the study area. Nomenclature used follows Kearney and Peebles (i960). Annuals, as well as perennials, were recorded by families (Appendix). Because of the extreme and unpredictable fluctuations in the amount, or even presence, of annual grasses and forbs as determined by the extremely variable yearly and seasonal rainfall, annuals provided little informa tion useful in characterizing the plant communities. The Hilaria mutica community, which occupied the lowest elevation in the study area, received runoff water from the slope above, and supported the greatest number of annual species.
A 3000-foot permanent line transect (Table 2) served as a ref erence for sampling the frequency and cover of perennials. Because of the permanent nature of the transect, future vegetational changes in species composition, frequency, and cover due to drought, grazing prac tices, shrub invasion, fire or other causes may be observed with reference to the present observations along this transect. The transect was marked at 100-foot intervals by either a wood stake or a rock monu ment. It was begun 100 feet south of the largest mesquite tree at the upper end and extended obliquely down-slope bearing North 23° West.
The change in altitude from one end of the transect to the other was
187 feet.
11 12
Table 2. Length of Transect Occupied and Percent Slope of Ten Plant Communities Along a 3000-Foot Transect on the North-Facing Slope of Turkey Creek Ridge
Length of Community Transect Slope
feet * Hilaria mutica ' 400 1
Hilaria-Flourensia 100 1
Flourensia 400 3
Flourensia-Larrea 200 3-4
Larrea 800 4—8
Larrea-Parthenium 275 8
Larrea-Bouteloua 325 5-9
Bouteloua 260 8
Hilaria belangeri 80 7
Bouteloua-Andropogon 160 22-27 13
Percent Frequency Analysis
A preliminary survey of frequency of perennials was conducted along the established transect. A quadrat frame, one meter square, was dropped at three-step intervals for the entire transect length and all perennials, the basal area of which fell inside the frame, were recorded.
The percent frequency thus obtained, and as used in this study, was de fined as the number of quadrats in which a given species occurred, in relation to the total number of quadrats sampled, expressed on a per centage basis, i. e., F = Humber of frames in which a species occurred x 100 4 total number of frames sampled (McGinnies, 1934). The validity of such a percent frequency analysis depends on a number of variables:
1) size of quadrat, 2) size of individuals, 3) number of individuals per unit area, and 4) pattern of distribution of individuals. Percent fre quency was used in this study to determine the perennial species distribution pattern.
On the basis of the frequency data thus obtained the vegetation was subdivided into 10 communities (or associations): Hilaria, HjJnria-
Flourensia, Flourensia. Flourensia-Larrea. Larrea. Larrea-Parthenium.
Larrea-Bouteloua, Bouteloua, Hilaria belangeri, and Bouteloua-Andropogon
(Fig. 2). Three of these were considered to be ecotones: Hilaria-
Flourensia, Flourensia-Larrea, and Larrea-Bouteloua.
Although the Hilaria belangeri association lay between the
Bouteloua eriopoda and Bouteloua—Andropogon communities, and contained species common to each, it was considered as a distinct association because of the high frequency of Hilaria belangeri, a species that z 130-
> 100-
1000 2000 3000 TRANSECT DISTANCE FEET
Fig. 2. Positions of ten plant communities along a permanent transect, Turkey Creek Ridge, 1961
1. Hilaria mutica 6. Larrea-Parthenium 2. Hilaria Flourensia 7- Larrea-Bouteloua 3. Flourensia 8. Bouteloua km Flourensia-Larrea 9. Hilaria belangeri 5. Larrea 10. Bouteloua-Andropogon
£ 15 occurred much less commonly in the other two. This Hilaria belangeri cammunity covered so small an area that no data, other than frequency, were collected.
Although a percent frequency analysis shows changes in the dis tribution of vegetation, these changes, as indicated by this method, are seldom abrupt enough to be measured with a high degree of accuracy.
Any consequent subdivisions of the vegetation, therefore, are somewhat arbitrary.
After the limits of the six communities (one of which was sub divided into: two fractions) and of three ecotone areas were visually defined, the percent frequency of perennials was recorded from 50, meter-square quadrat frames dropped randomly at three-step intervals in each area.
Cover Analyses
Two methods of measurement and several concepts of area were utilized in the cover analysis of five of the major plant communities:
Hilaria. Flourensia. Larrea, Bouteloua and Bouteloua-Andropogon. Area data included the cover of grasses and the canopy cover of shrubs, as well as the area occupied by annuals, litter, rock and bare soil.
Cover, as used in this study, was considered as the vertical projection of living aerial vegetation; canopy cover was the area of a circle just circumscribing a shrub and did not consider the spaces between or the overlapping of branches. As any point sampled may be covered by several layers of different plants, for example, a shrub canopy, a grass, and an annual forb, the total cover recorded was greater than 100 percent 16 in some instances. Shrub canopy cover was measured by the variable plot method (Cooper, 1957)• The data from 10 variable plots were recorded in both the Larrea and Flourensia communities. As the total shrub cover of the Bouteloua community was small, no record was made of shrub canopy cover in this predominantly grass community. Cover fractions, other than shrubs, were measured by the point-quadrat method (Goodall, 1952), the data from 10 point quadrats being recorded in each of the five communities RESULTS OF PLANT ANALYSES
The vegetation analyses provided information on the distribu tion or frequency of the perennial plant species (Table 3)> on the amount of cover contributed by each (Table 4) and on the component plant associations. It is contemplated that this information, useful initial ly in the current study, may also serve as a basis for determining future vegetational changes in the area. As total cover included annuals, lit ter, rock and bare soil, as well as perennial plant cover, the data also provided information concerning the soil surface. Because of species
overlap, the sum of all vegetation, plus rocks, in all cases totals more than would have been the case had there been no multistoried vegetation or had none of the plants formed an overstory over rocks.
Hilaria mutica Association
Hilaria mutica. with a percent frequency of 60, was the most
abundant perennial in this grass community (Fig. 3) followed by Pani-
cum obtusum with only six percent (Table 3)»
The community was unique among the five on which cover data
were obtained in that a single species, Hilaria mutica , contributed
all of the perennial cover recorded (44 percent) (Table 4). It will be
noted that annual cover was greater than in any other community. While
litter cover was greater than in the Larrea and Flourensia shrub com
munities, but less than in the Bouteloua and Bouteloua-Andropogon grass
communities. Coversely, there was less bare soil than in the shrub
17 18
Table 3* Percent Frequency of Perennial Plant Species in Ten Communities, Turkey Creek Ridge, Cochise County, Arizona
Community Species Frequency
&
Hilaria mutica Hilaria mutica 60 Panicum obtusum 6 Talinum angustissimum 4 Opuntia spp. 2 Bare ground or annuals 14
Hilaria-Flourensia Flourensia cemua 46 Hilaria mutica 26 Sporobolus wrightii 22 Kuhlenbergia porteri 14 Panicum obtusum 10 Enneapogon desvauxii 8 Tridens pulchellus 6 Bare ground or annuals 12 Flourensia Tridens pulchellus 64 Flourensia c e m u a 48 Larrea tridentata 16 Sporobolus wrightii 8 Muhlenbergia porteri 2 Opuntia spp. 2 Krameria lanceolata 2 Bare ground or annuals 0
Flourensia-Larrea Tridens pulchellus 96 Larrea tridentata 26 Dalea formosa 16 Croton texensis 16 Zinnia pumila 14 Z. grandiflora 6 Ephedra trifurca 4 Flourensia cemua 2 Bare ground or annuals 2 19
Table 3. (ContTd)
Community Species______Frequency
1
Larrea Larrea tridentata 78 Zinnia pumila 14 Partheniun incanun 10 Tridens pulcheUus 8 Dalea formosa 4 Bare ground or annuals 0
Larrea-Parthenium Larrea tridentata 56 Partheniun incanun 52 Zinnia pumila 34 Henodora scabra 28 Tridens pulchellus 24 Dalea formosa 12 Flourensia cemua 12 Muhlenber/d-a porteri 8 Croton texensis 2 Opuntia spp. 2 Bouteloua eriopoda 2 Bare ground or annuals 6
Larrea-Bouteloua Bouteloua eriopoda 60 Tridens pulchellus 52 Menodora scabra 52 Tridens muticus 22 Larrea tridentata 22 Croton texensis 20 Zinnia pumila 16 Bouteloua curtipendula 14 Dalea formosa 8 Hilaria mutica 6 Enneapogon desvauxii 6 Krameria lanceolata 2 Bare ground or annuals 0
Bouteloua Bouteloua eriopoda 100 Menodora scabra 24 Bouteloua curtipendula 18 20
Table 3. (Conttd)
Ccanmunity Species frequency
t
Bouteloua Croton texensis 14 (Cont’d) Zinnia pumj.la 8 Hilaria mitica 6 Acacia constricta 6 Aloysia wrightii 6 Tridens •pulchellus 2 Flourensia cernua 2 Prosopis .juliflora 2 Dalea fonnosa 2 Ephedra trifurca 2 Condalia lycioides 2 Parthenitna incanum. 2 Huhlenborgia porter! 2 Bare ground or annuals 0
Hilaria belangeri Hilaria belangeri 74 Bouteloua curtipendula 48 B. eriopoda 44 Andropogon barbinodis 28 Bouteloua gracilis 18 Croton texensis 4 Aristida haraulosa 4 Leptoloma cognatun 2 Bare ground or annuals 0
Bouteloua-Andropogon Bouteloua curtipendula 96 Andropogon barbinodis 78 Bouteloua gracilis 64 Eragrostis intermedia 32 Aristida hamulosa 20 Croton texensis 16 Lycurus phleoides 10 Bouteloua eriopoda 10 Bare ground or annuals 0 21
Table 4* Total Cover of Five Plant Communities, Turkey Creek Ridge, Cochise County, Arizona
Community Cover Fraction ______Cover Perennials Other
& 2 Hilaria mutica Hilaria mutica 44
Total perennials 44 Litter 24 Annuals 24 Rock 15 Soil 2? 534
Flourensia Flourensia cernua 19 Larrea tridentata 5 Tridens pulchAllnR 8
Total perennials 32 Litter 7 Annuals 10 Rock 20 Soil 61
130
Larrea Larrea tridentata 14 Flourensia cernua 2 Dalea formosa 3
Total perennials 19 Litter 5 Annuals 2 Rock 57 Soil 35
118
1. Fraction of the area occupied by vegetation, litter, peren nials, rock or bare soil.
2. Perennials are underlined. 22
Table 4* (Conttd)
Community Cover Fraction Cover Perennials Other
1
Bouteloua Bouteloua erionocia $2 Hilaria mutica 3
Total perennials 55 Litter 36 Annuals 4 Rock 27 Soil 17
139
Bouteloua-Andropogon Bouteloua curtipendula 42 B. eriopoda 1 B. gracilis 1 Aristida hamulosa 4 Andropogon barbinodis 13
Total perennials 61 Litter 37 Annuals 6 Rock ' 34 Soil 5
143 Fig. 3» Hilaria mutica community, Turkey Creek Ridge, study plot 24 communities, but more than in the other grass communities. There was also less rock or gravel than in any of the other four. The relative sparsity of rock presumably was due to two factors: the greater dis tance from the bajada that served as a primary source of rocks, and the relatively gentle slope that was less subject to erosion.
Hilaria-Flourensia Ecotone
Flourensia cernua, with a percent frequency of 46, was the most abundant perennial in this ecotone; Hilaria was second, with 26 percent.
Two other perennials helped characterize the ecotone— Muhlenbergia por ter!, which occurred only under Flourensia shrubs, and Sporobolus wrightii. a tall grass that requires relatively favorable moisture con ditions. The occurrence of Sporobolus in the narrow ecotone suggests that infiltration of runoff water may have been increased where its flow was restricted by the dense grass of the Hilaria community.
Flourensia Association
Flourensia, with a 48 percent frequency, was the dominant peren nial in this association (Fig. 4). Larrea. with 16 percent, occurred
at the lower limit of its local distribution in this shrub community.
A weak perennial grass, Tridens pulchellus, occurred more frequently
than any other species but comprised only a small fraction of the plant
cover.
Canopy cover of all shrubs was 24 percent; that of Tridens
eight percent (Table 4). Annual species cover (10 percent) was great
er than in any except the Hilaria community. Because of the absence to Fig. 4« Flourensia community, Turkey Creek Ridge study plot V t 26
of perennial forbs and grasses, litter cover was only seven percent, while rock and bare soil contributed a large fraction (81 percent).
Flourensia-Larrea Ecotone
The Flourensia-Larrea ecotone occurred on the lower bajada
(Fig.2). Larrea. with 26 percent frequency, was the dominant shrub;
Flourensia occurred almost as often. Tridens, with a 96 percent fre
quency, was more abundant here than in any other community. Two forbs,
Dalea formosa and Zinnia rrumila. appeared for the first time. The
growth of these species was restricted to depressions and small water
courses where soil moisture appeared to be relatively favorable. The
distribution of forbs throughout the area indicated that soil moisture
conditions became less favorable with increasing steepness and elevation
of the bajada.
Larrea- Larrea-Parthenium Association
The Larrea community, (Fig. 5), exclusive of the ecotones, was
subdivided into Larrea and Larrea-Parthenium associations and these
were separately analyzed as to percent frequency. Total cover was
sampled randomly over the community as a whole rather than separately.
Canopy cover, measured over the entire Larrea community, was
20 percent. Litter and annuals contributed very little to the ground
cover which was composed almost entirely of an erosion pavement of
small rock with a moderate amount of bare soil.
Larrea Association
Larrea, with a 78 percent frequency, greatly outnumbered all Fig. 5* Larrea community, Turkey Greek Ridge study plot 3 28
other perennial species in this association. Although Dalea, Tridens,
Parthenium incamm and Zinnia were recorded, none of these was sufficient
ly abundant to appreciably affect the floristics of the community.
Larrea-Parthenium Association.
Although Larrea was less abundant here than in the preceding
community, it was still dominant. Parthenium. while almost as abundant, was comprised of relatively small plants and was subdominant to the
Larrea. A more favorable soil-moisture relationship was indicated by the
appearance of two perennial grasses near the upper limits of the associ
ation: Huhlenbergia porteri and Bouteloua eriopoda.
Larrea-Bouteloua Ecotone
The percent frequency of Larrea decreased from $6 in the Larrea-
Parthenium association to 22 in this grass-shrub ecotone, while
Bouteloua eriopoda. the dominant grass, had a percent frequency of 60.
Tridens pulchellus was about twice as abundant; the number of Zinnia
plants decreased by about 50 percent and Parthenium totally disappeared.
Other perennial grasses, Bouteloua curtipendula (14 percent) and Tridens
muticus (22 percent), were recorded for the first time.
Larrea seedlings, abundant in the ecotone, suggest a possibili
ty of future increases in the shrub*s density and further encroachment
into the bordering grass community.
Bouteloua Association
The dominant perennial in the Bouteloua community was Bouteloua
eriopoda. a stoloniferous, sod-forming grass with a percent frequency 29 of 100 (Fig. 6). Several Chihuahuan Desert shrub species: Acacia constricta. Aloysia wrightii and Condalia lycioides, not present in any of the other communities further characterized this community. Flour- ensia ceraua was again present but occurred sparsely.
As shrubs were largely restricted to one small site in the
community, no canopy cover measurements were made. Fifty-five percent
of the area, on the other hand, was covered by perennial grasses. An nuals comprised only four percent of the ground cover but litter, pri marily from the previous year1s growth of grasses, became more important.
Rocks were larger on the upper bajada but contributed less ground cover
than in the Larrea community where rocks were smaller as well as in the
Bouteloua-Andropogon community, where rocks averaged larger.
Hilaria belangeri Association
Mo information other than percent frequency was recorded in
this very small association. Hilaria belangeri was the dominant and most characteristic grass species. Bouteloua curtipendula and B. erio-
poda, the dominant species in the two adjacent communities were also
important components although occurring less frequently than the Hilaria.
Three other perennial grasses, Bouteloua gracilis. Andropogon barbinodis
and Leptolcma cognatum were recorded for the first time.
Bouteloua-Andropogon Association
The Bouteloua-Andropogon association was a community of several
mid-grasses that were confined to an area of water concentration (Fig. 7).
Bouteloua curtipendula. with a frequency of 96 percent, was the dominant
species, followed by Andropogon barbinodis. Two grasses that appeared Fig. 6. Bouteloua community Turkey Creek Ridge study plot V) o Fig. 7* Bouteloua-Andropo/ron community, Turkey Creek Ridge study plot V) 32 for the first time were abundant: Eragrostis intermedia and Aristida hamulosa.
Sixty-one percent of the cover was comprised of perennial grasses, the highest percentage in any comnunity. Again, few annuals were present but there was a moderate amount (37 percent) of litter.
The large rocks present contributed 34 percent to the ground cover, while bare soil was recorded at only five percent. SOIL SAMPLING
After the boundaries of the five plant communities to be studied were defined by vegetational analysis, work was begun to determine edaphic differences between the communities. Because the soil characteristics of a specific plant community, rather than of the transitional areas (ecotones) between communities, were to be determined, three sampling sites were located randomly in each com munity but at least 50 feet from any ecotone.
Sampling sites were excavated, where possible, by pick and shovel to a depth of three feet. Soil samples were obtained from the surface inch, and at depths of 11-22 inches, 23-24 inches and 35-36 inches. Because of extremely hard caliche and rock in the Larrea com munity, it was hot possible to sample the soils of this community to a depth of more than four inches at two sampling sites and to 10 inches at the third site. At one sampling site in both the Flourensia and
Bouteloua-Andropogon communities rock was encountered at the 24-inch depth.
Physical Soil Analyses
Soil samples were first screened through a two mm. sieve. The rocks and pebbles thus removed were not reported as a percentage of the total sample. Analysis of the less-than-two mm. fraction was made by the hydrometer method (Bouyoucos, 1951) for the sand, silt, and clay 33 34 fractions. Twenty-five ml. of a solution containing 100 grams (Calgon**") in 500 ml. distilled water was used as a soil-dispersing agent.
To further characterize physical soil properties, the field capacity of each sample was determined at 3/3 atmosphere pressure and the permanent wilting percentage at 15 atmospheres. The pressure-plate and pressure-membrane methods were used (methods 30-31) (USDA, 1954)•
The difference between the moisture percentage at 1/3 and 15 atmospheres 2 pressure was considered to be the moisture available for plant growth.
Chemical Soil Analyses
Soil reaction was measured with a Beckman pH meter using a 1:5
soil-water mixture. The pH value was determined for duplicate soil
samples. In the few instances where readings differed, an average was
taken to represent the sample.
A one-normal solution of hydrochloric acid was used to test all
soil samples for free calcium carbonate (method 23a) (USDA, 1954)*
Calcium carbonate determinations were made on samples taken from one
sampling location in each of the study communities. The percentage of
calcium carbonate in the soils was determined by acid-neutralization
(method 23c) (USDA, 1954)* This method may result in a slightly high
reading because other alkaline-earth carbonates than those of calcium
may be present and react with the acid in the analyses.
1. A commercial water conditioner containing sodium hexameta- phosphate.
2. Available moisture is moisture the soil is capable of holding, not actual moisture present. RESULTS OF SOIL ANALYSES
Physical Soil Analyses
Mechanical Analyses. On the basis of mechanical analysis the soils of the five plant communities were subdivided into two distinct groups: (1 ) those soils that had more than 50 percent of the soil separates in the sand fraction and less than 25 percent in the clay fraction, and (2 ) those with 50 percent or less in the sand fraction and more than 25 percent in the clay fraction.
Although the percent composition of the soil separates varied with depth in both groups, the percent composition of the first group was the less variable with depth and all soil samples fell within the sandy-loam or sandy-clay-loam classification. Because of the more vari able percentage composition of the soils in the second group these were classified as sandy clay loam or clay loams at the surface to sandy clays and clays at greater depths.
The soils of the Larrea, Flourensia, and Bouteloua communities fell within the first group and were classified as sandy loams or sandy clay loams. The average percentage of the three soil fractions— sand,
silt and clay— obtained from three samples at each sampling depth is
shown in Table 5» The range (extremes), expressed in percent of each
soil fraction throughout the sampling depth, is shown in Table 6.
The soils of the Larrea community were sampled only to a depth
of 10 inches. As indicated in Table 5, percent sand decreased with
35 36
Table $. Mechanical Soil Analysis in Five Cochise County, Arizona Plant Communities. (Average of Three Samples at Each Depth)
Depth Size Classes, Expressed as % of Total Community of Sand Silt Clay Sample 2.00-0.05 0.05-0.002 0.002-0.000 mm. mm. mm.
inches larrea 0-1 68.2 20.3 11.5 3-4 58.2 26.3 15.5 9-10* 50.4 28.8 20.8
Flourensia 0-1 63.1 21.1 15.8 11-12 61.1 19.9 19.0 23-24 56.9 20.5 22.6 35-36* 62.0 15.4 22’. 6
Bouteloua 0—1 65.9 17.0 17.1 11-12 62.1 17.9 20.0 23-24 67.5 16.9 15.6 35-36 70.2 14.1 15.7
Hilaria mutica 0-1 56.8 18.8 24.4 11-12 45.6 14-5 39.9 23-24 35.5 19.9 44.6 35-36 42.0 19.6 38.4
Bouteloua- Andropogon 0-1 31.7 33.6 34.7 11-12 29.2 17.5 53.3 23-24 27.1 15.6 57.3 35t 36* 33.6 19.1 47.3
* Only one soil sample taken at this depth 37
Table 6. The Range in Percent of Each Soil Fraction Throughout The Sampling Depth of Five Plant Communities, Cochise County, Arizona
Size Classes as % of Total
Community Sand Silt Clay 2.00-0.05 0 .05-0.002 0 .002-0.0 mm. mm. mm#
Larrea 50-74 17-29 9-21
Flourensia 51-73 15-26 10-26
Bouteloua 59-77 12-21 11-23
Hilaria mutica 18-50 10-42 27-59
Bouteloua-Androno^on 29-62 5-27 22-57 38 depth of sample ■while silt and clay both increased. Because of the importance of the clay fraction in determining water-absorption and water-holding characteristics, the sand and silt fractions were com bined and expressed as a unit separately from the clay. When thus combined the sand-silt fraction of all soils in the Larrea community
(Larrea plus Larrea-Parthenium) represented 84 percent. The remaining
16 percent was clay.
The soils of the Hilaria and Bouteloua-Andropogon communities fell into the second group. These soils, as contrasted with those of group one, were characterized by a much higher clay content and more variability of soil texture between samples from different depths
(Tables 5 and 6).
In the Flourensia community only the clay fraction exhibited a consistent tendency to increase with depth, i. e., from 16 to 23 percent.
All other fractions were inconsistently variable with depth. The sand and silt fractions combined comprised 80 percent of the soil; clay made up the remaining 20 percent.
On a basis of mechanical analysis the soils of the Bouteloua community were rather similar to those of the Flourensia and Larrea com munities. Sand and silt comprised 83 percent and clay 17 percent.
The surface soil of the Hilaria mutica ; community was classified as a clay loam; below the surface as clays or sandy clays. The sand fraction measured 57 percent at the surface, 46 percent at the 11— to
12-inch depth and 36 percent at 23 to 24 inches. The sand fraction in
creased further at the 35- to 36-inch depth to 42 percent. The clay 39 content was greatest at the 11- to 12-inch depth (40 percent) and at the
23- to 24-inch depth (45 percent). It is a matter of rather common ob servation that Hilaria mutica occurs typically in much of its more arid range on fine-textured soils and in swales that receive runoff water from adjacent slopes. Mechanical analysis data from the study indicated that the soil of the community averaged less sand and silt (63 percent) and more clay (37 percent) than any community except the Bouteloua-
Andropogon.
In the Bouteloua-Andropogon community, the surface soil was classified as a sandy clay loam; deeper soils as clay loams to sandy clays and clays. As in the Hilaria mutica community, sand percentages tended to be highest in the surface inch of soil (32 percent) and at the
35- to 36-inch depth (34 percent). Clay fonaed the greatest fractions at depths of 11 to 12 inches (53 percent) and 23 to 24 inches (57 per cent). The average soil sand and silt fraction (52 percent) was less than in any other soil and the clay fraction was higher (48 percent).
Moisture Holding Capacity. There was an apparent correlation between moisture holding capacity and the presence of Larrea. Soils of the Larrea and Flourensia communities were characterized by a lesser ability to hold moisture than those that grew a relatively large amount of grass. A more limited range of moisture availability, as indicated by moisture differences recorded at pressures of one-third and 15 atmos pheres, was recorded for these shrub-type communities than for the grass communities (Tables 7 and 8 ). Table 7 indicates further that 40
Table ?• Available Moisture in Two Shrub Communities (Average of Three Samples at Each Sampling Depth)
Community Depth Moisture at Moisture at of 1/3 15 Available Sample Atmosphere Atmospheres Moisture
inches £ £ £
Larrea 0-1 14.21 7.34 7
3-4 18.94 10.41 9
9-10-::- 21.84 11.97 10
Flourensia 0-1 14.79 7-93 7
11-12 15.75 10.84 5
23-24 19-98 11.43 9
. 35-36 18.61 8.86 10
Only one sample taken at this depth 41
Table 8. Available Moisture in Three Grass Ccraunities (Average of Three Samples at Each Sampling Depth)
Depth Moisture at Moisture at Community of V 3 15 Available Sample Atmosphere Atmospheres Moisture
inches I & 2
Bouteloua 0—1 20.18 12.14 8 11-12 23>87 14.35 10 23-24 23.80 11.90 12 35-36 23-91 11.14 13
Hilaria mutica 0-1 27.39 16.83 10 11-12 30.52 20.73 9 23-24 34.56 21.75 13 35-36 31.52 21.25 11
Bouteloua- 0 15.19 7.43 8 Andropogon 11-12 23.97 13.71 10 23-24 29.27 16.29 13 35-36-"- 23.13 12.40 11
* Only one sample taken at this depth 42 potential moisture availability increased •with depth in both the Larrea and the Flourensia communities from seven percent at the soil surface to 10 percent at the nine- to 10-inch depth in the larrea community and from seven percent at the surface to 10 percent at the 35- to 36-inch depth in the Flourensia community. The Flourensia soil had an extreme
ly low (five percent) available moisture content at the U - to 12-inch
depth.
The average available moisture in each of the three grass
communities was 11 percent. However, in the Bouteloua community very
little root penetration was observed below the surface foot of soil,
presumably because of a hard packed soil high in calcium carbonate.
Because of the difficulty of root penetration it is doubtful whether
any moisture that might be contained in this deeper soil could be util
ized for plant growth. If this is true, then the eight percent moisture
available in the surface foot would be more indicative of plant moisture
relations than the average of 11 percent, and the available moisture of
the soil would compare more closely with that of the shrub associations
than with that of the other grass associations.
Chemical Soil Analyses
Soil Reaction. There was a high degree of similarity between
the soil pH values of four of the plant communities (Fig. 8). These
were the Flourensia and Bouteloua communities, which had an average
soil pH value of 8.5 for all samples at all depths, the Hilaria com
munity with an average pH value of 8.2 and the Larrea community with
an average soil pH value of 8 .4 . The soil pH value of the pH VALUES Fig. 8. Changes in soil pH values with sampling depth sampling values with soilpH Changesin 8.Fig. 10 OL DEPTH SOIL
12 INCHES Larrea Flourensia Hilaria mutica Comm'anities Bouteloua-Andropogon Bouteloua 43 44 community was included in this group even though, because of underlying caliche, the greatest depth sampled was 10 inches while the Flourensia,
Bouteloua and Hilaria communities were sampled to depths of 35 to 36 inches.
Only one soil, that of the Bouteloua-Andropogon community, was acid in reaction. In this case the pH value ranged from 5*4 on the soil surface to 7*3 at the 35- to 36-inch depth.
Calcium Carbonate. Although- free calcium carbonate was found in all samples of all soils except those from the Bouteloua-Andropogon community, the percentage and the form in which it occurred varied con siderably between communities (Table 9). No caliche was encountered at any depths sampled in the Hilaria muti-c a community, although percent calcium carbonate was only slightly less than in the adjoining Flouren sia community where a soft, unconsolidated caliche was typical.
In the larrea community the calcium carbonate content of 12 percent, less than that of any of the other basic soils tested, would have been increased considerably if soil samples could have been obtained from depths comparable with sampling depths in the other communities.
Calcium carbonate occurred here as a hard, consolidated caliche and rock mixture, that was relatively impermeable to water and that was
highly resistant to root penetration.
The soil of the Bouteloua community averaged 30 percent calcium
carbonate and ranged from 1? percent in the surface inch to 39 percent
at the 35- to 36-inch depth. This calcium carbonate in no way resembled 45
Table 9* Calcium Carbonate as Percent of Total Soil Sample in Four Communities. (Only One Sample Taken at Each Sampling Depth)
Community Depth of CaCO^ Sample
inches i
Larrea 0-1 9.95 3-4 11.97 9-10 15.37
Flourensia 0-1 20.11 12-12 18.85 23-24 22.62 35-36 23.65
Bouteloua 0-1 16.89 11-12 27.82 23-24 37.09 35-36 39.38
Hilaria mutica 0-1 14.60 11-12 8.79 23-24 11.44 35-36 23.94 46 the caliche found in the other areas. It was, instead, a hard-packed homogeneous mixture of calcium carbonate and red sand, which crumbled easily to a grey powder DISCUSSION
The vegetative cover on the north-facing slope of Turkey Creek
Ridge can be subdivided into distinctive communities that correspond closely "with similar communities described by observers of the Chihua- huan Desert in Mexico or of the desert grassland in Mexico and else where in the Southwest. The shrubs of the lower bajada, as well as of the Hilaria community which lies below the slope, comprise typical
Chihuahuan Desert associations. Muller (1947), for example, recorded larrea tridentata as the most characteristic species of the Chihuahuan
Desert in southern Coahuila, Mexico. Associated with Larrea was
Flourensia cernua and 17 less common species. Of these 17 species,
11 occur in southeastern Arizona and six of these were found in the
Turkey Creek Ridge study area. Shreve (1942) opined that the Hilaria flats of the Chihuahuan Desert are a desert association controlled by edaphic factors rather than a bona fide part of the climatic desert grassland. The other grasses, all of which occurred abundantly on the upper slope of the ridge, are true desert grassland species that are
common also to the grasslands of southern Coahuila (Topia and de Alba,
1955).
Yang (1961) reported the recent aggressive expansion of Larrea
from the Chihuahuan Desert of southeastern Arizona into neighboring
communities. The expansion was cited as occurring in grasslands where
Frosopis jnliflora had replaced the perennial grasses but was in turn
47 being replaced by Larrea. as well as directly into desert grassland
areas not pre-invaded by mesquite.
A hypothesis advanced by Tang was that Larrea expansion is
principally the result of an increase in aridity and greater fluctua
tions in the desert climate, during historical time, coupled with the
genetic adaptability of Larrea* This hypothesis is in disagreement
with the views of other investigators, such as Humphrey and Hehrhoff
(1958) who found no evidence that recent changes of climate had been a
factor in the invasion of grassland by shrubs.
The mean rainfall, for a 16-year period, recorded near the study
area over 60 years ago, compares closely with the mean recorded for the
past five years. This would seem to support the view that there has
been no appreciable change of climate, although the variability of the
rainfall and the cyclic nature of precipitation prevent the assumption
of complete stability.
On the basis of data obtained from the study, edaphic factors
would seem to be more directly involved in explaining the distribution
of Larrea than any change in climate, particularly those factors that
affect soil moisture. Larrea was found in the study area only where
slope, erosion pavement, caliche hardpan, shallow soil, or light-
texture soils resulted in well drained sites with relatively low avail
able moisture. Soil pH values may also be a factor in excluding Larrea
from at least one association. Dalton (1961) observed that Larrea seeds
failed to germinate at pH values of 6.2 or below. The pH value of the
surface soil in the Bouteloua-Andropogon community was found to be 5«4; 49 well below Dalton1 s minimum.
The form in which calcium carbonate occurred and its effect upon soil-moisture relationships and root penetration, rather than the percent present in the soil, appeared to be a major factor in determin
ing Larrea distribution. The Hilaria soil contained a greater percent
age of calcium carbonate than that of the Larrea community, but less
than the soil of the Flourensia community which did support some Larrea.
The absence of Larrea in the Hilaria mutica community may be
explained by the poorly drained, fine-textured soil and the competition
from Hilaria. in an edaphic environment well suited for the growth of
this grass,but poorly suited for the establishment of Larrea seedlings.
In the Flourensia association Flourensia shrubs were more
abundant and visibly more vigorous in that portion of the community
nearest the ecotone with Hilaria where soil moisture appeared to be
more favorable. Larrea. on the other hand, was more abundant in the
upper portion of the community where decreasing frequency and cover of
other perennials suggested less favorable moisture conditions.
Larrea was most abundant on the shallow soil of the bajada where
a caliche hardpan, which was relatively impermeable to moisture and root
penetration, and where increased runoff due to greater slope and an
erosion pavement combined to make moisture conditions for plant growth
the least favorable of any of the communities studied.
The establishment of many Larrea seedlings in an area of
abundant Bouteloua eriopoda indicated a recent, continuing invasion. 50
The soil of the Bouteloua community was deeper and had a higher moisture-holding capacity below the one-foot depth than that of the
Larrea community. However, a heavy calcium carbonate layer here tended to restrict root penetration to the surface foot and probably rendered this additional moisture largely unavailable for plant growth, thus permitting Larrea to compete with the grass under conditions of less favorable soil moisture.
In the Bouteloua-Andropogon association the deeper, fine- textured soils and relatively high available moisture appear to have favored the growth of grasses to the exclusion of Larrea and most other
shrubs except Prosopis .juliflora. Two individuals of this species were
conspicuous in the community. As mentioned earlier, the acid soil here may also have excluded Larrea by preventing seed germination. SUMMARY
Edaphic factors that might explain the distribution of Larrea
tridentata in the desert grassland of southeastern Arizona were inves
tigated on a site that supported an extensive stand of larrea. as well
as other Chihuahuan desert shrubs and several desert grassland associa
tions. The composition, distribution and total cover of the vegetation
were analyzed in the two major shrub communities, Larrea and Flourensia
cemua and in the three major grass communities, Hilaria mutica.
Bouteloua eriopoda and Bouteloua-Andropogon. A permanent line transect
that crossed a portion of each of the five communities was established
to record any future vegetations! changes.
Soils of the five major communities were mechanically analyzed
for textural class, water-holding percent and available moisture; and
chemically, for calcium carbonate content and pH value.
Results can be summarized as follows:
1. Soils of the Hilaria mutica and Bouteloua-Andropogon com
munities contained more clay (25 to 50 percent) and higher available
moisture (11 percent) than the soils of the Bouteloua-eriopoda and
the two shrub communities.
2. Larrea was most abundant on sites where shallow, sandy
soils with low available moisture, caliche hardpan, or erosion pave
ment resulted in relatively unfavorable moisture relationships for
the growth of other species.
51
l 52
3. The form of calcium carbonate, as affecting permeability and root penetration, was more a factor in determining Larrea distri bution than was percent of occurrence.
4* Factors that seem most likely to prevent the establishment of Larrea in the Hilaria mutica and Bouteloua-Andropogon communities are: poorly drained heavy soils, competition from better adapted species, and in the Bouteloua-Andropogon community a low pH value (5*4) that nay prevent seed germination.
5* Low available moisture (eight percent) in the surface foot and a calcium carbonate layer that restricted deeper root penetration, tended to cause the soil moisture relationships of the Bouteloua eriopoda community, the only grass community that Larrea was invading, to resemble those in the shrub associations more closely than in the other grass communities. REFERENCES CITED
Ashby, E. 1932. Transpiratory Organs of Larrea tridentata and Their Ecological Significance. Ecology 13:182-188.
Bouyoucos, G. J. 1951. A Recalibration of the Hydrometer Method for Making Mechanical Analysis of Soils. Agron. Jour. 43:434-438.
Cooper, C. F. 1957* The Variable Plot Method for Estimating Shrub Density. Jour. Range Mgmt. 10:111-115»
Dalton, P. D. 1961. Ecology of the Creosotebush (Larrea tridentata) DC. Cov. Ph. D. Dissertation. 13 + 162 pp. Processed. University of Arizona, Tucson, Arizona.
Duisberg, P. C. 1952. Some Relationships Between Xerophytism and the Content of Resins, II D G A Acid and Protein of Larrea divaricata Cav., Plant Physiol. 27:769-777*
Evan, J . 1938. Creosotebush and the Lower Sonoran Zone of the South west. The Amer. Midi. Nat. 2:334-335*
Fosberg, F. R. 1940. The Aestival Flora of the Mesilla Valley Region, New Mexico. Amer. Midi. Nat. 23:573-593.
Gardner, J. L. 1951. Vegetation of the Creosotebush Area of the Rio Grande Valley in New Mexico. Ecol. Mon. 21:379-403*
Good all, D. W. 1952. Some Consideration in the Use of Point Quadrats for the Analysis of Vegetation. Aust. Jour. Sci. Res. (Series B) 5:1-41.
Humphrey, R. R. 1958. The Desert Grassland— a History of Vegetational Change and an Analysis of Causes. Bot. Rev. 24:193-252.
Humphrey, R. R. and L. A. Mehrhoff. 1958. Vegetational Changes on a Southern Arizona Grassland Range. Ecology 39:720-726.
Kearney, T. H. and R. H. Peebles. I960. Arizona Flora. Univ. Calif. Press, Berkeley, Calif.
Keppel, R. V., J. E. Fletcher, J. L. Gardner and K. G. Renard. i960. Annual Progress Report. Southwest Watershed and Hydrology Studies Group. 25 pp. Processed.
53 54
McGinnies, W. G. 1934* The Relation Between Frequency Index and Abundance as Applied to Plant Populations in a Semiarid Region. Ecology 13:263-282.
Meinzer, 0. E. and F. C. Kelton. 1913. Geology and Water Resources of Sulphur Springs Valley, Arizona. U. S. Geological Survey Water Supply Paper 320. 225 pp.
Muller, C. H. 1947* Vegetation and Climate of Coahuila, Mexico. Madrono 9 :33-57 *
Rzedowski, J. and F. M. Leal. 1958. El Limite Sur de Distribucion Geografica de Larrea tridentata. Acta Cientifica Potosina 2: 133-147•
Shreve, F. 1940. The Edge of the Desert. Yearbook Assoc. Pac. Coast Geographers 6:6-11.
Shreve, F. 1942. Grassland and Related Vegetation in Northern Mexico. Madrono 6:190-198.
Spalding, V. M. 1904. Biological Relations of Certain Desert Shrubs. I. The Creosotebush (Covillea tridentata) in its Relation to Water Supply. Bot. Gaz. 38:122-138.
Topia, C. and J. de Alba. 1955. Species Survey of a Mexican Unfenced Range. Jour. Range Mgmt. 8:111-114.
U. S. Dept. Agr., Salinity Laboratory Staff. 1954* Diagnosis and Improvement of Saline and Alkali Soils. Agr. Handbook No. 60. 160 pp.
U. S. Dept. Agr., A. R. S. 1959. The Hydrology of Semi-Arid Watersheds as Influenced by Characteristics of Soil and Vegetation. Southwest Watershed Hydrology Studies Group Annual Progress Report. 37 PP* Processed.
U. S. Dept. Agr., A. R. S. I960. Walnut Gulch Experimental Watershed. Southwest Watershed Hydrology Studies Group. 5 pp. Processed.
U. S. Geological Survey. 1958. Topographic Map, Squaretop Hills Quadrangle.
Yang, T. W. 1950. Distribution of Larrea tridentata in the Tucson Area as Determined by Certain Physical and Chemical Factors of Habitat. M. S. Thesis. 47 pp* Processed. University of Arizona, Tucson, Arizona.
Yang, T. W. 1961. The Recent Expansion of Creosotebush (Larrea divari- cata) in the North American Desert. Western Reserve Acad. Nat. Hist. Mus. Spec. Publ. 1; 11 pp. APPENDIX
Species List of Plants (By Family) Found on the North-Facing Slope of Turkey Creek Ridge
FAMILY SPECIES
AKARAUTHACEAE Amaranthus palmeri Nats. Brayulinia densa (Humb. £c Bompl.) Small Tidestromia lanuginosa (Mutt.) Standi.
AMARYLLIDACEAE Zephyranthes longifolia Hemsl.
ASCLEPIADACEAE Asclepias asperula (Decne.) Woodson
CACTACEAE Mamnillaria sp. Opuntia leptocaulis DC. Opuntia spinosior (Engelm. & Bigel.) Tourney Opuntia sp. (Prickly pear type)
CHENOPODIACEAE Atriplex canescens (Pursh.) Nutt. Eurotia lanata (Pursh.) Koq. Salsola kali var. tenuifolia Tausch.
CQ1MELIMACEAE Comnelina erecta L.
CdlPOSITAE Baccharis pteronioides DC. Bahia absinthifolia Benth. Berlandiera lyrata Benth. Dyssodia acerosa DC. Erigeron schiedeanus Less. Flourensia cemua DC. Franseria confertifolia (DC.) Rydb. Gutierrezia sarothrae (Pursh.) Britt. & Rusby. Haplopappus spinulosus (Pursh.) DC. Haplopappus tenuisectus (Greene) Blake Parthenium incanun H.B.K. Perezia nana Gray Sanvitalia aberti Gray Senecio longilobus Benth. Stephanomeria pauciflora (Torr.) A. Nels. Verbesina encelioides (Cav.) Benth. & Hook. Verbesina rothrockii Robins. & Greene Zinnia grandiflora Nutt. Zinnia puraila Gray
55 APPENDIX 56 (ContTd)
GONVOLVULACEAE Convolvulus incanus Vahl. Evolvulus sp. Iponoea costallata Torr.
CUCURBITACEAE Apodanthera undulata Gray Cucurbita digitata Gray Cucurbita foetidissima H.B.K.
EPHEDRACEAE Ephedra trifurca Torr.
EUPHORBIACEAE Acalypha necsnexicana Muell. Arg. Croton texensis (KLotzsch) Muell. Arg. Euphorbia albomarginata Torr. & Gray Euphorbia sp.
GRAim-IEAE Andropogon barbinodis Lag. Aristida adscensionis L. Aristida glauca (Nees) VJalp. Aristida hamulosa Henr. Bouteloua barbata Lag. Bouteloua curtipendula (Michx.) Torr. Bouteloua eriopoda Torr. Bouteloua gracilis (H.B.K.) Lag. Bouteloua parryi (Fourn.) Griffith Bouteloua rothrockii Vasey Chloris virgata Swartz Enneapogon desvauxii Beauv. Eragrostis arida Hitchc. Eragrostis intermedia Hitchc. Eragrostis megastachya (Koel) Link. Eriochloa gracilis (Fourn.) Hitchc. Hilaria belangeri (Steud.).Hash Hilaria belangeri var. longifolia (Vasey) Hitchc. Hilaria mutica (Buckl.) Benth. Leptolcm cognatum (Schult.) Chase Lycurus phleoides H.B.K. Muhlenbergia asperifolia (Nees & Mey.) Parodi. Kuhleribergia porteri Scribn. Panicum capillare L. Panicum obtusum H.B.K. Scleropogun brevifolius Phil. Sporobolus contractus Hitchc. Sporobolus wrightii Kunro. Sporobolus cryptandrus (Torr.) Gray Tridens muticus (Torr.) Nash . Tridens pulchellus (H.B.K.) Hitchc.
LABIATAE Salvia sp 57 APPENDIX (Conttd)
LEGUMINOSAE Acacia constricta Benth. Astragalus sp. Cassia bauhinioides Gray Crotalaria pumila Ortega Dalea fornosa Torr. Dalea lachnostachys Gray Dalea polygonoides Gray Desmodium retinens Schlecht. Hoffmanseggia densiflora Benth. Hoffmanseggia janesii Torr. & Gray Krameria lanceolata Torr. Prosopis juliflora (Swartz) DC.
LUJACEAE Dasylirion wheeleri Wats. Yucca elata Engeln.
MALVACEAE Anoda cristata (L.) Schlecht. Anoda pentaschista Gray Hibiscus denudatus Benth. Sida lepidota Gray . Sida neonezicana Gray Sida physo calyx Gray Sida procumbens Sw. Sphaeralcea subhastata Coult.
NYCTAGDIACEAE Allionia incamata L. Boerhaavia sp.
OLEACEAE Mendora scabra Gray
OUAGRACEAE Gaura coccinea Ilutt.
POLEHCMIACEAE Gilia longiflora (Torr.) G. Don.
POLYGALACEAE Polygala obscura Benth.
POLYGONACEAE Eriogomm abertianun Torr.
PORTULACACEAE Portulaca sp. Talinum angustissimum (Gray) Vfoot. & Standi.
RHAMHACEAE Condalia sp.
SCROPHULARIACEAE Castilleja sp.
SOLANACEAE Solanun elaeagnifoliun Car. APPENDIX (Conttd)
VEREENACEAE Aloysia wrightii (Gray) Heller Tetraclea coulter! Gray Verbena sp.
ZYGOPHYLLACEAE Kal1stroenia grandiflora Torn. Kallstroeuia parviflora Norton Larrea tridentata (DC.) Coville