The Presettlement Vegetation of the California Desert

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BLM LIBRARY

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THE PRESETTLEMENT VEGETATION

OF THE CALIFORNIA DESERT

June 1980

Sys

THE PRESETTLEMENT VEGETATION

OF THE CALIFORNIA DESERT

June 1980

W. Geoffrey Spauling College of Forest Resources

and

Quaternary Research Center University of Washington Seattle, Washington 98195

for

Bureau of Land Management 1695 Spruce Street Riverside, California 92507

Contract CA-060-CT8-65

Bureau of Land Management Library Bldg. 50, Denver Federal Center Denver, CO 80225

THE PRE SETTLEMENT VEGETATION

OF THE CALIFORNIA DESERT

Submitted to the California Bureau of Land Management, Desert Planning

Staff Under Contract // CA -DGe>' Ct8 - Co S

W. Geoffrey Spaulding College of Forest Resources and Quaternary Research Center University of Washington Seattle, WA 98195 ACKNOWLEDGEMENTS

The following people have offered their kind assistance and advice

of this research: , , ln8 the course

Ken Cole Peter Rowlands

Hyruin Johnson Sue Spaulding

Estella Leopold Ray Turner

Paul Martin Roger Twitchell

Jim Mead Tom Van Devender

ii TABLE OF CONTENTS

ACKNOWLEDGEMENTS ii

LIST OF TABLES v

LIST OF FIGURES vi

INTRODUCTION 1

METHODS 2

Field Techniques 5

Laboratory Techniques 6

Data Treatment 6

Age Determination 7

A Chronological Framework 7

THE STUDY AREA. 9

The Marble Mountains 10

Marble Mountains Locality A 10

Marble^ Mountains Locality B 12

< Marble Mountains Locality C 13 The Eureka Valley And Vicinity 13

Horse Thief Hills Locality A 15

Horse Thief Hills Locality B 16 The Eureka View Locality 17

THE FOSSIL RECORDS 18 The Marble Mountains 18

Marble Mountains Locality A *. 18

Marble Mountains Locality B And C 24

The Eureka Valley And Vicinity ; 35 Horse Thief Hills Locality A... 35 Horse Thief Hills Locality B 38 The Eureka View Locality 41

iii The Regional Fossil Record 49 The Latest Wisconsin At Low Elevations 49

The Latest Wisconsin At Intermediate Elevations .53 The Early Holocene at Low Elevations ,...55 Early Holocene Middens From Intermediate Elevations 60

Middle and Late Holocene Packrat Middens 63

DISCUSSION 69 Vegetation Dynamics 70 The Marble Mountains 70 The Eureka Valley 75

The Sheep Range 83

Precipitation Gradients '. 84 Creosote Bush and Glacial Refugia 86

LITERATURE CITED 93

iv A

LIST OF TABLES.

Table No. Page No,

1. Packrat midden sites, samples, and radiocarbon dates 19 from the Marble Mountains, San Bernadino County, California.

2. Plants from Marble Mountains Locality A. 22

3. Plants from Marble Mountains Locality B. 25

AA. Plant s pecies from Marble Mountains Locality C, Sites 29

6 and 7.

AB. Plants from Marble Mountains Locality C, site 9. 31

5. Packrat midden sites, samples, and radiocarbon dates 36 from the northeastern Eureka Valley, Inyo County, California.

6. Plants from the Horse Thief Hills 1 site. 39

7. Plants from the Horse Thief Hills 2 site. . AO

8. Plants from the Horse Thief Hills 3 site. A2

9. Plants from the Eureka View Locality. . A

10. Plants from the Owl. Canyon 1 midden, Amargosa Desert, 52 Nye County, Nevada.

11. Plants from late Wisconsin and early Holocene packrat 56 midden sites in the Sheep Range, Clark County, Nevada.

12. Plants from middle and late Holocene packrat midden 6A sites in the Sheep Range, Clark County, Nevada.

13. Tossil records of Larrea tridentata from the Wisconsin 87 Glacial Age and early Holocene. LIST OF FIGURES

Figure No. Page No,

1. The California Desert and adjacent areas. 3

2. The Marble Mountains study area. 11

3. The Eureka Valley Study area. 14

A. The estimated relative abundance of selected plants 71

in Neotoma midden samples from Marble Mountains

Locality B and C.

5. Packrat midden samples from the CDCA and adjacent 73 regions plotted against radiocarbon age.

6. The total number of identified plant species (N) in 76 Neotoma midden samples from Marble Mountains Locality

B and Locality C plotted against radiocarbon age.

7. The estimated relative abundance of selected plant 78 species in macrofossil assemblages from the Eureka View Locality plotted against radiocarbon age.

8. The total number of identified plant species (N) in 82

macrofossil assemblages from the Eureka View Locality '. plotted against radiocarbon age.

9. Fossil sites of Wisconsin and early Holocene age con- 90 taining creosote bush.

vi INTRODUCTION

The biotic provinces of North America, as we know them now, are relatively recent developments. The environment of the last Ice Age was drastically different than today's and, during that time, the distribution and composition of natural communities differed correspondingly.

For at least the last million and a half years, a period known as the

Pleistocene, the global climate has alternated between long glacial and shorter interglacial intervals. At least six glacial to interglacial cycles have occured in the last half million years. The average length of the cold, glacial periods exceeds 50,000 years while that of the warmer, interglacials is less than 15,000 years. Superimposed on these long term oscillations are fluctuations of shorter frequency and lower amplitude. The climate of the present interglacial, the Holocene, has been subject to frequent changes of relatively low intensity (Imbrie and Imbrie 1980; Turekian 1971; Wright 1976; Davis 1976).

The last shift in global environment from an Ice Age to the warmer climate that we enjoy today occured between 14,000 and 8,000 years ago. One result was the last major episode of plant species migration ' and the ultimate segregation of those species into plant communities of modern aspect. In the strict biogeographic sense, the

Mojave Desert has existed for only the last 8,000 years or so. The regional vegetation and climate of the preceding Wisconsin glacial age was different. Hence, at the outset, we are presented with the fact that the ecosystems under scrutiny in the California Desert Conservation

Area (CDCA) are subject to low frequency perturbations of sufficient amplitude to alter their aspect considerably. The desert-adapted

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plants and animals being studied are subject to, and are the product of, selective pressures exerted by a long series of major environmental changes. This study examines the impact of the last such change, the end of the Wisconsin Glacial Age, on vegetation of the present Mojave and Colorado (northwestern Sonoran) Deserts (Fig. 1). The development of postglacial desertscrub (sensu Brown and Lowe 1974) vegetation through the last 8,000 years is also discussed.

METHODS

Paleoecological reconstruction depends upon a reliable source of fossil material. In temperate regions depositional basins such as lakes and bogs yield well preserved organic remains. Sites such as these can provide stratigraphically continuous records of vegetation that stretch back tens of thousands of years. However, sedimentary deposits containing well preserved organic remains are scarce in the deserts of North America.

In the early 1960 f s a new paleoecological tool was introduced

(Wells and Jorgensen 1964; Wells 1976) and its use has shed a great deal of light on the biotic history of arid North America. Packrat

( Neotoma spp.) midden analysis (paleonidology) is the reconstruction of past vegetation using the mummified plant remains gathered and secreted in rock crevices and caves by packrats. The habit of col- lecting massive amounts of vegetation and constructing a dwelling with that debris is a generic trait of this cricetid rodent (Finley

1958; Van Devender 1973). Within a packrat stickhouse or den there are various functionally specific areas including nesting chambers, -3-

Figure 1. The California desert and adjacent areas. Subdivisions of

the CDCA defined by the California Bureau of Land Management,

Desert Planning Staff. A dashed line delimits the Colorado

(Sonoran) and Mojave Deserts. EV, Eureka Valley; MM, Marble

Mountains. Other fossil sites are: 1, Funeral Range; 2, Kofa

Mountains; 3, Lower Grand Canyon; 4, Lucerne Valley and vici-

nity; 5, southern Nevada Test Site; 6, Newberry Mountains;

7, New Water Mountains; 8, Ord Mountain; 9, Owl Canyon; 10,

Picacho Peak; 11, Scodie Mountains; 12, southeastern Sheep

Range; 13, western Sheep Range; 14, Titus Canyon, Inyo Moun-

tains; 15, Turtle Mountains; 16, Wellton Hills; 17, Whipple Mts, ^ '

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food caches, and trash middens. Vegetable debris from the latter areas

is the focus of this study. In rocky habitats packrats often live in

rock shelters or crevices, using the naturally occuring shelter to

augment that provided by their den. Middens often serve as urination

and defecation points and the vegetable debris in sheltered middens fre-

quently becomes cemented in a matrix of crystalized urine (amberat)

These dessicated masses of plant debris, encased in rock-hard amberat,

occur in cavities in the hills and mountains of the North American deserts,

Middens that are protected in a rock shelter may persist as long as the

shelter lasts. Cavities in slow weathering rock types have yielded

middens and associated paleobotanical data for the last 50,000 radio-

carbon years. Ancient middens normally have a thick weathering rind

composed of degraded midden debris, adhering dust, and packrat urine.

Amberat is hygroscopic, it absorbs water from the air during periods

of high atmospheric humidity and, during such times, becomes partially

| fluid again; Fresh breaks in a midden are quickly sealed by a new

layer of amberat drawn by capillary action from the interior of the

midden. With time, the amberat becomes convoluted and takes on the

aspect of the original weathering, rind. This self-sealing contributes

to the long-term viability of fossil middens.

Several factors combine to make packrat midden analysis a power-

ful paleoecological tool. The most striking is the n^ar perfect pre-

servation of leaves, flowers, seeds, and other delicate plant parts

that may be tens of thousands of years old. Given an adequate reference

collection, more than 60% of the fossil taxa in a midden can be iden-

tified to the species level. Packrats are comprehensive collectors of -5- vegetation within their limited (generally less than 30 m; Stones and

Hayward 1968; Bleich and Schwartz 1975) home range. Modern middens invariably contain all the plants that are common at a site as well as most of the uncommon and rare species. Age determination of fossil middens is achieved through radiocarbon dating.

Field Techniques . Field searches for midden sites were made along outcrops of shelter forming rocks, particularly along cliff bases. Carbonaceous sediments and certain igneous rocks yield ancient middens for this study. Shelters that contain fossil packrat middens occur only in rocky habitats and this study is necessarily restricted to rocky slope vegetation.

Once a midden is located its position in the shelter and exposed stratigraphy is carefully examined. Ancient middens that appear complex or jumbled, without easily discernable layering, are disregarded.

Any indurated midden with a heavy weathering rind and of uncomplicated aspect is considered suitable for sampling. The weathering rind is removed from selected middens and they are again examined for evidence of multiple layering. Samples from individual midden units are removed with a hammer and cold chisel or crowbar and carefully wrapped. A list of all the plants occuring within 30 m of the midden site is made.

The distribution of plant communities, principal species, and their relationships to the mosaic of available habitats is noted for the entire locality. The quantitative relationships between dominance of species in a plant community and importance of plant fragments in a midden are poorly understood. Quantitative data gathered on packrat midden site vegetation is restricted to qualitative ordinations of abundance. -6-

Laboratory Techniques . Once in the lab midden samples are thor-

oughly cleaned of weathering rind and other contaminants that adhere to

their surfaces. They are then placed in a bucket of water to disolve

the amberat and release the plant fragments. The resultant wet mass

is poured from the bucket through a nested pair of six mesh (3.35 mm)

and 20 mesh (0.8A mm) soil sieves and flushed with water. The captured

material is dried and the identifiable plant .fragments, bones, etc....

are sorted into plastic vials. The plant fragments are examined under

moderate magnification and compared with modern reference specimens.

A list is compiled with estimates of the relative abundance of each

plant taxon in the fossil assemblage.

Data Treatment . Possible relative abundance values for the

fossil plants in a midden sample range from 5 (very abundant), to 4

(abundant), to 3 (common), to 2 (occasional), to 1 (rare), to (one

or two fragments, a possible contaminant). The lowest number of fossil

taxa (N) in , any macrofossil assemblage in this study is 8, the greatest

number recorded is 33. The total number of woody perennials, excluding

grasses, (N ) in each macrofossil assemblage is presented as well as P the percentage of woody perennials (N/N x 100). Similarity indices

(IS) are used to express the percentage of shared taxa between the

macrofossil assemblage and the plants at the site today. The similarity

index chosen for this study is that proposed by Soren§en (1948; Mueller-

Dombois and Ellenberg 1974) and takes the form :

IS ' x 10° I tVb

where A is the total number of species (N) noted within 30 m of the midden -7-

site, B is the total number of species occuring in the midden sample, and c is the number of plants common to both the modern vegetation and the midden sample. Possible values range from (no species in common) to 100 (all species shared) although test comparisons between recent middens and the surrounding plant community have failed to yield IS values greater than 85. Invariably there seem to be a few plants in the midden that were not noted on the field list, and a few noted on the list that are not found in the midden debris.

Age Determination . I prefer to use a single extralocal plant

species from a midden sample for radiocarbon ( C) dating. This method,

suggested by Van Devender (1977a) , is desirable since the date is directly associated with at least one plant species and there is no chance of contamination by recent plant debris. But desertscrub middens frequently lack sufficient mass of any one species to radiocarbon date. Many of the age determinations used for this study are on undifferentiated twigs or packrat fecal pellets. Whatever the material dated, in the absence of discordant multiple dates and given a favorable stratigraphic setting, it is assumed that the C date reflects the age of the entire macrofossil assemblage. A C date is expressed as a mean date with a standard deviation reflecting various uncertainties including background radiation at the counter, sample size, counting time, number of counts, etc.... (Long and Rippeteau 1974; Sheppard 1975).

A Chronological Framework . Packrat midden analysis is used to study late Quaternary climates but, for the purpose of this vegetation analysis, the chronology of climatic change, as it is known, is presented first. The chronological divisions of the latest Wisconsin and early -8-

Holocene follow those presented by Van Devender and Spaulding (1979) and Phillips (1977). The latest Wisconsin (Stage III of Phillips 1977) extends from ca. 14,000 B.P. (radiocarbon years before present) to ca. 11,000 B.P. and is the waning phase of the last Ice Age. The start of the early Holocene at ca. 11,000 B.P. is marked by the retreat of many mesophytic species from the interior basins of the southwestern

United States (Van Devender and Spaulding 1979; Martin and Mehringer

1965) . The extirpation of pinyon pines from the low elevation woodlands of the present Chihuahuan, Sonoran, and Mojave Deserts was a near synchronous event at ca. 11,000 B.P. (Van Devender and Spaulding 1979).

The early Holocene, persisting to ca. 7800 B.P. (Stage IV of Phillips

1977), was a time of transition characterized by an increasingly xeric climate. Desertscrub species assume gradual dominance throughout this period, culminating at ca. 7800 B.P. with the disappearance of the last of the woodland plants (Van Devender 1977b, Phillips 1977). The middle

Holocene bioclimatic phase begins at this time and terminates ca. 3700 B.P

At about this time in the White Mountains at the northern border of the CDCA upper tree line retreats dramatically (La Marche 1973, 1974).

The White Mountains record of higher spring through autumn temperatures during the middle Holocene supports Antevs' (1948, 1955) concept of an

Altithermal climatic phase in this region. Martin (1963) suggests that the middle Holocene may have been a period of increased summer precipitation in the monsoonal Southwest. This hypothesis is not inconsistent with a climate characterized by increased warm season temperatures and,

perhaps, increased dryness in the Great Basin Desert. The late Holocene, . from ca. 3700 B.P. to present, appears in the White Mountains chronology -9-

(La Marche 1973, 1974) to be a time of generally cooler temperatures.

There is evidence for a more mesic late Holocene climatic regime in pollen and packrat midden data from the northern Mojave and southeastern

Great Basin Deserts of Nevada (Spaulding 1977, unpub. ; Madsen 1973).

THE STUDY AREA

The fossil plant data from two areas, one at the southern boundary of the Mojave Desert and one in the northern Mojave Desert, are the basis for this study. The significance of other fossil sites in the vicinity is also discussed. Climatically induced changes in plant species distribution and abundance may be more pronounced in boundary region vegetation where more species are at the limits of their habitat tolerance. Hence, the two main study areas were selected to be near the bio tic boundaries of the Mojave Desert.

The southern limit of the Mojave Desert is gradational and poorly defined. Describing the borders of the Colorado and Mojave subdivisions of the CDCA, the floristic boundary is defined by the decreasing number and importance of species adapted to a bimodal, winter-summer precipitation regime (Fig. 1; Shreve and Wiggins 1964; Hastings et al. 1972).

Plants such as Dalea spinosa , Beloperone calif ornica , Cercidium f loridum ,

Fouquieria splendens , Hyptis emoryi , and Simmondsia chinensis occur in the Sonoran Desert (of which the "Colorado Desert" is but a subdivision)

1 to the south and east. In this southerly region summer rainfall comprises not only a significant percentage of the annual total but is also pre- dictable. The staggered distribution limits of these monsoonal-adapted species form the biogeographic criteria for the southern Mojave boundary. -10-

In contrast, the Mojave Desert's northern boundary is based on the limits of one species, Larrea tridentata (creosote bush), and is correspondingly well defined. A shrub that is ubiquitous in the warm deserts of North America, the northern limit of Larrea tridentata is controlled by minimum temperatures (Beatley 1974, 1975) and defines both the northern limit of the Mojave Desert and the southern boundary of the Intermountain Region (Cronquist et al. 1971).

The Marble Mountains

The Marble Mountains trend from northwest to southeast and lie at the northern end of the Cadiz Valley (ca. Lat. 34 40' N, ca. Lon.

115 35' W) . The mountain range is immediately south of the juncture of the southern limits of the Central and Eastern Mojave subdivisions of the CDCA (Fig. 1). Where the mountain slopes meet the bajada elevations range from ca. 300 m (1000 ft) in the south to ca. 920 m

(3000 ft) at the northern terminus (Fig. 2). Castle Peak at 1170 m

(3842 ft) is the highest point in the Marble Mountains and the majority of the range lies below 900 m (2950 ft) elevation.

Marble Mountains Locality A . Near the Iron Hat Mine in the southwestern Marble Mountains (Fig. 2) is an area of steep limestone slopes and ledges designated Locality A (Cadiz, CA, 15' quadrangle: T6N, R14E,

NW*£, NW"J£, Sec. 19). These limestone outcrops provide anomalous habitats in a mountain range of Tertiary (?) rhyolite, rhyolitic breccia, and associated volcanic rocks. The sere south to southwest facing slopes, from 365 m to 490 m (1200 ft to 1600 ft) elevation, support widely scattered vegetation. The only common shrubs on the rocks are Encelia -11-

Figure 2. The Marble Mountains study area. The Marble Mountains 1, 2

and 3 sites are at Locality A, the MM 4 and 5 sites at Locality

B, and the MM 6, 7, and 9 sites at Locality C. GARBLE MOUNTAINS STUDY AREA 4 mi .

Contours at 400 feet o / 115 35 -12-

farinosa (brittle bush), Larrea tridentata , and Salvia cf. funerea , the latter occuring ca. 275 km (165 mi) southwest of its main range in the Death Valley region (Munz 1974). Its restricted occurence on calcareous substrate in the Iron Hat Mine area is a hitherto unre- ported disjunct record of this species. Cacti in the vicinity include

Ferocactus acanthodes (California barrel cactus) , Echinocactus poly-

cephalus (cotton-top barrel cactus) , and Opuntia basilaris (beavertail prickly pear). Pleurocoronis pluriseta (arrow leaf) and Physalis crassifolia (ground cherry) are common at the cliff bases. In the more mesic canyon bottoms and on the upper bajada away from the rocky slopes such species as Ambrosia dumosa (white bursage), Hyptis emoryi

(desert lavender) , Bebbia juncea (sweet bush) , Dalea spinosa (smoke tree), and Peucephyllum schottii (desert spruce) occur.

Marble Mountains Locality B . This area in the northern Marble

Mountains is at the head of a small canyon ca. 3 km (1.8 mi) east of the Brown Buttes (T^S, R13E, NWk, SE*£, Sec. 7; Fig. 2). The locality presents southwest to west facing talus slopes and cliffs with elevations varying from 850 m to 915 m (2800 ft to 3000 ft). The substrate of rhyolitic breccia supports well developed creosote bush - brittle bush vegetation. Trails and dung piles indicate moderate use of the area

by feral burros ( Equus asinus) , particularly under the cliff overhangs.

The vegetation is more diverse than at Locality A. Shrubs occuring on

these slopes include Bebbia juncea , Ambrosia dumosa , Hy*ptis emoryi , and

Eriogonum fasciculatum (California buckwheat). Pleurocoronis pluriseta and Physalis crassifolia again occur at the cliff bases along with Ditaxis

lanceolata. The cacti at this locality are Ferocactus acanthodes , . ,

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Opuntia basilaris and, in more open habitats away from the cliffs,

Opuntia ramosissima (diamond cholla) and 0_. echinocarpa (golden cholla)

The mesic wash bottoms and upper bajada habitats support such shrubs

as Acacia greggii (catclaw acacia) , Lycium californicum (boxthorn) and Brickellia desertorum (brickel bush)

Marble Mountains Locality C . Locality C is 2.4 km (1.5 mi) southeast of the Brown Buttes and 1.4 km (0.9 mi) southwest of Locality

B (T7N. R13E, SW^, NW*£, Sec. 18). It is an area of cliffs and talus slopes about 100 m above the bajada at ca. 865 m (2840 ft) elevation

(Fig. 2). The slopes are derived from rhyolitic breccia and their south to southwest facing aspect produces particularly xeric habitats. Ercelia farinosa, Larrea tridentata, and Ambrosia dumosa are codominants here

with Peucephyllum schottii , Lycium californicum , and Bebbia juncea being common shrubs. Pleurocoronis pluriseta and Ditaxis lanceolata are occasional smaller perennials. A few isolated plants of Yucca schidigera may be seen about half a kilometer away in mesic wash habitats of the upper bajada.

The Eureka Valley And Vicinity

An unnamed series of hills that trends northwest to southeast rims the northeastern Eureka Valley. They form a nearly continuous highland connection from the north end of the Inyo Mountains to the northwest end of the Last Chance Range. The hills are upthrust on the west side of the Furnace Creek Fault Zoneand also form the southwest _j . border of the Fish Lake Valley (Fig. 3). Horse Thief Canyon runs through the center (Lat. 37°22' N, Lon. 117°50' W; Soldier Pass, CA.-NV., 15' -14-

Figure 3. The Eureka Valley study area. EV, Eureka View; HTH, Horse

Thief Hills.

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quadrangle) of this mountain mass and, for the purpose of this study,

lends its name to the Horse Thief Hills. The northern limit of Larrea

tridentata and, by definition, the northern boundary of the Mojave Desert

lies in the Horse Thief Hills. Creosote bush has been noted as far north as the upper Willow Wash Canyon on south facing alluvial slopes at ca. 1635 m (5360 ft) elevation. Elevations in the Horse Thief Hills range from ca. 1430 m (4700 ft) at their southern terminus to 1990 m

(6520 ft) near their northern end at Sugarloaf Mountain. Most of the

topography lies below ca. 1710 m (5600 ft) elevation. The roughly

1000 m gain in elevation from the Marble Mountains at the north end of the Cadiz Valley (Lat. 34 40 f N) to these hills at the northern end of the Eureka Valley (Lat. 37 22' N) reflects a general physiographic trend in the Mojave Desert. The increase in base level with increasing latitude is an important factor controlling the northern extent of

Mojavean flora (Beatley 1974, 1975). To the north, the next higher tier of valleys (Deep Springs, Fish Lake) have, at least as far as winter months are concerned, Great Basin temperature regimes that

effectively exclude many warm desert species, such as Larrea tridentata .

Horse Thief Hills Locality A . This area of rugged dolomitic cliffs and talus slopes, outcrops of the Bonanza King Formation (McKee and Nelson 1967), forms the south wall of Willow Wash (Fig. 3). Eleva- tions range from 1585 m to 1645 m (5200 ft to 5400 ft) on these north and east facing slopes that are less than half a, kilometer southwest of the foot of the Sylvania Mountains (T7S, R38E, SE^s, NW^, Sec. 10).

Atriplex confertifolia (shadscale) and Chrysothamnus viscidiflorus ' ,

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(rabbit brush) are the most frequent plants on the calcareous slopes.

Dther shrubs that are common in the diverse mixed desertscrub vegetation

nclude Ephedra nevadensis (Mormon tea), Eurotia lanata (winter fat),

riogonum heermannii , and Tetradymia axillaris (horsebrush) . Symphori-

carpos longiflorus (snowberry), Ephedra viridis (joint-fir), Cercocarpus

intricatus (littleleaf mountain mahogany) , Hecastocleis shockleyi

(Mojave prickle-leaf), and Forsellesia nevadensis (greasebush) occur

in mesic habitats. Xeric species such as Dalea fremontii (indigo bush),

Artemisia spinescens (bud-sage) , Encelia virgin ensis (rayless encelia)

Menodora spinescens ( ground thorn) , and Acamptopappus sphaerocephalus

(goldenhead) occur on the xeric exposures. Creosote bush appears

restricted to the alluvial slopes of Willow Wash, only one plant was noted on the hills above. This also seems to be the case with Yucca brevifolia (Joshua tree). It is scattered on the xeric exposures of

Willow Wash Canyon but not on the bedrock slopes.

Horse Thief Hills Locality B . This low series of hills juts out from the floor of Willow Wash, equidistant between the Horse Thief Hills

to the southwest and the Sylvania Mountains to the northeast (T7S, R38E,

SW*s, SW*£, Sec. 11; Fig. 3). With a maximum elevation of 1575 m (5160 ft), it rises only 25 m above the floor of the wash. Metamorphosed siltstone and shale of the Campito (?) Formation (McKee and Nelson 1967) support a relatively sparse desert community dominated by Atriplex confertifolia .

Dalea fremontii , Hymenoclea salsola (cheeseweed) , and Ephedra nevadensis are occasional on these shadscale dominated west facing slopes. Larrea tridentata and Yucca brevifolia occur sporadically on the alluvial fan below. -17-

The Eureka View Locality . These midden sites are at the extreme

: southern end of the Horse Thief Hills (T7S, R38E, S^. NW-£, Sec. 23;

Fig. 3). The locality is named for the spectacular view from these walls that tower above the northeast end of the Eureka Valley. South facing slopes of limestone and dolomites of the Bonanza King Formation,-, from 1390 m to 1555 m (4560 ft to 5100 ft) elevation, provide a rugged

habitat of ledges and talus slopes. Atriplex conf ertif olia and Larrea » tridentata share dominance in the surprisingly diverse desertscrub communities occuring here. Amphipappus f remontii (chaff -bush) and Eriogonum fasciculatum are subdominants at this locality while Brickellia arguta

(brickel bush), Erioneuron pulchellum (fluff grass), Stipa sp. (perhaps

S_. arida , Mormon needlegrass) , Haplopappus brickellioides , and Nicotiana trigonophylla (tobacco) are rather common on these slopes. Viguiera

reticulata occurs infrequently, as does Ephedra nevadensis , Ambrosia

dumosa , and Scopulophila rixfordii . Encelia virginensis is restricted to the west facing wall of a nearby canyon that dissects a large talus

slope. With the exception of Echinocactus polycephalus , cacti do not occur on these rock exposures. Opuntia basilaris and 0. echinocarpa are scattered on the alluvial fan, ca. 60 m from the nearest fossil site,

as is Dalea fremontii . All three species occur on the upper bajada below Eureka View. The Eureka Valley bajada supports a well developed creosote bush-white bursage community at elevations exceeding 1220 m

(4000 ft), an unusually high occurence of this vegetation type. In contrast to the Eureka View Locality, the upper bajada substrate is gruss derived from extensive outcrops of quartz monzonite in the Sylvania

Mountains and Last Chance Range to the east (McKee and Nelson 1967;

Fig. 3). This may account for the occurence of Atriplex polycarpa -18-

here and its absence on the nearby limestone slopes. Yucca brevlfolia ,

restricted to the Willow Wash and Cucomungo Canyon to the east (Fig. 3),

does not occur in this part of the Eureka Valley.

THE FOSSIL RECORDS

Twenty-six macrofossil assemblages were recovered from 19 ancient

packrat middens found near the northern and southern boundaries of the *

California Mojave Desert. The samples date frpm the late Wisconsin,

the final phase of the last Ice Age, through the Holocene, to the sub-

recent. At many localities the midden sites lie in close proximity to

one another with similar habitats and vegetation. Where this occurs

the species lists from each site are combined in a single table that

applies to the entire fossil locality. The fossil sites from Marble

Mountains Locality A are considered in one group, those from Localities

B and C in another, and the sites from the Eureka View in a third.

Only the habitats of the Horse Thief Hills sites varied sufficiently

from one another to be considered separately.

The Marble Mountains

The age distribution on the Marble Mountains packrat middens

is ideal for the study of early Holocene vegetation dynamics. Six of the 11 midden samples that are dealt with in detail radiocarbon date between ca. 11,000 B.P. and ca. 7800 B.P. (Table 1). These macrofossil assemblages provide data on the transition from early Holocene vegetation to plant communities of essentially modern aspect.

Marble Mountains Locality A . Three fossil packrat middens provide information on the past ecology of these limestone slopes at i i' -19-

CO I o u Ph CO *3 •O 01 3 0) c ' c w CO CO CO CO CO CO Rj CU 3 0) a in i-l o o CJ o co to o o J= JJ 01 OJ * u 0) CU 0) too 14-1 •H CO 14-4 CO c c co c CO CO a> •H T3 c CO P O) OB $1 wi o • 0> 4J E u E td OJ CO • -H OJ CO 4-1 4J 4J CO CO o o toO 4-1 CJ 00 o o 4-1 CJ too c c X CO ID H o CO «H CO CO o CO •H o 3 u s •H c 0) •H ^ H tO OJ > CJ 4J 5S 2 4J S "*-• 5S s 4-1 ^^ «;; a) c

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-20-

CO CO CO • OJ to 3 3 CD o o a CO CO o S *-3 S *J :s 55

PQ Pm

PQ /-s PQ *~> PQ /-> PQ /-v O *-s PQ /-s PQ /-v m vO co rH CTl O *H m 00 O 00 o oo m co CN 00 CT\ CT> -H O O CO SD r-t vD •— VO rH cr\ •— CM vD cn vo <—i vD CN vO + |VD CN vD t—t vO

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CN CN

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to co co CO >H c c B e H «H H CU CO QJ to QJ to cu to QJ (0 QJ tO OJ CO rH 4-» rH 4-t H 4J rH 4J fH 4J X> C X C X C -Q C X C XI C X) C CJ In - M 3 U 3 M 3 M 3 jh 3 U 3 O C0 O tO O tO O tO O tO O fO O to o XX XX X 2C £ X -21-

ca. 475 m (1560 ft) elevation (Fig. 2). Marble Mountains 2 is the oldest midden, with a radiocarbon age of 10,465 + 330 B.P. (GX-6178; Table 1).

The fossil assemblage differs considerably from the present sparse creosote

bush-brittle bush vegetation (Table 2) . MM 2 contains higher elevation

desertscrub species and is dominated by Ephedra cf . calif ornica and

Lycium sp. (perhaps L, californicum ) , neither of which presently occur near the site. Ephedra californica does occur within 6 km (3.6 mi) on bajadas as low as 490 m (1600 ft) elevation, but never on the low, bedrock

slopes. Lycium californicum occurs in the Marble Mountains at least as

low as 790 m (2600 ft) elevation on south facing rhyolite slopes. The leaves of Salvia cf. funerea and S. mohavensis (Mojave sage) occur in

the MM 2 assemblage. The former persists at the site today but the latter does not. Salvia mohavensis occurs today in mixed desertscrub vegetation above ca. 1100 m (3700 ft) elevation in the Van Winkle Mountains, ca. 18 km (11 mi) to the north. Not only does' this macrofossil assemblage lack

Larrea tridentata and Encelia f arinosa , the most important plants at the

site today, but it also lacks the woodland species that are common in contemporaneous middens from low elevation sites further east.

The Marble Mountains 1 and 3 middens contain late Holocene plant

14 macrofossils. The former midden has a C date of 3080 + 165 B.P.

(GX-6177), and the latter, 1465 + 180 B.P. (GX-6179; Table 1). Both assemblages contain abundant Larrea tridentata and Encelia f arinosa ,

the current dominants at the sites (Table 2). Neither Suggests appreciable

change in the local vegetation in the last 3100 years except, perhaps,

in the distribution of Bebbia juncea and Peucephyllum schottii . Both 1 1 \' iI 1 1 i1 i

-22-

CO CO e 4J o c • CO •— c u to — u-l 2: y H 0- o P-> lt-1 U bO U-l 4J o X c CO 0) «H a en o O c o 0) "O CO CU -H CO i—l <— eo to - .l-i c 3 CO C M N 0) •H >— JO 5 CO XI M • l-^lltlllroiiii^niiimii CM I liH •• £ CO >% CO 4J 3 CJ C 5 2 y o c C -H y o> E a; c o v E .H CO 3 CO CO C E S CO CJ 4J cu e -H T> o l-i o u C - 4-1 o o C 3 <"> CO #s 4J OJ .o 0) cO •" ^ * c 0' *J CO CO HOC t-l i-l Cu a > co t a H T> 4.) • »• CN CQ CO 4J C c co C 01 CO 3 CO > c JO iH CO .O D •H o JO CO * oj < CO CO CO c bO 4J O CO >> c >% a •> i-l U CO M CO JO •H C OJ CO H E H i-l > CO CU « E •> -H CO CQ u » CO c co 09 o "o m 4-1 C to oj c m J c 0) iH -H VD • * • a) J-l i-l jD CO •— M OJ QJ «H 4J CN r-4 X E M I I l«—i—i| | | | ftOI I I I | fl I I I N I HH c u bO CX CO CO C + -H C CO co CO S 3 CO Q u >, o U 0) U-l •O U-l S oo c >* c o o 3 JD -H o O 0) CO 3 3 J= X T> E 4J 4J X a> cu U-l Vu M »-l o c «H CO o 0) X CO ^^ /-N JO C U p a) N-^ /-N T-l : »- to c OJ OJ > /-s X ^H CO V-'^-N^-s X <8 «H CO c X to ^ o 3 XX ^ ^v o *E 0) w ^| 14-1 iH to CO V-/N-/ oj a 3 JO i-H 3 •H /—N CO CJ CU *-* JO * c CO >l C 4-1 X JO •H a, E CO ^-\ /—s 4-1 H §^h o >» H l-i CO w D. ^. c •H o 3 QJ •H XI to c •H U X X 0) i-t i-l 4J C o« o 3 QJ X l-i -H JO r-l TO CO >-^ 1-1 w CO 4.) <""s ^ »* CO •> tfl «H o >> • o 00 CO u w o c u iH O CO iH O 4J X to 4J CO OJ 4J t-l •H 4-1 a th C >> U-l C •H 0) JO ^ «H CO /-s O CO CO O » -t-i 4-1 u o u O CO a CO CO > CO CO iH CO •H re JO X CO 4J X u-i H >v niM CO 4-1 tH d .-( 01 iH O CO ^H Eg 4-1 CJ c w O re >-• -H h i-l CJ •H C 01 t-l «H 0) a CO l-i • O a CO c rH CO 4J 4-1 c re o CO >> Quo) 6 o CO -H JO /-». Uj OJ C o QJ • 3 u /-s -H CU c • iH iH c- I-l m D--H • 0) CO QJ c X cj u CO -H OJ u-i a CO X >. CO OJ a. H — E o ft. CO 4J i— >-l • >^ Z en o- *o •H O o OJ w C 3 JO u l-i -a CO CO • 3 E CO to 1-1 c CO J= CO co 4J CO U-l c CO O CO •H CO re U-l i-l cu QJ CO 4-1 CO ^H U-l • •H U Cu CO CJ i-l 3 JO o o E E o 3, . E — JO CU tfl CO i-) •H c -o 4-1 CO 3 3 I-l 4> a. cu CO 4-t CO i-l l-i >J OJ CO c TJ CO •H c CO CJ CO CO C C 3 CJ CO -H rH O CO CO — — a) o i-l -C CO CO N QO CO -H O -H >-i O O CU CO CO o re E QJ E i-t •H Bi >•> I) •H 4-1 o •H c_> •H •H 4-1 X c i-4 -a Cp bO c CJ CO — l—l QJ O N C u 4-1 0) 4-1 o QJ CO •H JO I-l M Cu CO •H QJ QJ o O o o •H 4-1 bo M H 4-1 cx C c CJ H bO •H 1-1 JO • t^ 4-1 JO U JO H •H •H I-l — CU C u CJ c •H o t-l « a> o 9 3 JJ a' U-l J= • l-i •H u c U I-l l-i CU -H >> CO re >> CU — D. 01 QJ hi (o-l M U a a C/j < <

-23-

to OJ C H •H ^ rt 4-J r-< COCO P I | CM I I I I Oh sr co c o ^ S o S

en 0) c rH H >Q cc 6^ -h «d" rH o-\ in J-l aJ I hN I I —I CM I CO C HHSfO S =5 SO

CO OJ C rH «H .O CO &m> .- | —< en I I < O — O <— CO C •-h < r» so S 3 O

vo * X CO oo — I /— CO ^^ u cm w m i •u X OJ X CO td S*> CO H •H •H 3 r-l CO S-i CO x eq C O H 3 0) «-* c m «4-l l-i H ^ o p •H CO a.

-24-

are presently restricted to the mesic upper bajada in the vicinity of

A (Fig. Locality 2) , but Bebbia juncea occurs in both samples and Peuce-

phyllum schottii in MM 3

Marble Mountains Localities B And C . The Marble Mountains 4 and

5 sites lie at 885 m and 900 m (2900 ft and 2950 ft) elevation, respec-

tively. Samples from these two middens provide early, middle, and late

Holocene-age macrofossil assemblages. The oldest, MM 5(1), is radio-

carbon dated twice. Miscellaneous twigs and Prunus fasciculata (desert- 14 almond) seeds yield a C date of 10,090 + 380 B.P. (GX-6182) while a

second date on Neotoma feces is contemporaneous within one standard

deviation at 10,325 + 350 B.P. (GX-6183; Table 1). A single age, aver-

aged from these two values (Long and Rippeteau 1974), is 10,210 + 260 B.P,

Like MM 2 in the southern Marble Mountains, the MM 5(1) assemblage

provides evidence for early Holocene mixed desertscrub vegetation at a

site currently dominated by brittle bush and creosote bush (Table 3)

The MM 5(1) midden sample lacks woodland plants, with the exception of

the rare occurence of Cowania mexicana (Table 3). Higher elevation desert shrubs, such as cf. Haplopappus laricifolius (turpentine brush),

H. cuneatus , Prunus fasciculata , Coleogyne ramosissima (blackbrush)

Salvia mohavensis , and Yucca schidigera (Mojave yucca), are most common

in the MM 5(1) assemblage. The diversity of this macrofossil assemblage

is striking compared to the younger midden records from the same locality or the modern vegetation (Table 3). It is, however, no? unusual relative

to the present mixed desertscrub communities on the north slopes of the

Van Winkle Mountains at ca. 1220 m (4000 ft) elevation, 10 km (6 mi) to » '

-25-

c IT) /— 4360 + «-l I i CM I I CM I I I I NHHH | | — I | < I O I I I CM o 210 B.P. x» oj e m « c ,£) CO c a p j~»* o CO C /- + o iH 10,210 —I a S H |rt |HHCSrtN-l |Nh I | ! I O CN I I I I w x 260 B.P. o J~ C a

a H

0) to

CO en 1680 + ^-' >> CO I I I I ~H | rH CM I I l l I I I l I I I I l B.P. I I en x» 0) c <* 150 ^ iH •H •H .O ctj a M XJ u CO C ^s S 3 CM 9515 + •J --^ O >— X CJ CO -s 6 w 03 H -a CO X 3 0) •H > >H E W r-s. XJ 0) «»-s CO e a) 60 •H Xl CO *o »-» o u •H CO /—\ CO >-* rH o CO X w CO •H /^<^s ,fl M CO X •-» 3 s: •H to "H CO w CO . CO CJ /-N XJ xi o XX CO 0) c s-' • • CO iH Cl CO •H ^-n C jo ^ C X C •H . to to 0) . . u-i U-i o CO O CO O CJ X x to •H Cl O CO u • Cl Cfl cl u cj e o c * o> c CO •H >-" u 60 to £ QJ CO • Cl CO CJ CO CO •H CO Cl U CO •H CO •H CO CO b. CO 60 3 XJ TD o. CO u aj OJ rJ X CO C 3 M • Uh c E 01 0) CO •o CO CO CO c CO J= ,C a> CO CO 0) CO a c 3 Cl CO XJ U-l XJ M 3 cfl CO 3 3 •H XJ QJ E A tO rH t-i co e CO O 3 60 JC CO •H CO CO CO rH C C c XJ XJ OJ 3 CO s: XJ t| H r E CO XJ •H ^ *o "O H CO 1—1 CO CO >! CO C •H co o CO CO 3 p rH u o 0) fO C CO o •H •H D. to CO QJ N N 60 •H cfl X) •H o •H •H O OJ U-I •H CO •H •H CO o c XJ XJ 01 CO H ^ •H •H O c XJ U X c rH iH 60| 01 c l-i o JO CJ U M M •H CO CO rH J- -O CJ J-i H 0) Cfl a 3 rC •H O) OJ O J= •H XJ o ' 0) CO CO J3 CO •H •H O XJ ,£ •H O O rH 5 >* U XJ j3 CJ CJ •H >-l Cm y U i-j CO CO a) U J= JS O o u 3 •H CJ c c U •, •. «l 01 Vj < 1 1 1 < < < < ea PQ CJ u U O U U a U w UJ ux w W U b -- s

-26-

m /— CNI I I I t i«-i| -3- icni I i i i i i i s^ 111—«tcsi i r^ o Cl (0 •-h mm CD c in H •H X> CD M J-J CO c s-~>.

H | I I I ^h I l r-> 1 S 3 NN co i n h i i m i

CO e^? till I I I ST I I I I I I I *H I *-4 I I | I | | h» m m CT\ OJ c

u 4J « C /-> 0) 2 3 CM *«? i-l *-* I I I -H O I I — I I I O —t I ICNI I -* I I —I CN CO O vO ON .Q a <* .—i m CN CO H

4-1

«4-l o

OJ 00 CO X l-l v^ OJ > X CO 4-1 X X CO /— ^-n B^S c .H -' J-l CO u 3 a. oj (X C o • • »H 03 U 03 03 to c c X a (0 0) • U-J 03 3 o o 00| a- i-i 03 03 03 a, c CO CJ 3 OJ-H a. o CO 03 iH CO CO 03 O • a> u -»-N 14 CA CO CO CO JO o. o CO O CO to u o 03 H CJ o CJ CI 03 CO to CO a u CO o rH o CD o J= u o to c V^ •H =3 o Mk3 CD CO T3 O 4-1 4J CO 03 z CO JQ 0) CD o >J O ^*"^ C£ o\ OJ CO 01 3 D. Ctn 5 CO U cj x c/3 >- 2 z z M « -27-

the northeast. Larrea tridentata is lacking in MM 5(1) and Encelia farinosa is represented by a single achene, a possible contaminant. — «, The Marble Mountains 4(2) macrofosssil assemblage, at 9515 +

185- B. P. (GX-6180), is ca. 700 radiocarbon years younger than MM 5(1) and reflects considerable change in the vegetation. Many mesophytic species that are important in MM 5(1) are rare or lacking in this younger assemblage (Table 3), only Yucca schidigera and Brickellia sp. persist in significant quantities. During this ca. 700 year interval

Encelia farinosa apparently became a common plant at this site.

A few fragments of Larrea tridentata indicate that this species may have just arrived at the site.

Marble Mountains 5(2) is the only middle Holocene-age sample from

Locality B and, at 4360 + 210 B.P. (GX-6184; Table 1), it is the only

one to contain Ambrosia dumosa , Cucurbita sp. (gourd) , and Ferocactus acanthodes (Table 3). The extralocal mesophytes, present in the early

Holocene samples, are missing and the current dominants at the site, creosote bush and brittle bush, are the primary constituents of the midden, MM 4(3), at 1680 + 150 B.P. (GX-6181; Table 1), is a creosote bush-brittle bush assemblage quite similar to MM 5(2). Curiously, it lacks white bur sage, gourd, and California barrel cactus and is of unusually low diversity (Table 3).

The Marble Mountains 6, 7, and 9 sites are at the head of steep, south facing talus slopes at ca. 850 m (2790 ft) elevation (Fig. 2).

One of the most diverse assemblages, Marble Mountains 9(1) , is radio- ? carbon dated twice. This replicate dating demonstrates contamination.. -28-

Undifferentiated twigs yielded a radiocarbon age of 10,550 + 210 B.P.

(GX-6189) while associated Neotoma fecal pellets were dated at 7935 +'

260 B.P. (GX-6190; Table 1). The species list from MM 9(1) is pre- 2 sented but it is excluded from any consideration of vegetation succession

at this site. The MM 9(2) macrofossil assemblage is also rejected.

Positioned below MM 9(1) in a stratigraphically complex setting, it 2 contains extralocal mesophytes such as Ephedra cf . viridis , Artemisia

sec. Tridentatae, and Salvia mohavensis associated with a radiocarbon

age of 5520 + 190 B.P. (GX-6191; Table 1). It is most unlikely that

these plants persisted into the middle Holocene at this locality.

The oldest reliably dated samples from Locality C are Marble'

Mountains. 7(1) and MM 9(1).. Contemporaneous radiocarbon dates of

8925 + 360 B.P. (GX-6186) and . 8905 + 265 B.P. (GX-6188) , respectively,

indicate that these assemblages are roughly 600 radiocarbon years

younger than MM 4(2) and 1200 radiocarbon years younger than MM 5(1).

Like the early Holqcene Locality B assemblages, MM 7(1) and MM 9(1).

are dominated by desert shrubs that are found only at higher elevations

today. Haplopappus cuneatus , cf. Haplopappus laricifolius , and Salvia

mohavensis are among the most abundant (Table AA, AB) . The leaves of

.Yucca whipplei , not found in the Locality B middens, are common in

MM 9(1). (Table AB) and occasional in MM 7(1) (Table AA) . Brickellia sp.

flowers are abundant iij MM 7(1) only, while Yucca brevifolia , Coleogyne

ramosissima , and Opuntia acanthocarpa occur only in MM 9(1).. Paradoxi-

cally, MM 7(1) and MM 9(1). contain two woodland species that are lacking

in the older Locality B samples. Juniperus sp. twigs and seeds are 1

-29-

c CO <^v M eC o • U-i 05

CM cs o

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vJD CO 05 4J CO c

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-31-

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occassional in MM 7(1) and abundant enough in MM 9(1). to provide suf-

ficient mass for radiocarbon dating (Table 1). Ribes cf. velutinum '

in MM 9(1) represents an intriguing early Holocene occurence (Table 1 AB)

The- leaves of a sagebrush ( Artemisia sec. Tridentatae) occur in both

MM 7(1) and MM 9(l)j and, while not strictly a woodland species, it is an occurence not replicated by the older Locality B samples.

The xeric desertscrub species that characterize the Locality C vegetation today are rare or lacking in MM 7(1) and MM 9(1)., with the

exception of Encelia farinosa (Table AA, AB) . Brittle bush is the first of the warm desert dominants to appear in the Locality B fossil record.

Such appears to be the case at Locality C as well. Marble Mountains 6, at 7930 +285 B.P. (GX-6185; Tbale 1), is ca. 1000 years younger than

MM 7(1) and MM 9(1). and is a mix of some extralocal desertscrub species and the xeric shrubs that occur at the site today. Larrea tridentata is the most abundant fossil species in MM 6. Salvia mohavensis and cf

Haplopappus laricifolius are common and Opuntia acanthocarpa occassional

extralocals (Table AA) . Juniperus sp. , Artemisia sec. Tridentatae,

Coleogyne ramosissima , Haplopappus cuneatus , Yucca brevifolia , and Y_. yhipplei are lacking. Accompanying the dramatic increase of Larrea in the fossil record, and the demise of most mesophytic species, is the appearance of Peucephyllum schottii and an apparent increase of

Ambrosia dumosa. Two annuals occur in relatively large amounts in MM 6,

Eschscholtzia sp. (California poppy) and Filago sp. (Ta*ble AA) .

Marble Mountains 7(2), at AA75 + 170 B.P. (GX-6187; Table 1),

is the youngest reliably dated Locality C midden sample.- There is little discernable difference between this assemblage and the present vegetation -35-

Both are dominated by Larrea tridentata and Encelia farinosa at the site. ,

with smaller amounts of Ambrosia dumosa, Bebbia juncea , and Peucephyllum

frhottii (Table 4A) . A trace amount of Salvia mohavensis occurs, but this may .well be a contaminant. MM 7(2), as well as the older MM 7(1) assem-

blage, contains the seeds and spines of Ferocactus acanthodes , which does not occur at Locality C today.

The Eureka Valley And Vicinity

The Horse Thief Hills localities (Fig. 3) are unproductive areas for midden sites, the rocks are fractured and metamorphosed with few cavities. This is unsurprising since these localities are within half a kilometer of the Furnace Creek Faultzone. The activity of the fault probably produces an unstable geomorphic setting. However, the cliffs on the northeast border of the Eureka Valley, several kilometers further south (Fig. 3), are a more productive midden area.

Horse Thief Hills Locality A . Horse Thief Hills 1 is a small solu- tion cavity ca. 30 m above the bed of an arroyo tributary to Willow Wash

(Fig. 3). The east sloping talus below the cliff containing the cavity, at 1615 m (5300 ft) elevation, is derived from dolomite of the Bonanza

King Formation. The local plant community is dominated by the Great

Basin shrubs Atriplex conf ertifolia and Chrysothamnus viscidiflorus .

In contrast to the present Great Basin desertscrub community, the Horse

Thief Hills 1 midden contains abundant woodland species. The twigs and seeds of the most common fossil, Juniperus osteosperma (Utah juniper),

14 are C dated at 10,690 + 280 B.P. (GX-6217; Table 5). Symphoricarpos c ^« longiflorus is also abundant while extralocal woodland taxa include, i1 l I i ' ' 'i i i

-36-

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-37-

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-38- besides Utah juniper, Amelanchier utahensis (service-berry), Chamae- batiaria millefolium (fern-bush), Purshia cf. tridentata (bitterbrush)

Ephedra cf. viridis , and Cercocarpus intricatus (Table 6). The last two species occur in restricted mesic habitats within half a kilometer of

HTH 1. Other plants that do not occur at the site today, such as

Artemisia sec. Tridentatae, Prunus cf. andersonii (desert-peach), and

Salvia dorrii, have affinities with mesic desertscrub communities although many are also common in xeric woodlands. Atriplex confertifolia is the only plant common at the site today that is also abundant in the macrofossil assemblage (Table 6).

The Horse Thief Hills 2 site, at 1635 m (5360 ft) elevation, is near a crest overlooking Willow Wash. The site is not as protected as

HTH 1 and the plant community is correspondingly more xeric. Atriplex

confertifolia dominates the local community with Eurotia lanata , Ephedra

nevadensis , and Hecastocleis shockleyi being common. The HTH 2 packrat midden is a partly indurated mass with a radiocarbon date of <200 B.P., confirming its young appearance (GX-6218; Table 5). The most common plants in the midden appear to be the most common in the surrounding vegetation (Table 7).

Horse Thief Hills Locality B . The single midden site at Locality B,

HTH 3, is isolated in the middle of Willow Wash canyon with a less rugged topography than the preceding sites. A west sloping alluvial fan leads away from the rock outcrop, coalescing with the Willows/ash fan ca. 35 m from the midden site. Atriplex confertifolia is the dominant shrub with

Da lea fremontii and Hymenoclea salsola common associates. The latter is )

Table 6. Plants from the Horse Thief Hills 1 site, See Table 2 for an explanation of the symbols.

Horse Thief Hills 1

Sp_e< es (10,690 + 280 B.P.)

Anelanchier titahensis 2 Amsinckia intermedia 1 Aristida sp. (X) Artemisia sec. Tridentatae 3 Astragalus sp. 1 Atriplex confertifolia (X) A Erickellia sp. 1 Cercocarpus intricatus 2 Chamaebat iar ia millefolium 1 Chenopodium sp. 1

Chrysothamnus viscidif lorus ( X_ cf. Chrysothamnus sp. 1 Crypt ant ha sp. 1 Dalea f remontii (X) Ephedra nevadensis (X) Ephedra cf. viridis Eriogonum heermannii (X) Eriogonum sp. 1 Eurotia lanata (X) 2 Fendlerella utahensis (X) Forsellesia nevadensis (X) Forsellesia sp. Galium sp. (X) Hecastocleis shockleyi (X) Hilaria jamesii (X) Juniperus osteosperma 5 Mirabilis multiflora (X)

Mortonia utahensis *l 1 Opuntia basilaris (X)

Opuntia cf . polyacantha 3 Oryzopsis hymenoides (X) 1 Penstemon sp. 1 cf . Prunus andersonii 3

Purshia cf . tridentata 1 Salvia dorrii 1 Scopulophila rixfordii 1 Sphaeralcea sp. 2 _Stanleya cf. pinnata (X) Symphoricarpos cf. longiflorus (X) Tetradymia axillaris (X) Tetradymia cf. canescens (X) T etradymia sp.

N (19) 27 N P (14) 19 VN (100) (74%) 70% IS 35 -40-

Table 7. Plants from the Horse Thief Hills 2 site, See Table 2 for an explanation of the symbols.

Horse Thief Hills 2 S£ecies (<200 B.P.)

Apropyron sp. 1 Artemisia spinescens (X) 2 Atriplex conf ertifolia (X_) 5 Brickellia sp. 1 Chrysothamnus viscidif lorus (X) 2 Ephedra nevadensis (X) 3 Eriogonum heermannii (X) Eurotia lanata (X) 3 Fendlerella utahensis (X) 2 Forsellesia nevadensis 1 Gilia cf. latifolia (X) 2 Hecastocleis shockleyi (X) 2 Hilaria jamesii (X) Lepidium cf. fremontii 1 Menodora spinescens (X) 2 Mirabilis sp. 1 Opuntia basilaris 2 Oryzopsis hymenoides (X) 1 Prunus f asciculata (X) Scopulophila rixfordii (X) 2 Sphaeralcea sp. 1 Stanleya cf. pinnata 1 Symphoricarpos cf. longiflorus (X) 3 Tetradymia axillaris (X) 3

N N (16) 21 Np (13) 16 Nn/N (100) (81%) 76% IS 70 -41-

a disturbance plant normally restricted to washes. Both midden samples

are well indurated but both yield subrecent radiocarbon dates (<200 B.P.;

Table 5). The date on Horse Thief Hills 3(1) is on Neotoma feces

(GX-6219) while the two subrecent dates on HTH 3(2) are on Neotoma feces

and undifferentiated twigs (GX-6221 and GX-6220, respectively; Table 5).

Both midden assemblages agree with the present plant community in that

they are dominated by Atriplex conf ertifolia with common Dalea fremontii

and Ephedra sp. (probably E_. nevadensis ; Table 8). However, both samples

diverge from the modern vegetation in other aspects. Opuntia echinocarpa , present at the locality but not within 30 m of the midden site, is abundant in HTH 3(1) and probably represented by abundant Opuntia sp. spines in HTH 3(2) (Table 8). HTH 3(1) contains no Hymenoclea salsola but

does contain the frequent remains of Lycium sp. and Menodora spinescens , both absent at the locality today. Hymenoclea salsola and Menodora spinescens are occassional in HTH 3(2) and Lycium sp. represented by rare fragments (Table 8). Larrea tridentata and Yucca brevifolia are scattered at Locality B, although none occur at the HTH 3 site. The remains of these two plants are found only in the HTH 3(2) assemblage.

The Eureka View Locality . Eureka View is a network of midden sites on rugged exposures of limestone and dolomite rising above Eureka

Valley (Fig. 3). The desertscrub communities on the dark, south to west facing calcareous slopes are diverse. All the sites occur within

100 m of each other and offer a time transgressive serfes of fossil oiddens from one locality and one community type, dominated by Atriplex confertifolia and Larrea tridentata. -42-

Table 8. Plants from the Horse Thief Hills 3 site, See Table 2 for an explanation of the symbols.

Horse Thief Hills 3 HTH 3(1) HTH 3(2) Species (<200 B.P.) (<200 B.P.)

Acamptopappus sphaerocephalus (X) Amsinckia tesselata Aristida sp. (X) Artemisia spinescens Atriplex conf ertifolia (X) Chaenactis sp. Chrysothamnus viscidif lorus (X) Dalea fremontii (X) Ephedra nevadensis (X) Ephedra sp. Eriogonum fasciculatum (X) E. heermannii (X) E. inflatum (X) Eriogonum sp. 1 Erioneuron pulchellum (X) Eurotia lanata (X) 1 Grayia spinosa (X) Hapiopappus laricifolius (X) cf . Hapiopappus laricifolius 2 Hilaria jamesii (X) Hymenoclea salsola (X) 2 Larrea tridentata Leucelene ericoides (X) Lycium sp. 2 Menodora spinescens 3 Opuntia basilaris (X) (h echinocarpa (X) 4 Opuntia sp. Oryzopsis hymenoides (X) 1 Prunus fasciculata (X) Scopulophila rixfordii Sphaeralce a sp. (X) Stanleya cf. pinnata (X) Tetradymia axillaris (X) T. canescens (X) Yucca brevifolia (X)

M (25) 13 1? N 10 13 p (19) Np/N (100) (76%) 77Z 762 I? •» A7 48 -43-

Eureka View 5, at ca. 1430 m (4690 ft), is the lowest elevation site and provides the two oldest macrofossil assemblages. It is about

10 m above the head of the alluvial fan that slopes away to the Eureka

Valley bajada. EV 5A contains abundant Utah juniper twigs and seeds,

1/+ C dated at 14,720 + 530 B.P. (GX-6230; Table 5). Other extralocals

in this late Wisconsin woodland assemblage include Ephedra cf . viridis ,_

Artemisia sec. Tridentatae, Forsellesia nevadensis , Salvia dorrii,

Hecastocleis shockleyi and, remarkably, Pinus cf. f lexilis (limber pine; Table 9). Atriplex confertifolia is the only common desertscrub species in the EV 5A sample and the only fossil plant that is well- represented in the modern vegetation. Eureka View 5B, at 8330 + 250

B.P. (GX-6231), is the only early Holocene macrofossil assemblage and documents an apparent decline in species diversity at this site

(Table 9). The woodland and Great Basin desertscrub species, common in the late Wisconsin EV 5A sample, are lacking in EV 5B along with most of the species present at the site today. Shadscale is the abundant fossil in EV 5B and Ephedra sp. is common with the extralocal Lepidium cf. fremontii (Mojave peppergrass) occassional (Table 9).

The Eureka View 4 site, located ca. 20 m west of the EV 5 site, contains the next oldest suite of middens (Table 5). EV 4(2), at

6795 +190 B.P. (GX-6228 on cf. Atriplex confertifolia wood), is dominated by shadscale and contains common Haplopappus brickellioides and

Encella virginensis (Table 9). The latter does not occur at the site tQday. Opuntia basilaris , Lycium sp., and Dalea fremontii are other frequent extralocals in the EV 4(2) assemblage. Ambrosia dumosa first

•PPears in this sample. * ( i H

-44-

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Eureka View 4(1) and Eureka View 4(3) have radiocarbon dates that are contemporaneous within one standard deviation and are ca. 1300 years

K, younger than EV 4(2). A date of 5595 + 210 B.P. (GX-6227) for EV 4(1) is slightly older than a date of 5435 + 220 B.P. (GX-6229; Table 5) for

EV 4(3). Atriplex confertifolia is the primary constituent of both assemblages with Encelia virginensis and Opuntia basilaris being abundant extralocals in EV 4(3) and common in EV 4(1) (Table 9). EV 4(1) is the oldest midden sample to contain the spines of Opuntia sp.

(perhaps 0. echinocarpa ) . Erioneuron pulchellum , which made its earliest appearance in EV 4(2), is occassional in both EV 4(1) and EV 4(3).

Dalea fremontii , common in EV 4(2), is represented by only rare occur- ences in these two younger samples. A few fragments of Larrea tridentata occur in the youngest assemblage, EV 4(3) (Table 9). A single twig of Juniperus osteosperma in EV 4(1) is a presumed contaminant.

The Eureka View 2C macrofossil assemblage, dated at 3930 + 180 B.P.

(GX-6225; Table 5), is ca. 1500 radiocarbon years younger than EV 4(3) and the youngest middle Holocene sample. Like the preceding assemblages,

EV 2C is dominated by Atriplex confertifolia and contains common Opuntia

basilaris . It contains two other cacti, Opuntia sp. and Echinocactus

polycephalus (Table 9) . This is the oldest midden sample to contain

cotton-top barrel cactus as well as appreciable quantities of Larrea

trldentata and Peucephyllum schottii . Desert spruce was not found in

the census of the modern vegetation. The nearest population known to me

occur* on south facing slopes in lower Hanging Rock Canyon at ca. 1370 m

(ca. A500 ft) elevation, 12 km (7 mi) to the southwest in the Last

Chance Range. ,

-48-

Eureka View 1, at 1510 m (4950 ft), is the highest elevation site.

At 2635 + 140 B.P. (GX-6222; Table 5) it is the oldest assemblage with an array of species closely resembling the modern community (Table 9).

The twigs and leaves of creosote bush and shadscale are abundant.

Viguiera reticulata and Ambrosia dumosa are both common in this sample, the former appearing in the fossil record for the first time. Echino-

cactus polycephalus , Lepidium cf . fremontii , and Opuntia echinocarpa are occassional in EV 1, but they do not occur at the site today.

A radiocarbon date of 1580 + 140 B.P. (GX-6224) for EV 2B places it ca. 2400 radiocarbon years younger than EV 2C, its companion midden from the same site (Table 5). The most important plants in EV 2B are the dominant species in the present vegetation, although it contains

common beavertail prickly pear, Opuntia sp. (probably Ch echinocarpa )

Peucephyllum schottii , and Dalea fremontii , desertscrub species not pre-

sent at the site today. (table 9). Amphipappus fremontii , a common plant at the Eureka View Locality today, makes a belated appearance in the fossil record in EV 2B.

s' Eureka View 3 is the youngest radiocarbon dated midden from this locality. A C date of 535 + 150 B.P. (GX-6226; Table 5) is associated with a macrofossil assemblage containing abundant Larrea tridentata

and common Atriplex confertifolia (Table 9) . It contains only two species that cannot be found at the site today, Opuntia sp. and Peucephyllum

schottii . The last midden from this series, EV 2A, ha* no radiocarbon date. This sample of Larrea twigs was lost at the radiocarbon lab. Its similarity to other late Holocene macrofossil assemblages suggests that

*t is less than 3500 years old.. -49-

The Regional Fossil Record

Late Quaternary age packrat middens have been found throughout- the

Southwest, from Mexico as far south as Lat. 26 N (Van Devender 1978),

to Trans-Pecos Texas and southern New Mexico (Van Devender et al. 1978,

1979; Van Devender and Everitt 1977; Wells 1966), west to the inland

flanks of the Sierra Nevada (Van Devender and Spaulding 1979) , north to east-central Nevada (Thompson 1978). Fossil packrat middens from within the California Desert Conservation Area and adjacent Nevada and

Arizona provide a biogeographic and temporal framework from which to assess the records presented up to this point. The latest Wisconsin

(14,000 B.P. to 11,000 B.P.), early Holocene (11,000 B.P. to 7,800 B.P.), and then middle and late Holocene (7,800 B.P. to present) midden records will be dealt with in that order. The presentation is further separated

into low ( < 1000 m; < 3280 ft) and intermediate (1000 m to 1800 m;

3280 ft to 5900 ft) elevation sites. Ancient packrat middens have been recovered from elevations as great as 2400 m (7870 ft) in Mojave Desert mountain ranges (Van Devender and Spaulding 1979).

The Latest Wisconsin At Low Elevations . Five midden sites from

the Kofa (Lat. 33°20' N, Lon. 114°00' W) and New Water Mountains (Lat.

33 36' N; Lon. 113 55' W; Fig. 1) of Yuma County, Arizona were studied

by Van Devender (1973). The sites range in elevation from 550 m to

860 m (1800 ft to 2820 ft) in paloverde-saguaro (Cercidium micro phyllum-

Cereus giganteus ) vegetation more characteristic of the Arizona Uplands

•ection of the eastern Sonoran Desert (Shreve and Wiggins 1964). Palo-

v erde-saguaro communities are zoned at higher elevations in the mountains

t0 the east of the Colorado River Valley (Brown and Lowe 1977). The . .

-50-

fossil middens provide evidence for a xeric juniper woodland with lesser

amounts of shrub live oak ( Quercus turbinella ) . Only the oldest mid^deh from the highest elevation site (Burro Canyon #1(1)), C dated at

14,400 + 330 B.P. (A-1315), contains abundant Pinus monophylla . Other woodland plants such as Rhus trilobata (skunk-bush), Berberis sp.

(barberry), and northern desert species like Thamnosma montana (turpentine-

broom) , Yucca brevifolia , and Atriplex conf ertifolia are present in the middens. The data also suggests the absence of typical, frost-sensitive

Sonoran Desert species including Cercidium microphyllum and Cereus giganteus (Van Devender 1973)

Packrat middens from the Newberry Mountains in extreme southern

Nevada (Lat. 35°15' N, Lon. 114°37' W; Fig. 1) were studied by Leskinen

(1975) . A midden sample from 850 m (2790 ft) elevation in modern creosote bush vegetation documents an ancient pinyon-juniper-live oak woodland. The oak species are identified as Quercus chrysolepis and G> dunnii and are associated with a radiocarbon date of 13,380 + 300 B.P. (GaK-1988)

The most comprehensive study of Mojave Desert paleocommunities to date is that by Phillips (1977). His Lower Grand Canyon study area

(Lat. 36°06' N; Lon. 113°55' W; Tig. 1) encompasses 25 midden sites ranging from 440 m to 675 m (1440 ft to 2215 ft) elevation on calcareous

slopes of the Inner Gorge. The present open slope desertscrub communi-

ties are often dominated by Larrea tridentata and Encelia farinosa .

Other important species include Fouquieria splendens (ocotillo) , Echino-

cactus polycephalus , and Viguiera deltoidea (desert marigold) . The

The latest Wisconsin of the Lower Grand Canyon, corresponding to Phillips'

v«getational Stage III, was characterized by a woodland dominated by . . ,

-51-

juniper (as Juniperus sp.) and single-leaf ash ( Fraxinus anomala )

Important associates include Atriplex confertifolia , Symphoricarpos sp.

Mortonia scabrella (tick-weed) , Nolina microcarpa (bear-grass) , Prunus

fasciculata , and Yucca baccata (banana yucca) . Various Mojave Desert plants that do not occur in the Inner Gorge today were widespread in-

cluding Coleogyne ramosissima , Yucca brevifolia , and Salvia dorrii

(Phillips 1977). Only one high elevation midden from a north slope records a pinyon-juniper woodland. Desert Almond //10A at 635 m (2080 ft) contains both Pinus monophylla and P_. edulis (Rocky Mountain pinyon) associated with a radiocarbon date of 12,650 + 380 B.P. (A-1720;

Phillips 1977).

The Owl Canyon midden site is on the southeastern edge of the

Amargosa Desert (Lat. 36 25' N, Lon. 116°15 f W; Fig. 1), immediately

outside the CDCA in the low hills east of Ash Meadows , Nevada (Mehringer and Warren 1976). Creosote bush and white bursage are widely scattered on west facing calcareous slopes at ca. 790 m (2600 ft) elevation.

The macrofossil assemblage contains abundant Juniperus osteosperma and

Agave utahensis (Utah agave), the former yielding a "" C date of

13,150 + 500 B.P. (1-4237; Mehringer and Warren 1976). Other extralocal

perennials that are common in this fossil midden include Ribes cf yelutinum and Petrophytum caespitosum (Table 10)

Few macrofossil records date from 14,000 B.P. to 11,000 B.P. at

low elevation sites within the CDCA. A midden from 730 m (2400 ft)

•levation in the Turtle Mountains of the Eastern Colorado Subdivision

(Ut. f 34°15 N, Lon. 114°50' W; Fig. 1) contains both pinyon and juniper.

C date of 13,900 + 200 B.P. is reported for this assemblage but a

-52-

Table 10. Plants from the Owl Canyon 1 midden, Amargosa Desert, Nye

County, Nevada, ca. 790 m (ca. 2600 ft) elevation. Radiocarbon

'' age: 13,150 + 500 B.P. (1-4237) on Juniperus osteosperma

(Mehringer and Warren 1976) . See Table 2 for an explanation of the symbols.

Relative Species abundance

Agave utahensis 4

Ambrosia dump s

Artemisia sec. Tridentatae 1

Atriplex confertifolia 2

Chrysothamnus cf . viscidiflorus 2 cf. Eriastrum sp. 1

Eriogonum heermannii 1

Gutierrezia cf . microcephala cf . Haplopappus nanus 1

Juniperus osteosperma 5

Larrea tridentata 1

Opuntia sp. 1

Petrophytum caespitosum 3

Ribes cf . velutinum 3

Salvia dorrii 1

Scopulophila rixfordii 1

Sphaeralcea sp. 1

Symphoricarpos cf . longiflorus 1

N - 18; N =18 P .

-53-

few species are recorded as associates (Wells and Berger 1967). The

o * Whipple Mountains of eastern San Bernadino County (Lat. 34 16 f N, Lon.

114°25 I W; Fig. 1) have yielded a number of ancient packrat middens

(Van Devender 1977b, King and Van Devender 1977; Mead et al. 1978) but the bulk of the data from these sites remains unpublished. The Redtail

Peak #5 midden, radiocarbon dated at 12,960 + 210 B.P. (A-1666) , is the* only midden to contain both juniper and single-needle pinyon. At an elevation of only 510 m (1670 ft) it is the lowest altitude record of Ice Age pinyon- juniper woodland (Van Devender and Spaulding 1979)

Other associates of the latest Wisconsin woodland in the Whipple Moun-

tains include Yucca brevifolia , Y_. whipplei , Nolina bigelovii , and

Cercocarpus intricatus (Mead et al. 1978; Van Devender and Spaulding 1979).

The Latest Wisconsin At Intermediate Elevations . Two packrat midden sites in the South Central Mojave Subdivision of the CDCA document xeric juniper woodlands in the Lucerne Valley and on Ord

Mountain, north of the San Bernadino Mountains (Fig. 1). The fossil midden from the north side of Ord Mountain at an elevation of 1220 m

(4000 ft; Lat. 34°40' N, Lon. 115°50' W) contains abundant Juniperus

14 psteosperma and Purshia glandulosa (cliff-rose) associated with a C date of 11,850 + 550 (UCR-149; King 1976 a,b). Pinus monophylla occurs in the midden. Larrea tridentata and Haplopappus cooperi are the dominant shrubs at the site today while Yucca schidigera and Y_. brevifolia

are common (King 1976 a,b) . None of these species were noted in the oacrofossil assemblage. Southwest of Ord Mountain, the Lucerne Valley bidden at 1010 m (3300 ft; Lat. 34°35* N; Lon. 115°50' W) elevation -54-

provides a complex sequence of macrofossil assemblages (King 1976 a,b).

The two oldest samples are radiocarbon' dated at 12,100 + 400 B.P.

(UCR-181; Samp. #13) and 11,100 + 420 B.P. (UCR-187; Samp. #10) and

contain abundant juniper but no pinyon. The present xeric desertscrub

vegetation is dominated by Larrea tridentata and Ephedra californica .

The Scodie Mountains lie on the western edge of the South Western

Mojave Subdivision of the CDCA (Lat. 35°36' N, Lon. 117°57' W; Fig. 1),

forming part of the southern terminus of the Sierra Nevada. Three midden

samples from the Robber's Roost area at 1130 ra (3710 ft) elevation are

radiocarbon dated from 13,800 + 400 B.P. to 12,820 + 400 B.P. (A-1763

and A-1762, respectively; Van Devender and Spaulding 1979). They docu-

ment the occurence of a pinyon-juniper woodland in what is now creosote

bush desert. A detailed analysis has yet to be published but the middens

contain a diverse fossil flora including Cercocarpus ledifolius , Yucca

brevifolia , Quercus turbinella , Purshia glandulosa , and Artemisia

tridentata (Van Devender and Spaulding 1979).

Numerous ancient packrat middens have been collected in the Sheep

Range of southern Nevada (Fig. 1; Spaulding 1977; Van Devender and Spaul-

ding 1979). However, only one midden from intermediate elevations is

of latest Wisconsin age. The Penthouse 2 site, at 1580 m (5180 ft)

elevation, is at the crest of a limestone ridge in the southeastern Sheep

Range (Lat. 36°28' N, Lon. 115°15' W) . The xeric east slope about the

site supports a mixed Mojave desertscrub community dominated by Gutierrezia

l_arothr ae (snakeweed) , Ephedra torreyana , Coldenia canescens , and Stipa

a rida» The Penthouse 2(2) macrofossil assemblage is radiocarbon dated -55-

at 11,550 + 150 B.P. (A-1774) and contains abundant Utah juniper and

Forsellesia nevadensis (greasebush) . The remains of Pinus monophylla

and Ephedra viridis are. common while Ceanothus greggii (Mojave buckbrush)

and Prunus fasciculata occur occasionally (Table 11).

Wells and Berger (1967) report a midden site from 1280 m (4200 ft)

elevation in the Funeral Range in the Eastern Mojave Subdivision of

the CDCA (ca. Lat. 36°50' N, ca. Lon. 117°05' W; Fig. 1). A woodland •

assemblage with abundant Juniperus osteosperma and Prunus fasciculata

14 and common Symphoricarpos longif lorus is associated with a C date of

11,600 + 160 B.P. . No pinyon was found although the presence of other

mesophytes, such as Chamaebatiaria millefolium , Fraxinus anomala, and

Cercocarpus ledifolius , is extraordinary in this arid range in the

Death Valiey region. Another ancient midden from 1250 m (4100 ft)

elevation on Mercury Ridge in the Nevada Test Site of south-central

Nevada (ca. Lat. 36 40' N, ca. Lon. 115 50' W; Fig. 1) contains abundant juniper but no pinyon. The radiocarbon date for this low diversity midden sample is 12,700 +200 B.P. (Wells and Berger 1967).

The Early Holocene At Low Elevations . Fossil records of desert-

scrub vegetation from the Wellton Hills in Yuma County, Arizona are

reported by Van Devender (1973; 1977b). These low hills at the

north end of the Lechuguilla Desert (Lat. 32°36' N; Lon. 114°11* W;

Fig. 1) are surrounded by sparse creosote bush-white bursage vegetation.

The Wellton Hills //l midden at 160 m (530 ft) elevation is dated by

two statistically equivalent radiocarbon ages run on Larrea tridentata

(A-1407) and extralocal Ephedra nevadensis (A-1406; Van Devender 1973; s * 1 1 1 —t ) i 1

-56-

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-59-

Mead et al. 1978). They yield an average date (Long and Rippeteau 1974) of 10,665 + 325 B.P. Ambrosia dumosa is also present while other extra-

local species, besides Ephedra , are Encelia frutescens (common)

Brickellia atractyloides , and Holacantha emoryi (both rare) . Extralocal shrubs are the most important component of the Wellton Hills //l midden although it contains the plants that dominate the vegetation today.

Other dates on extralocal Ephedra nevadensis from the Wellton Hills are

8750 + 320 B.P. (A-1399, WH #2) and 8150 + 320 (A-1364; WH #5) while creosote bush fossils yield dates of 7950 + 370 B.P. (A-1400, WH #2) and

6600 + 370 B.P. (A-1365, WH //5; Van Devender 1973, 1977b; Mead et al. 1978)

An early Holocene midden from 245 m (800 ft) elevation on Picacho

Peak, Imperial County, California (Fig. 1) contains a desertscrub assemblage radiocarbon dated at 8650 + 280 B.P. (Van Devender and Spaulding

1979). Of the common species in the midden, only Ephedra nevadensis is lacking in the modern flora (K.L. Cole, pers. comm.). This contrasts with the early Holocene Marble Mountains desertscrub assemblages which are dominated by extralocal plants.

Other packrat midden sites that date from 11,000 B.P. to 7800 B.P.

document the occurence of juniper woodland (Van Devender 1977b). Ancient

oiddens from the Lower Grand Canyon in Arizona (Fig. 1) provide an ela-

borate record of this period (Phillips 1977). They contain abundant

woodland species although Phillips (ibid.) notes a progressive decrease

*a mesophytic plants and a corresponding increase in xecic species through

the early Holocene. In this area, many of the desert shrubs destined to

dominate the middle and late Holocene vegetation were apparently present

<^ring the preceding woodland phase. Phillips (1977, 87-88) summarizes :

-60-

this phenomenon in the Lower Grand Canyon thusly ( parentheses mine)

"The Stage III and Stage IV (latest Wisconsin and early .,

Holocene, respectively) changes in vegetation involved

not a replacement of woodland by desert from other areas,

but a gradual decrease in abundance and number of wood-

land species and a relative increase in importance of

desert species, most of which were already there."

Fouquieria splendens is one of the desert plants that apparently did not

enter the area until after the close of the early Holocene.

Fossil sites from 365 m (1200 ft) to 520 m (1700 ft) elevation

in the Whipple Mountains of the Eastern Colorado Subdivision of the CDCA

(Fig.* 1) document early Holocene juniper woodland (Van Devender 1977b;

King and Van Devender 1977; Mead et al. 1978). Associated extralocal

species include Salvia mohavensis , Nolina bigelovii , Yucca brevifolia ,

and Y_. whipplei . A complete analysis of these sites has yet to be pub-

lished. Extensive early Holocene juniper woodland in the Whipple Moun-

tains contrasts with the fossil record of desertscrub vegetation from

the higher elevation Marble Mountains to the northwest (Fig. 1).

Early Holocene Midden's From Intermediate Elevations . A packrat midden from modern creosote bush desert at 850 m (2790 ft) elevation

in the Newberry Mountains of extreme southern Nevada contains the fossils

chrysolepis of Juniperus sp. , Quercus dunnii , and perhaps (£. (Leskinen

1975). A radiocarbon date on oak acorns from this assemblage is 9500 +

240 B.P. (A-1077).

Further north in Nevada the Sheep Range and several small ranges

in the southern Nevada Test Site have early Holocene midden records (Fig.

1). The Basin Canyon 2 site, in mixed Mojave desertscrub vegetation on -61-

36°43' the west side of the Sheep Range (Lat. N, Lon. 115°15* W) , is at

1630 m (5340 ft) elevation, Gutierrezia sarothrae , Artemisia bigelovii ,

Fallugia paradoxa , and Yucca brevifolia are common today on this northwest - facing limestone slope. The Basin Canyon 2B midden, radiocarbon dated at 19,200 + 580 B.P. (A-1741), provides a full glacial perspective

with an assemblage containing abundant Juniperus osteosperma , common

Atriplex conf ertifolia , and some montane species (Table 11). The early

Holocene BC 2A midden still contains abundant extralocal juniper, dated at 9365 + 320 B.P. (UCR-727). However, the other common fossil plants,

Prunus fasciculata , Atriplex canescens (four-wing saltbush) , and Symphori-

v carpos longiflorus , persist at the site today. Atriplex canescens appears more abundant in BC 2A than in the present community and Gutierrezia saro-

thrae , the most common plant at the site today, is lacking in BC 2A

(Table 11).

Penthouse 1(1), at an elevation of 1600 m (5240 ft) on a xeric lime-

14 stone ridge in the southeastern Sheep Range, has a C date of 8100 + 120

B.P. (A-1771). Gutierrezia sarothrae is the most abundant species in the present community and the midden sample. Ph 1(1) contains extralocal species that can be grouped into. three types. The occasional twigs of

Juniperus osteosperma and Forsellesia nevadensis , among others (Table 11), represent plants that were important in the preceding late Wisconsin but

•re now restricted to woodland communities some distance away. A second group is represented by Coleogyne ramosissima and Buddie ja utahensis , desert shrubs that occur nearby but only in more mesic habitats. The third and most unusual group is represented by the remains of Larrea illdentata and cf. Lycium andersonii (Table 11). Neither species occurs

n *«r the site today and their presence in the Ph 1(1) assemblage represents -62-

a displacement upward relative to their current position on the lower,

warmer talus slopes in this part of the Sheep Range.

Wells and Berger (1967) discuss a suite of six early Holocene fossil

assemblages from the mountain ranges in the southern Nevada Test Site from

Lat. 36°37' N to Lat. 36°55' N (ca. Lon. 115°50' W; Fig. 1). The midden

sites range from 1100 m to 1830 m (3610 ft to 6000 ft) elevation and record

juniper woodland in what is now desertscrub vegetation. An exception is

the Spotted Range //2 midden (9450 + 90 B.P.; 1550 m, 5080 ft) which con-

tains common Pinus monophylla . Other early Holocene variations include

the Ranger Mountains //2 midden (10,100 + 160 B.P.; 1100 m,v3610 ft eleva-

tion) in which Atriplex confertifolia shares dominance with juniper and

Mercury Ridge #1 (9000 + 250 B.P.; 1390 m, 4560 ft elevation) in which

Cowania mexicana exceeds juniper in abundance (Wells and Berger 1967).

The Spotted Range #2 occurence of Pinus monophylla is anomalous. The ma-

crofossil record from throughout the Southwest indicates that pinyon was

eliminated from low elevations by 11,000 B.P. (Van Devender and Spaulding

1979).

The Lucerne Valley midden in the South Central Mojave Subdivision of

the CDCA at 1010 m (3300 ft; Fig. 1) elevation records a juniper woodland 14 in current creosote bush desert (King 1976 a,b). Two samples with " C dates

of 8300 + 780 B.P. (UCR-186) and 7800 + 350 B.P. (UCR-249) contain common

Juniper and Encelia frutescens but no Larrea tridentata* is reported. One

•ample also contains common Purshia glandulosa (King 1976 a,b) . The Lucerne

v*lley midden substantiates the early Holocene record from Negro Butte, ca.

5 km northwest of King's site at 1070 m (3510 ft) elevation (Wells and

^tger 1967). The Negro Butte midden contains abundant Juniperus osteosperma

* nd Purshia glandulosa as well as some Yucca brevifolia (ibid.) . The midden

i -63-

samples discussed by King (1976 a,b) contain a yucca identified only to the generic level.

Middle and Late Holocene Packrat Middens . Other than the Marble

Mountains and Eureka Valley fossil sites, only middens from the Lucerne

Valley (King 1976 a,b) and the Sheep Range provide vegetational data on this time period. The Sunset Cove (5880 +250 B.P.; UCR-134) and Lucerne

Peak (5800 + 250 B.P.; UCR-135) middens from above Lucerne Valley at 970 m

(3080 ft) and 1100 m (3610 ft) elevation, respectively, suggest some vegetation change over the last 5800 years (King 1976 a,b). The Lucerne Peak

midden contains little Larrea tridentata , in contrast to its present abun-

dance at the site, and Encelia frutescens , common in the midden, is not noted in the modern species list. The Sunset Cove midden contains Opuntia basilaris and Opuntia sp. which are both lacking in the modern flora (King

1976 a,b). Ambrosia dumosa and Opuntia sp. are not noted for the present vegetation of the Lucerne Valley midden site but it is common in midden samples ranging in age from ca. 4300 B.P. to ca. 3600 B.P.

Middle to late Holocene records in the Sheep Range span the last 5200 radiocarbon years. The Desert View midden site, on the range T s west flank at 1810 m (5940 ft) elevation, lies near the upper limit of blackbrush de- fiertscrub. A single pinyon tree grows in a crack above the midden site today but the lower limit of woodland lies at about 1950 m (6400 ft) elevation, ca. h km away. The Desert View macrofossil assemblage is radiocar-

bon dated at 5210 + 95 B.P. (WSU-2044) on extralocal Juniperus osteosperma .

Prickly pears ( Opuntia phaeacantha and 0_. polyacantha) in the Desert View

•ample do not presently occur on the bedrock slopes near the site (Table 12).

The Willow Wash 2 packrat midden site (not to be confused with the

Ulow Wash at the northern Eureka Valley) is in the bottom of a canyon at < « t < ii i «i > 1i

-64-

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1540 m (5040 ft) elevation in the southeastern Sheep Range. The site is surrounded by diverse ripparian desertscrub vegetation. The Willow Wash 2 macrofossil assemblage is radiocarbon dated at 4125 + 85 B.P. (WSU-2039) and is dominated by the species that presently occur at the site (Table

12). Four taxa that are in the midden but not at the site now are Chilop-

sis linearis (desert willow), Brickellia sp., Opuntia basilaris , and Agave

utahensis (Table 12) . Two frost damaged desert willows grow at the mouth of the canyon, ca. 1 km downstream at 1470 m (4880 ft) elevation and are

the only known representatives of this tree from the main mass of the Sheep

Range. ^

The Canyon Two midden at 1800 m (5920 ft) elevation, again in the

southeastern Sheep Range, provides assemblages that are radiocarbon dated

at 3310 + 100 B.P. (A-1531, top layer) and 1990 + 70 B.P. (A-1532, bottom

layer; Spaulding 1977). Like Desert View, the site is near the present

lower limit of woodland which extends down to 1980 m (6500 ft) elevation.

The midden samples contain the desertscrub species that dominate the modern

vegetation but Juniperus osteosperma occurs in both and is one of the most

Important speices in the younger, bottom layer (CT(2)), which also contains

Pinus monophylla (Table 12). The presence of these conifers at the Canyon

Two site suggests a slight downward displacement of the lower limit of wood-

land at ca. 3300 B.P. with a larger displacement occuring by ca. 2000 B.P.

(Spaulding 1977).

The Basin Wash midden site, from the west flank of the Sheep Range

»t 1650 m (5420 ft) elevation, is near the bottom of an arroyo in a rip-

parian desertscrub community. It contains the shrubs currently important

*t the site as well as abundant Utah Juniper and common single-needle pin- 14 " yon (Table 12). Juniper twigs and seeds yield a C date of 3520 +100 -69-

B.P. (WSU-1854). Both conifers occur sporadically in the vicinity of the site. Modern midden debris from the Basin Wash site reflects their

rarity (Table 12) . The present lower limit of woodland in this part of the Sheep Range lies at ca. 1790 m (5860 ft) elevation, about 1.5 km west of the Basin Wash site. Like the Desert View and Canyon Two middens, the data from Basin Wash suggests a depression of the lower woodland boundary by at least 100 m.

The Sawmill Canyon middens on the east flank of the Sheep Range at 1755 m (5760 ft) elevation provide late Holocene macrofossil records from within the current pinyon-juniper woodland. Located immediately east of the highest part of the Sheep Range, the site is im an area where forest and woodland vegetation extends to unusually low elevations.

The Sawmill Canyon 1A and ID macrofossil assemblages are radiocarbon dated at 3530 + 90 B.P. (WSU-2045) and 3040 + 75 B.P. (WSU-2041), respectively. They show no appreciable departure from the current vegetation (Table 12).

t DISCUSSION

The last 14,000 years have seen the decline of Ice Age plant asso-

ciations and the ultimate demise' of many mesophytic species in the

interior basins of western North America. The concommitent expansion

of xerophytic plants led to the development of the present interglacial

plant communities. The fossil packrat midden record is complex and

reflects changes that are, to some degree, unique to each area. The

tlaing of the retreat of mesic plants, as well as the advent of xeric

Pecies, varies from place to place and from plant to plant. Some -70- desert shrubs apparently migrated considerable distances in post-Wisconsin times, while others were present in the preceding late Wisconsin vegetation. v

Vegetation Dynamics

The Marble Mountains . The early Holocene vegetation of this area

(Fig. 1,2) was dominated by mesic desertscrub plants. The fossil record at Marble Mountains 2 (465 m elevation) implies vegetation in which

Ephedra cf . californica and Lycium sp. were important. Larrea tridentata

and Encelia farinosa , the dominant taxa today, are not in the fossil sample. At higher elevations, from 840 m to 900 m elevation, contemporaneous early Holocene middens record diverse, mixed desertscrub vegeta-

tion in what is now creosote bush-brittle bush desert. Salvia mohavensis ,

Yucca schidigera , cf . Kaplopappus laricifolius , and H. cuneatus are among the most common species in these shrub assemblages (Fig. 4). None of these early Holocene plants occur near the fossil sites today, although most may be found in higher elevation (ca. 1400 m) desertscrub communities with similar aspect.

An exception to the early Holocene record of desertscrub vegetation is found in two Marble Mountains middens, both dated at ca. 8900 B.P.

They are the only samples to contain Juniperus sp., Yucca brevifolia , and

Y. whipplei (Fig. 4) and represent either a reversal in the early Holocene trend toward a xeric climate, or a stochastic, developmental phase of vegetation not directly related to any forcing climatic-factor. The

Juniper represented in these two assemblages may be either the mesic

Juniperus osteosperma , or the xerophytic £. californica (California juniper). The latter is found In higher elevation desertscrub vegetation -71-

Figure 4. The estimated relative abundance of selected plants in

Neotoma midden samples from Marble Mountains Locality B and C

v plotted against radiocarbon age. 5, very abundant; 4, abun-

< dant; 3, common; 2, occasional; 1, rare; closed circle, one

or two fragments, a possible contaminant. IO

Ji • • I

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within the CDCA today, associated with many of the plants found in these early Holocene middens. Reversals in the early Holocene drying-warming trend have yet to be consistently defined in the Southwest. Mehringer

(1967) suggests that such a mesic phase occured between ca. 8500 B.P. and ca. 8,000 B.P. based on pollen stratigraphic data from Tule Springs in the Las Vegas Valley of southern Nevada.

The minor role played by woodland species in the early Holocene vegetation of the Marble Mountains is surprising. The three oldest macrofossil assemblages, dating from ca. 10,500 B.P. to ca. 9500 B.P., are virtually devoid of any woodland indicators (Table 2,3). However, contemporaneous early Holocene woodland is well documented v in the eastern Colorado Subdivision of the CDCA as well as the Mojave Desert of Arizona (Fig. 5). Woodland records range upward from elevations as low as 365 m (1200 ft) on igneous substrate of the Whipple Mountains

(King and Van Devender 1977) and 460 m (1520 ft) on limestone of the

Lower Grand Canyon (Phillips 1977). Only middens from the low elevation Wellton Hills sites in Yuma County, Arizona and Picacho Peak in

Imperial County, California contain additional evidence of early Holo- cene desertscrub communities (Van Devender 1973, 1977b; Van Devender

and Spaulding 1979; Fig. 1,5).

The conclusion appears inescapable that, during the early Holocene,

the vegetation of the Marble Mountains was desertscrub while sites of

•lmilar age and aspect to the east supported xeric woodland. However,

• contributing cause to these discrepancies may lie in a bias introduced

ty midden sampling technique. Extralocal woodland species have been

•aployed as a field index of antiquity. In the Whipple Mountains and — r

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Figure 5. Packrat midden samples from the CDCA and adjacent regions

plotted against radiocarbon age and elevation. Open circles

indicate desert scrub assemblages, closed circles juniper

woodland, and closed squares- pinyon-juniper woodland. The

broad dashed line indicates the present lower limit of

woodland in the Sheep Range of southern Nevada. The short

dashed line, the approximate lower limit of woodland in

the Granite Mountains, ca. 5 km north of the Marble Mountains. -74-

Lower Grand Canyon only middens containing obvious extralocal woodland taxa were chosen for analysis. The consequence is sampling biased,

against ancient middens that may contain records of desertscrub vegetation. Sampling bias cannot account for differences between the Marble Mountains record and contemporaneous woodland samples from the systema- tically excavated Lucerne Valley midden (King 1976 a,b; Fig. 5). The

Lucerne Valley site is at ca. 1010 m (3310 ft) elevation, ca. 110 m (400 ft) higher than the Marble Mountains sites, and apparently lay above the lower limit of woodland during the early Holocene.

Questions of sampling aside, it is plausible that an early Holocene

east-west gradient of increasingly xeric vegetation existed in the southern Mojave and eastern Colorado Deserts. Low elevations may have

supported xeric juniper woodland in the Colorado River Valley but only desertscrub vegetation further west. Today perennial vegetation becomes

increasingly sparse and of more xeric aspect on a similar east-west gradient. Excepting an apparent reversal about 8900 B.P. (Fig. 4,5), the early Holocene, fossil record from the Marble Mountains reflects the progressive development of xeric desertscrub vegetation Of the peren-

nials that occur at the sites today, only Bebbia juncea is reliably

documented in the oldest fossil sample. Most of the current dominant

•Pedes were probably absent during the latest Wisconsin and immigrated

to the area during the early Holocene. Encelia farinosa was the first

to appear, in trace quantities at ca. 10,200 B.P., and then commonly

the « next youngest fossil sample at ca. 9500 B.P. (Fig. 4). It is

*o important element in all younger midden samples. The apparent im- igration | of Larrea tridentata , Ambrosia dumosa , and Peucephyllum -75- schottii occured later (Fig. 4). Trace amounts of creosote bush first appear at ca. 8900 B.P. but it apparently did not become important until sometime between ca. 8900 B.P. and ca. 8000 B.P.

Figure 6 presents the total number of fossil taxa in midden samples from Marble Mountains Locality B and C. It is assumed that the sample diversity is directly proportional to the diversity of the vegetation at the sites. There is no dramatic early Holocene species diversity decrease since the local extinction of mesophytes was accompanied by the near synchronous immigration and expansion of xeric plants (Fig. 4,6).

There is a notable diversity decrease in the middle and late Holocene, but more samples are needed to establish the validity of this trend.

The little evidence for vegetation change at the Marble Mountains during the middle and late Holocene is equivocal. More samples of this age are needed for analyses of vegetation changes which, at this area, appear to have been quite subtle.

The Eureka Valley . Plant macrofossils from the northern boundary of the California Mojave Desert (Fig. 1,3) document changes in high elevation (1430 m to 1635 m) desertscrub vegetation. Ancient middens of latest Wisconsin and early Holocene age are scarce, but the record of the middle and late Holocene is good.

The two oldest midden samples are dated at ca. 14,700 B.P. (Eureka

View 5A) and ca. 10,700 B.P. (Horse Thief Hills 1) and suggest the presence of a juniper-shadscale woodland. Juniperus osteosperma and

Atriplex confertifolia are abundant in both samples while other shared taxa include desertscrub (Salvia dorrii , Artemisia sec. Tridentatae) and woodland ( Symphoricarpos cf. longiflorus ) species. The HTH 1 site is -76-

Figure 6. The total number of identified plant species (N) in Neotoma

midden samples from Marble Mountains Locality B and Locality

C plotted against their radiocarbon age.

S I 01

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— - 10 /£ 20 25 30 -77- more mesic and the midden contains records of Amelanchier utahensis ,

Chamaebatiaria millefolium , and Prunus cf . andersonii . Hecastocleis shockleyi is present at the Horse Thief Hills 1 site today but absent from the midden. It is common, however, in the Eureka View 5A assemblage implying that, during the Ice Age, the distribution of Mojave prickleleaf had shifted from north facing to south facing slopes. The remains of

Pinus cf. flexilis in EV 5A suggest a nearly continuous extension of this subalpine species from the Inyo Mountains to the arid Last

Chance Range.

The abundance of Atriplex confertifolia in the two oldest Eureka

Valley middens points to an important difference between the record of changing vegetation here and that from the Marble Mountains. In the latter area there was a near complete turnover in the composition of plant communities from the early Holocene to present. Plants that are important in the current Marble Mountains vegetation are almost totally lacking in the earliest fossil assemblages (Fig. A). In contrast, for the last ca. 15,000 years shadscale has played an important role in the plant communities bordering the northern Eureka Valley. Terminal Wis- consin and Holocene vegetation change took place within a persistent matrix of Atriplex confertifolia (Fig. 7).

The only early Holocene sample from Eureka View lacks Juniperus

osteosperma . It is uncertain when, between ca. 1A,700 B.P. and the

EV 5B date of ca. 8300 B.P., juniper-shadscale woodland gave way to

•hadscale desertscrub (Fig. 7). It appears that desertscrub vegetation ttay have developed earlier here than further to the southeast in the highlands of the Nevada Test Site (Wells and Berger 1967), Sheep Range, -78-

Figure 7. The estimated relative abundance of selected plant species

in macrofossil assemblages from the Eureka View Locality

plotted against radiocarbon age. Note the change in time

scale. 5, very abundant; A, abundant; 3, common; 2, occasional;

1, rare; Closed circle, one or two fragments, a possible

contaminant. IT)

I_^_i O

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or the Lower Grand Canyon (Phillips 1977; Fig. 1,5). Such a phenomenon would not be unusual considering the Eureka Valley's position in the immediate lee of both the Sierra Nevada and the Inyo Mountains and the subsequent intense rainshadow# The EV 5B assemblage is of low species diversity (Fig. 8), lacking the mesophytes that were present during the preceding late Wisconsin, as well as many of the desert shrubs occur ing today. It does contain two shrubs that are not found here today, Lepi-

dium cf . fremontii and Dalea fremontii .

Four macrofossil assemblages ranging in age from ca. 6800 B.P. to ca. 3900 B.P. provide evidence for an anomalous shadscale desertscrub association at Eureka View. The array of species from these midden samples is anomalous relative to the current vegetation since several desert .shrubs that were apparently important then are no longer present at the fossil sites while other plants, common at the site today, were lacking. Encelia virginensis and Opuntia basilaris are the most common mid-Holocene extralocals, while Lycium sp. and Dalea fremontii occur frequently (Fig. 7). Opuntia basilaris and Dalea fremontii persist into the late Holocene while the youngest sample to contain Encelia virginensis and Lycium sp. is dated at ca. 3900 B.P. (Fig. 7). Compounding the unusual floristic aspect of these middle Holocene assemblages is the nature of the samples themselves. Hidden samples dating from ca. 6800

B.P. to ca. 3900 B.P. are rich In calcium carbonate. The carbonate is presumed to be of eolian origin, rather having been leached into the middens, since both older and younger samples from the same sites contain little carbonate. Leaching would also lead to rapid degradation of the middens but these samples are well preserved. The edaphic effect of a -80-

carbonate dust fallout may have contributed to the unusual aspect of

the middle Holocene vegetation at Eureka View. The nearest areas *,

supporting mid-Holocene elements such as Encelia virginensis , Dalea

fremontii , and Opun tia basilaris are the caliche-rich alluvial fans

below the bedrock outcrops.

At least two species of Opuntia , (). basilaris and £. echinocarpa , .

are important in the middle and late Holocene Eureka View assemblages.

Neither can be found at the locality today. Peucephyllum schottii

first appears in the fossil record at ca. 3900 B.P. (Fig. 7) and occurs

in all younger assemblages, but like the cacti, could not be located

in the modern vegetation at the sites.

The middle Holocene abundance of Opuntia spp. at the Eureka View

Locality is not unique. A similar expansion of Platyopuntia is recorded

In the mid-Holocene Desert View (0. phaeacantha , £. polyacantha) and

-billow Wash 2 (0. basilaris) packrat middens from the Sheep Range (Table 12).

Middle Holocene midden samples from the Lucerne Valley area display a

rather dramatic increase in Opuntia (0. basilaris and Opuntia sp.) as

well as Encelia (E_. farinosa and E_. frutescens ; King 1976 a,b).

The Eureka View fossil data reflect the differing migrational

histories of the plants currently at the sites. Only Atriplex con-

fertifolia is present through the entire ca. 14,700 years of record.

Haplo pappus brickellioides is the first immigrant to appear, by ca.

8300 B.P. , and Erioneuron pulchellum and Ambrosia dumosa appear next

by ca. 6800 B.P. (Fig. 7). Trace quantities of Larrea tridentata

appear at ca. 5400 B.P. and creosote bush is frequent ca. 1500 years

later in EV 2C (Fig. 7). Amphi pappus fremontii and Eriogonum fasciculatum -81-

are among the latest bo appear in the fossil record at ca. 1600 B.P. The development of vegetation of essentially modern aspect, that is the Remise of most extralocal plants and the dominance of both shadscale and creosote bush, occured sometime between ca. 3900 B.P. and ca. 2600 B.P. (Fig. 7).

The total number of fossil taxa in each Eureka View midden sample is plotted against radiocarbon age in Fig. 8. A drop in diversity at the end of the early Holocene, followed by a gradual increase during the last ca. 5000 radiocarbon years, reflects the general vegetation history. It is a classic response. Low diversity communities are expected following major, rapid climatic change since there are few species present (such as

Atriplex confertif olia) that are preadapted to the new environmental regime (Wolfe 1978). Diversity gradually increases at the Eureka View Lo- cality as xerophytes migrate into the area. As previously mentioned, the

Marble Mountains samples show no drastic early Holocene diversity decrease

(Fig. 6) , due to the relatively early arrival of xerophytic plants at the

Marble Mountains (Fig. 5). The early establishment of vegetation of essentially modern aspect at the Marble Mountains is consistent with trends described by Van Devender and Spaulding (1979). The anomalous nature of mid-Holocene vegetation at Eureka View and the late arrival of important perennial plants to this locality is unexpected. The delayed development of modern plant communities here is contrary to a model calling for little middle and late Holocene vegetation change (Van Devender and Spaulding

1979).

Three midden samples from the Horse Thief Hills Localities A and B

(Fig. 3) are associated with subrecent radiocarbon ages (Table 5). None

•re more than 200 radiocarbon years old and it is impossible to determine -82- (

Figure 8. The total number of identified plant species (N) in macro-

fossil assemblages from the Eureka View Locality plotted

against radiocarbon age. Note the change in time scale. r

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where they fall within that interval. The HTH 2 assemblage (Locality A) reflects the current vegetation at the site (Table 7), but HTH 3(1) and

HTH 3(2) vary more from the present plant community and each other (Table 8).

It appears that the plant associations in the bottom of this canyon (Fig. 3) have undergone considerable alteration in the last ca. 200 years. These changes may reflect the impact of historic overgrazing but it is a prema- ture assumption without finer chronological co'ntrol. Similar changes are not evident in young middens from hillslope sites at Eureka View and HTH locality A.

The Sheep Range . Latest Wisconsin and early Holocene packrat middens from the Sheep Range reflect the chronology of vegetation development proposed by Van Devender and Spaulding (1979). Diverse woodland vegetation with juniper and pinyon gave way to xeric juniper woodland by ca. 9500 B.P. which in turn gave way to middle Holocene desertscrub at elevations below ca. 1740 m (5700 ft). A single high elevation (2080 m; 6820 ft) midden in the present pinyon- juniper woodland, dated at ca. 9550 B.P. (Fig. 5), contains abundant juniper but only rare pinyon. It lacks any extralocal mesophytes and is similar to the present woodland at the site. Contemporaneous records from lower elevations document woodland in what is now desertscrub vegetation.

Middle to late Holocene middens from the upper elevation desert communities (1540 m to 1810 m; 5050 ft. to 5940 ft.) suggest a shifting lower woodland boundary. It appears that, at present, the lower limit of woodland is ca. 100 m to 150 m (330 ft to 490 ft) higher than it was for much of the

preceding 5000 radiocarbon years. The only middle to late Holocene -84- midden not documenting woodland is from the lowest elevation site. It provides evidence for the prior expansion of Chilopsis linearis and suggests the presence of a more mesic middle Holocene ripparian desertscrub community than currently exists at the site.

Precipitation Gradients

Wisconsin and early Holocene middens south of ca. Lat. 36 N document the occurence of evergreen oaks from the Chihuahuan Desert

(Wells 1966; Van Devender et al. 1978; Van Devender and Riskind 1979) west to extreme southern Nevada (Van Devender 1973; Van Devender and

Spaulding 1979; Leskinen 1975; Wells 1979). Ancient packrat middens north of ca. 36 N Lat., as well as west of the Colorado River in the

CDCA, yield no evidence for the Ice Age occurence of live oaks (Van

Devender and Spaulding 1979; Wells and Berger 1967; Wells 1979; King

1976 a,b). The discovery of Quercus turbinella in the Scodie Moun- tains packrat middens, near the southern terminus of the Sierra Nevada, is the single exception (Fig. 1; Van Devender and Spaulding 1979).

The apparent westerly limit of fossil live oaks along the Lower Colo- rado River Valley may reflect the extent of significant summer precipitation during the early Holocene and late Wisconsin, roughly con- gruent (Wells 1979) or some 100 km to the east (Huning 1978) of the current boundary of the monsoonal precipitation regime.

Additional evidence for the former boundary between summer-wet and summer-dry bioclimatic regions may be found in the distribution of fossil pinyon pine (Wells 1979). Pinus monophylla occured in Ice

Age woodlands to elevations as low as 510 m (1670 ft) in the Whipple -85-

Mountains and 615 m (2020 ft) in the New Water Mountains (Fig. 1; Van

Devender 1973; Van Devender and Spaulding 1979). However, Pinus monophylla is lacking in almost all fossil assemblages below ca. 1550 m

(5080 ft) elevation north of ca. 36° N Lat. (Spaulding 1976; Wells

1979). There is a modern corollary to the southerly decrease in the lower elevational limit of pinyon during the Wisconsin. Wells and

Berger (1967) point to the decline in the elevational limit of current woodland from north to south and suggest that increasing effectiveness of summer precipitation is the cause. While speculation as to whether this cline was steeper during Wisconsin and early Holocene times (Wells

1979) begs an incomplete fossil record, the northwestern and western boundary of monsoonal climate appears to be a persistent feature of the late Quaternary climate of the Southwest (Wells 1979).

As mentioned previously, the records of early Holocene desertscrub in the Marble Mountains apparently indicate a gradient of decreasing precipitation effectiveness west of the Colorado River Valley. This may reflect a more intense rainshadow caused by the proximity of the

Peninsular and Transverse Ranges (Fig. 1). But, of perhaps greater significance, is the distance from the Colorado River Valley to the

Marble Mountains and the valley's function as a trough for the northward movement of moist tropical air from the Gulf of California (Huning 1978;

Hales 1974). Sites near the Colorado River Valley receive a greater proportion of late Bummer and fall precipitation from this source than areas further inland (Huning 1978) . The current limit of this secondary

Summer precipitation maximum delineates the western boundaries of the -86-

Eastern Colorado and Eastern Mojave Subdivisions of the CDCA (Fig. 1;

Huning 1978). The contrast during the early Holocene between the Marble

Mountains desertscrub and contemporaneous lower elevation woodland to the east (Fig. 5) implies that the former area may have been west of the bounadry of effective summer precipitation. That is, prior to ca. 8000 B.P., the effective limit of precipitation associated with maritime tropical air from the Gulf of California (Houghton et al. 1975) appears to have been to the east of its current limit.

Creosote Bush And Glacial Refugia

Wells and Hunziker (1978) point to the "intriguing possibility of a late, intercontinental dispersal (of Larrea tridentata) from

South America" (parentheses mine) . Cytological and biogeographic evidence indicate that creosote bush may well have originated in South

America (ibid.). But, however intriguing the possibility may be, the assertion that "Larrea has not been detected in the full-glacial Neotoma record" (ibid.) is inconsistent with the fossil data (Table 13). The fossil record offers unequivocal evidence for the Ice Age presence of

Larrea tridentata in North America. Of particular interest is the presence of Larrea cuticles in middle and late Wisconsin-age ground

sloth ( Nothrotheriops shastense ) dung from Rampart Cave in the Lower

Grand Canyon (Table 13; Hansen 1978), Small amounts of creosote bush in Neotoma middens may always be attributed to post-Pleistocene contamination but its inclusion in extinct herbivore dung cannot.

There are only ten fossil records of creosote bush spanning the ca. 20,000 year duration of the late middle and late Wisconsin Glacial i i ' ' • i

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Age (Table 13). An additional 10 sites document creosote bush during the brief ca. 3000 year span of the early Holocene, reflecting the post-Pleistocene expansion of this xerophyte. During the last Ice Age

Larrea appears to have occured sporadically as far north as the Lower

f Grand Canyon (Lat. 36°06 N) , the Amargosa Desert (Lat. 36°25' N) , and the Sheep Range (Lat. 36°38' N; Table 13; Fig. 9). Nor is this record for creosote bush an exception; desert plants such as Encelia spp.,

Ferocactus acanthodes , Ambrosia dumosa , and Yucca brevifolia occur sporadically in Wisconsin-age packrat middens as far north as ca. 36 30' N

Lat. (Van Devender 1973; Phillips 1977; Van Devender and Spaulding 1979).

In many cases the documented southward displacement of desert plants appears to have been unaccompanied by an equivalent contraction of their northern range (Spaulding 1976).

The macrofossil evidence for glacial refugia, areas of favorable

Ice Age climate into which desert plant communities were displaced, is non-existent. Glacial-age woodlands occured in areas that presently

support xeric desertscrub vegetation as far south as Lat. 26 K in

Coahuila, Durango (Van Devender 1978), Sonora, and Baja, California

(Wells 1976, 1979). There is abundant evidence for the southward displacement of many Mojave and Great Basin Desert species but they are

inevitably associated with woodland fossil assemblages.

Van Devender (1978) suggests an alternative to the classic concept

of a desert refugium that is consistent with the fossil record. In

discussing the occurence of characteristic desert species in xeric Ice

Age woodlands far south of the U.S. -Mexico border, he suggests "a biotic

refugium in dramatically altered communities rather than a relict core L

oj tO KM

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o> W • CO CT)-D

tf>~p£'O-' Oi' n* -90-

Figure 9. Fossil sites of Wisconsin and early Holocene age containing

creosote bush. Numbers refer to the radiocarbon ages of the

3 sites in thousands of years brfore present (x 10 B.P.).

AM, Artillery Mountains; LGC, Lower Grand Canyon; MM, Marble

Mountains; NWM; New Water Mountains; OC, Owl Canyon; PP,

Picacho Peak; SR, Sheep Range; WH, Wellton Hills; WM, Whipple

Mountains. The Eureka Valley study area (EV) is added for

reference.

N -91- desert" (ibid.). This concept is equally applicable to current warm desert areas north of the Mexican border. Desert species, 6uch as Larrea

tridentata , may have persisted throughout much of their present range in restricted habitats. This would result in a sporadic fossil record

(Table 13) and is not inconsistent with the observed cytogeographic differentiation of creosote bush (Barbour 1969; Yang 1970; Wells and * Hunziker 1976). A widespread but spotty Wisconsin distribution of creosote bush may account for its relatively early advent at sites spread over a wide geographic area (Table 13; Fig. 9). The delayed arrival of Larrea at the northerly Eureka Valley study area (Fig. 7,9) may reflect the distance between that area and the nearest Wisconsin-age population.

CONCLUSIONS

1. Prior to ca. 11,000 B.P. desertscrub vegetation was lacking or extremely rare in the California Desert Conservation Area. Juniper and pinyon-juniper woodland dominated the present California desert.

2. The developmental trend toward desertscrub vegetation of modern aspect began at the close of the late Wisconsin, at least 11,000 years ago. In some areas, such as the Marble Mountains in the central

CDCA, the transition may have been completed by ca. 7800 B.P. In other areas, such as the Eureka Valley, vegetation of essentially modern aspect did not develope until ca. 2500 B.P.

3. There is no evidence for desert refugia during the last Ice

Age. Instead, many desert plants appear to have persisted throughout ouch of their present range in xeric woodland vegetation. This may not be true for certain frost sensitive Sonoran Desert species, but -92- it appears to be the case for many xerophytes important in the CDCA.

4. The boundary between summer-wet (monsoonal) and summer-dry climatic regimes in the CDCA has persisted throughout the last ca.

14,000 years. During the early Holocene it may have lay ca. 100 km to the east of its present position.

5. Differential and lengthy migration rates are indicated for . certain desert plants. Creosote bush, with a widespread but sporadic distribution, does not appear in the fossil records of many sites until well into the early Holocene. Its arrival near its present northern limits was delayed until well into the middle Holocene.

6. The history of vegetation development is unique to each site.

Climatic changes may be widespread and broadly synchronous, but their impact at each locality is modulated by unique biotic and environmental factors.

7. Desert plants communities are in a state of flux. Perturba- tions within the present climatic regime result in vegetation changes that may be marked at sites that lie near current vegetation boundaries.

In the Eureka Valley, mid-Holocene environmental changes and migrational lag times resulted in anomalous desertscrub communties. In the Sheep

Range the lower limit of pinyon-juniper woodland fluctuated more than

100 m during the last ca. 5000 years.

V LITERATURE CITED

Antevs, E. 1948. The Great Basin with emphasis on glacial and postglacial times. Univ. Utah Bull. 38: 1-24.

. 1955. Geologic-climatic dating in the West. Amer. Anti-

quity 20: 317-335. Barbour, M.G. 1969. Patterns of genetic similarity between Larrea divaricata of North and South America. Amer. Midi. Nat. 81: 54-67.

Beatley, J.C. 1974. Effects of rainfall and temperature on the distribution and behavior of Larrea tridentata (creosote-bush) in the Mojave Desert of Nevada. Ecology 55: 245-261.

. 1975. Climate and vegetation pattern across the Mojave/

Great Basin transition of southern Nevada. Amer. Midi. Nat. 93: 53-70. Bleich, V.C. and O.A. Schwartz. 1975. Observations on the home range

of the desert woodrat, Neotoma lepida intermedia . J. Mammalogy

56: 518-519. Brown, D.E. and C.H. Lowe. 1974. The Arizona system for natural and potential vegetation- illustrated summary through the fifth digit

for the North America Southwest. . J. Ariz. Acad. Sci. 9: Suppl.

3. 56 pp.

• 1977. ^ Biotic communities of the Southwest. U.S.D.A. Gen.

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