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PROCEEDINGS

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Founded IS74

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San Diego Society of Natural History OCT 1 5 1990

HARVARD

UNIVERSITY

Nunibcr 2

15 Seplember 1990

Holocene Vegetation of the Hornaday Mountains

of Northwestern Sonora, Mexico

Thomas R. Van Devender

Arizona-Sonnni Desert Museum. 2021 N. Kiiiner RJ.. Tucson. Arizoiici S5743. USA

Tony L. Burgess

U.S. Geological Survey. 1675 W. AiikUiin Rd . Tucson. Arizona S5705. USA

Richard S. Felger

Drylands Institute. 2509 N. Campbell Aveiuie ft 176. Tucson. Arizona S57I9. USA

Raymond M. Turner

U.S. Geological Survey. 1675 W. Anklam Rd.. Tucson. Arizona <S5705. USA

ABSTRACT. — The 94 laxa of phnis idcntiricd Irom 10 packrat (Neolonw sp. ) middens from 240 to 260 m elevation m the granitie Hornaday

Mountains provide an excellent history of vegetation and climate lor the last 10.000 yr B.P. in the Pinacale Region ol northwestern Sonora, Mexico.

Rocky slopes in this arid area (ca. 120 mm/yr precipitation) in the Lower Colorado River subdivision of the Sonoran Desert have supported

desenseruh communities dominated by Emelia farinosa (brittlebush). Larrea divaricata (creosotebush). and Carnegiea gigantea (saguaro)

throughout the Holocene. Community composition changed continuously on both long (10,000 yrs) and shorter time scales.

Ephedra nevadensis (Mormon tea), codominant with Encelia farinosa, Larrea divaricata. and Carnegiea gigantea in early Holocene (8910 to

10.000 yr B.P.) samples, and rare Juniperus cf. californica (California juniper) imply a continuation of the cool summers and primarily winter

rainfall of the late Wisconsin glacial. Middle Holocene (4430 to 8660 yr B.P.) samples contained trees and shrubs such as Acacia greggii (catclaw

acacia). Cercidium foridiim (blue paloverde). and Prosopis velutina (velvet mesquile) that are now restricted to relatively mesic washes, implying

summer rainfall greater and freezes more frequent than today. Late Holocene ( 1720 to 2320 yr B.P.) samples dominated by Encelia farinosa and

OIneya lesota (ironwood) withC. m/(;v)/'/n7/H»M foothills paloverde ) are similar to present Sonoran Desert vegetation and imply a relatively modem

biseasona! rainfall regime with the wannest winters of the Holocene. In the last 1700 years, O. tesola disappeared an. Ambrosia t/cZ/on/cu (triangleleaf

bursage), Bwsera microphylla (elephant tree). Jatropha cuneata (cuneate limberbush). and L. divaricata appeared or increased at the sites. The

modem vegetation of the Homaday Mountains is as sparse and the climate is as hot and dry as at any time in the last 10,000 years. The variability in

community composition through the Holocene was similar in magnitude to the modem variability among different midden sites and appears to

reflect the highly variable climate. Responses of individual species to climatic fluctuations on scales from years to millennia may prevent desertscrub

communities from ever reaching equilibrium.

Disjunct populations of .Artemisia bidoviciana (white sage). Berlwris haemalocarpa (barberry). Opiinlia chlorotica (silver dollar cactus). Rhus

aromatica (skunk bush). Salvia mohavensis (Mohave sage), and Stipa speciosa (desert needlegrass) above 630 to 800 m elevation on the north side

of Pinacate Peak are relicts from ice age woodland expansions into the Sonoran Desert. In the late Wisconsin and early Holocene. Pinus monophylla

(singlcleaf pinyon), several species of Juniperus (junipers), and Yucca brevifolia (Joshua tree) were widespread down to 460 m elevation in the

Tinajas Altas Mountains of southwestem ."Arizona. An 1 8,700 yr B.P. radiocarbon date on twigs o( Larrea divaricata documents its regional presence

at 330 m in a full-glacial juniper woodland dominated by J. californica and Y. brevifolia. Creosotebush desertscrub samples with abundant J.

californica have been found as low as 240 m in the Butler Mountains just to the west. Ice age desertscrub dominated by Larrea divaricata without

woodland plants was apparently restricted to below 250-300 m in the Lower Colorado River Valley and the Gran Desierto. A lowering of sea level

of about 100 m during the Wisconsin exposed additional lowlands around the head of the Gulf of California. Mohave Desert plants such as

Coleogyne ramosissima (blackbmsh). Salvia mohavensis. Y. brevifolia. and K whipplci (Whipple yucca) expanded their ranges southward and to

lower elevations. Dominants of arid central Baja California communities such as Finiquieria coluinnaris (boojum tree) and Pachycornuis discolor

T. R. Van Devender el al-

(elephanl tree) were conspicuousl> absenl Irom low-elevalion Louer Colorado River \alle\ and Gran Desierlo midden assemblages. Arid

desertscrub comniunilies have probabl> been in these Sonoran Desert lowlands throughout the Pleistocene.

Resume)!. Las 94 laxa de plantas identificadas en 10 depositos de Nenloma sp. (tori) de 240 a 260 m de altitud en las montanas graniticas de

Homaday proveen una historia excelenle de la vegetacion y el clima para los ijliimos 10.000 aiios en la region del Pinaeate en el noroeste de Sonora.

Mexico. Las pendientes rocosas en esta area arida (ca. 120 mm/ano de precipitacion pluvial 1, en la subdivision del Rio Colorado Bajo del Desierto

Sonorense, ban soslenido comunidades de niatorral xcrotilo. siendo las dominantes F.mclia faiiiiiisa (incienso). Lunea divariccita (gobemadora) v

Caniegiea !^if;ciiilea (saguaro) durante todo el Holoceno. La composicion de las comunidades cambio continuamente con una tendencia general

desde el glacial tardio al presente y tambien a escalas de tiempo mas cortas.

Ephedra n('V(«A';i.v/,i- (tepopote), codominante con Enceliafarinosa. Larrea divaruulu. y Ccinh't;ieci iiiatiiiWii en mucstras del Holoceno temprano

(8910 a 1(1.000 aiios antes del presente = A. P.), y el raro Junipeius cf. californica (enebro) indican una continuacion de los veranos frescos y las

lluvias principals invemales del glacial de Wisconsin tardi'o. Las muestras de! Holoceno medio (4430 a 8600 anos A.P.) contenian arboles y arbustos

como Acacia gregf/ii (una de gato), Cercidium floridum (paloverde azul). y Prosopis vcliilina (mezquite) los cuales ahora estan restringidos a los

arroyos relalivamente hiimedos, indicando precipitaciones veraniegas mas elevadas y heladas mas frecuentes que en la actualidad. Las muestras del

Holoceno tardio (1720 a 2.'?20aiios A.P.) en las que dominan Enceliafarinosa y Olneya tesota (palo tierro) con C microphvUiim tpMoxerde} refiejan

una vegetacion relativamenle modema del Desierto Sonorense e indican un regimen de lluvias bieslacional relativamente niodemo con las

condiciones mas subtropicales en el Holoceno. Durante los liltimos 1700 aiios. O. resata desaparecio, en tanto que Amlvusia delloideci (chicurilla).

Biirsera microphwila (torote). .lalropha cuncata (sangregrado) y L. divaricata aparecieron o aumentaron en los sitios. La vegetacion es tan escasa en

las montafias Homaday y el clima moderno tan caliente y arido como en cualquier tiempo durante los liltimos 10.(K)0 alios. La variabilidad de la

composicion de las comunidades durante el Holoceno era semejanle en magnitud a la variabilidad modema entre los sitios de depositos de Neoloma

y parece retlejar el clima altamente variable. Las respuestas de especies individuales a las fluctuaciones climaticas en escalas de aiios a milenios

puedan evitar que las comunidades de matorral xerofilo lleguen a alcanzar el equilibrio.

Las poblaciones disyuntas de Arlemisia ludoviciana (estafiate). Berheris liaematDcarpa (agrito), Optinlia ch/oroiica (nopal). Rluis aromatica

(agrillo). Salvia mohavensis (salvia de Mojave), y Stipa speciosa (flechilla del desierto) en elevaciones mayores de 650-800 m. al lado norte del

Cenro Pinacate. son relictos de las expansiones de los bosques del periodo glacial hacia el Desierto Sonorense. En el Wisconsin tardi'o y el Holoceno

temprano. Pinits manopliylla (piiion). varias especies de Jiinipenis. y Yucca Ivevifolia (arbol de Josue) esiaban ampliamente distribuidas, bajando

hasta los 460 m de elevacion en las montatias Tinajas Altas del sudoeste de .Arizona. Una lecha de radiocarbon de 18.700 afios A.P., en ramitas de

Larrea divaricata. documenta su presencia a una altura de 330 m. en bosque glacial de enebro dominado por7. californica y Y. hrevifolia. Se han

encontrado muestras de matorral xerofilo de gobemadora con abundanteV. californica a no mas de 240 m en las montafias Butler a poca distancia al

oeste. Las comunidades de matorral xerofilo del periodo glacial dominadas par Larrea divaricata sin plantas de bosque, aparentemente esiaban

liinitadas a altitudes menores a 250 a 300 m en el Valle del Ri'o Colorado Bajo y el Gran Desierto. El descenso del nivel del mar de aproximadamente

100 m durante el periodo Wisconsin expuso tierras bajas adicionales cerca de la cabecera del Golfo de Califomia. Plantas del Desierto de Mojave.

tales como Coleogyne ramosissima (arbusto negro). Salvia mohavensis, Y. hrevifolia. y Y. whipplei extendieron su area de distribucion hacia el sur y

a elevaciones menores. Las plantas dominantes de las comunidades aridas de Baja Califomia central, tales como Foiic/iiieria cohiinuaris (cirio) y

Pachycormus discolor (copalquin) estaban conspicuamente ausentes en depositos de Neotonia del Valle de Rio Colorado Bajo y el Gran Desierto.

Las comunidades aridas de matorral xercifilo probahlemente han eslado en eslas tierras bajas del Desierto Sonorense durante todo el Pleistoceno.

INTRODUCTION

about 30-50 m of the shelter (Van Devender. 1990). Radiocarbon

Plant remains preserved in ancient packrat (Neoloma sp.) dating allows the niitlden assemblages to be placed in time. Series

middens allow detailed reconstructions of the local history of veg- "^ radiocarbon-dated midden assemblages from a single cave or

elation and climate for the last 30,000 years in the North American several caves in a .small local area provide detailed records of the

deserts (Van Devender et al., 1987). Chronological sequences of 'ocal vegetation history.

middens typically begin in the late Wisconsin, when woodlands '" '^is study, we analyzed plant macrofossils in a series of

dominated by pinyons, junipers, and oaks were widespread, and Holocene packrat tniddens from the Homaday Mountains in the

continue through the Holocene as modern desertscrub communities ^fid Pinacate Region, northwestern Sonora.

developed. In the Sonoran Desert in Arizona, pinyon-juniper

woodlands with Pinus monophylla (singleleaf pinyon) at 610-1535 ENVIRONMENTAL SETTING

m elevation occurred above xeric woodlands dominated by

Juiupenis californica (Califomia juniper) prior to 11,000 yr B.P. The Sonoran Desert is the subtropical arid region centered

(radiocarbon years before 1930; Van Devender , 1990). During the around the head of the Gulf of California in western Sonora, south-

last glacial, and probably much of the Quatemary, lower areas western Arizona, southeastem California, and much of the Baja

along the Colorado River in Arizona, California, and adjacent Califomia peninsula (Shreve, 1964; Fig. 1 ). Elevations range from

Mexico supported desertscrub dominated by Larrea divaricata below sea level in the Imperial Valley of California to about 1000 m

(creosotebush) without woodland plants. al the northeastern margin in Arizona. Along the Colorado River

Packrat middens are hard, dark, shiny organic deposits built by they range frotii 278 in at Needles to sea level in the Gulf of

rodents of the genus Ncolonui. In open areas, packrats construct Califoi-nia (Turner and Brown, 1982). Subdivisions of the Sonoran

houses near shrubs or large rocks by using a variety of local plant Desert based on vegetation and climate were proposed by Shreve

materials, including thorny branches and cactus stems and pads. (1964) and refined by Turner and Brown (1982). Most of the

Middens are fonned near entrances to internal passages where fecal Sonoran Desert occasionally experiences freezing temperatures,

pellets, plant fragments, and general debris are deposited during although rarely for more than a single night. In general, the fre-

house cleaning (Finley, 1958). In dens in dry rockshelters where the quency and intensity of freezes increases to the north, toward the

amount of house material can be reduced or absent, the midden can Mohave Desert. Rainfall ranges from a biseasonal regime with

be cemented with urine and preserved for tens of thousands of strong summer nioiisoons in Sonora and Arizona to a winter-rainfall

years. Packrats gather represenlalive samples of the plants within regime on the Pacific coast of Baja California. Total precipitation

Holoccnc Vcgclalion of Ihc Homaduy Mountains

■Bujj^e'P* '

CALIFORNIA

ARIZONA

Figure 1. Location of Homaday Mountains and other areas discussed in text. Limits of Sonoran Desert after Shreve (1964). Arizona Upland

subdivision in heavy stipple; Lower Colorado River Valley in light stipple. Branch, Burscia niiirophyUu (elephant tree).

decreases with decreasing elevation from the Arizona Upland and

Plains of Sonera west to the Lower Colorado River Valley and the

Gulf of California. Except in the most arid portions of the Sonoran

Desert, trees and arborescent cacti are common and reflect strong

affinities with subtropical thomscrub to the south.

The hottest and driest climate in North America is in the

Sonoran Desert lowlands of Sonora. Baja California. Arizona, and

California in the Lower Colorado River Valley. Precipitation is

highly variable and at lower elevations averages less than 100 mm/

yr (Ezcurra and Rodrigues, 1986). We estimated precipitation for

the Homaday Mountains at 230 m elevation by using regional

regression equations derived from climatic data compiled by the

U.S. Geological Survey: 1 19,3 mm/yr with .'^LO'/f in summer (June-

August), 60.67f in summer-fall (June-November), ?<\.17c in winter

(December- February), and 39. 4*^ in winter-spring (Decetiiber-

May). The estimates for summer and fall may be slightly high

because the Homadays are on the drier western edge of the Pinacate

Region (Table 1 ). The driest portion of the year is from the middle

of March through June.

The vegetation in the Lower Colorado River Valley subdivision

is mostly an open desertscrub structurally dominated by Lanea

divavicata. Ainhrosia diimosii (white bursage), Encelta fariiinsa

(brittlebush). and Hikma ligida (big galleta grass). Felger (1980)

described the vegetation, flora, and environment of the Gran

Desierto (Desierto de Altar), the sandy lowlands of northwestern

Sonora. The Homaday Mountains (Sierrita el Temporal) are a gra-

nitic ridge at 210-440 m elevation on the northweslem edge of the

Pinacate Volcanic Complex. 56 ktn north of the Gulf of California

(Lynch, 1981). MacDougal Crater (Volcan el Verdugo) in the

Pinacates is just to the south. The area is 14 km south of the Tule

Table L Climatic records for stations in the Pinacate Reaion of northwestern Sonora. Mexico."

"Data from Hastings and Humphrey ( 1969).

''Distance (km) and direction from the Homaday Mountains.

'Calculated for July to October for stations and June to November (summer-fall) for the Homaday Mountains. Homaday Mountains values were

estimated by means of regression equations denved from climatic data compiled by the U.S. Geological Survey.

T. R. Van Des ender el al.

Desert, which lies between the Cabeza Prieta Mountains and the

Sierra Pinta in the Cabeza Prieta National Wildlife Refuge in Yuma

County. Arizona. The massive Pinacate volcanic complex, with an

area of about 2000 km-, is a stark landscape of volcanic craters and

lava flows, many of Pleistocene age. bounded by sand dunes

(Homaday. 1908;'lves. 1964; Figs. 1,2). Pinacate Peak (28 km SSE

of the study area) rises from an elevation of 100 m at its south-

western base to 1290 m on top. The Homaday Mountains and

MacDougal Crater were named by the geographer Godfrey Sykes

for William T. Homaday and Daniel T. MacDougal. leaders of a

1907 expedition to explore the Pinacate sponsored by the Carnegie

Desert Botanical Laboratory (Homaday. 1908).

The general environment of the Homaday s is similar to that of

the more arid Sierra del Rosario {50 km W; Felger. 1980). The

vegetation near the midden rockshelters is a desertscrub dominated

by Biirsera microphylla (elephant tree). Cercidium micidphylluin

(foothill paloverde ). Eiue/iu fciriiiDsii, and Lavrea clivaricatu (Table

1). This association was included in the Lanea divaricata-

Fouquieria splendens (ocotillo) communities described in the

Pinacate by Ezcurra and Lopez-Portillo (1987). Important shrubs,

succulents, and grasses include Ambrosia deltoidea (triangleleaf

bursage). A. dumosa, Caine^iea gigaiuea (saguaro). Fouquieria

splcndcus. Hilaria rigida. Hyplis enioryi (desert lavender),

Jairoplia cuncata (cuneate limberbush). and Opunlia higelovii

(teddy bear cholla). Herbaceous perennials and annuals are less

diverse in rocky habitats than on nearby bajadas and dunes.

METHODS AND RESULTS

Ten packrat middens were collected from granitic rockshelters

at 240-260 m elevation from five areas on the southwest, south, and

east slopes of the Hornaday Mountains (31°59'N. ll.^°36'W;

Table 2). Arthropod remains from the samples were reported by

Hall et al. (1988). We listed all plants occurring within 30-50 m of

each rockshelter on rocky slopes. Table 3 lists the flora of the entire

study area by growth form and habitat (rocky slopes, bajadas. and

stabilized sand dunes). Voucher speciinens of 64 species of plants

were deposited into the Llniversity of Arizona Herbarium. The flora

totals 104 species including trees and shrubs (17.3%). subshrubs

( I5.47f ). stem and rosette succulents (8.7%). grasses (10.6%), and

herbs (48.1%^). especially annuals (36.5%). Sampling periods were

restricted to the winter-spring season ( 1 9-20 February 1 984. 22-24

March 1985); a few additional summer annuals would be expected.

Annuals include both the winter and summer seasonal ephemerals

Figure 2. Aerial view toward the southeast of the Homaday Mountains. MacDougal Crater, and the Pinacate volcanic field and peaks. Photo hy Peter

L. Kresan.

Holocene Vegelalion ol'lhc Hciniiuiuy Mounlains

Table 2. Radiocarbon dates on Ncoloma sp. fecal pellets, sample

weights after washing, and site elevation and slope for packrat

middens from the Homaday Mountains, Sonora. Mexico.

of Shreve (1964) and plants that germinate in one cool season and

survive to the next.

Samples from packrat middens were washed in water, oven-

dried, and sorted. Sample weights after washing ranged from 72 to

194 g. Plant macrofossils were identified by reference to specimens

in the University of Arizona Herbarium and Desert Laboratory.

Internal relative abundances (RA) were assigned to each plant

taxon in an assemblage on a scale of I to 5. On this scale a single

specimen is a 1. the most common taxon is a .5. and other abun-

dances are ranked between. Although the number of plants in ranks

2 to 5 varies according to the total number of identified specimens,

the relative abundances of different samples are readily comparable.

Ten radiocarbon dates on fecal pellets of Neoloma sp. from the

samples yielded ages from 10.000 to 1720 yr B.R with fairly

continuous coverage through the Holocene (Table .3). Careful strati-

graphic collection and removal of outer surfaces minimized con-

tamination by younger materials. The chronology presented by Van

Devender et al. (1987 ) is followed in this paper, i.e., late Wisconsin,

22,000-11,000 yr B.R; early Holocene, 11,000-8900 yr B.R;

middle Holocene, 8900-4000 yr B.R; late Holocene, 4000 yr B.R

to present.

A total of 94 taxa with 23-38 (av. 30.4) taxa per assemblage was

identified from the midden assemblages (Table 4). The midden taxa

included trees and shrubs (19.1%), subshrubs (12.8%), stem and

rosette succulents (9.6%), grasses (9.6%-), and herbs (48.9%), es-

pecially annuals (35.1%). A chronological summary of selected

perennial species is presented in Figure 3. Distributions of plants

were determined through personal observations, notes, and collec-

tions, surveys of the University of Arizona Herbarium, which

houses Richard S. Felger's extensive regional collection, an atlas of

Sonoran Desert plant distributions (Hastings et al., 1972), and

regional floras (Bowers, 1980; Felger, 1980, unpublished; Kearney

and Peebles, 1964; Munz, 1974; Shreve and Wiggins, 1964;

Wiggins, 1980).

VEGETATION HISTORY

Early Holocene

Three early Holocene (8910 to 10,000 yr B.R) samples record a

desertscrub dominated by Ephedra itevadensis. Encelia fannosa.

and Lanea dnaricata in association with various shrubs. A single

twig of Junipevus cf. califoniica was in the 9370 yr B.R sample.

The nearest population of./, califonuca is in the Sierra Juarez (26.5

km W) in northern Baja California (Wiggins, 1980). Chiyso-

thammis lerelifolius (rabbit brush), represented by a leaf in the same

sample, is a shrub found in crcosolcbush and Joshua tree {Yucca

hicvlfolia) deaenscTub and pinyon-juniper woodland in the Mohave

Desert in California (Munz, 1974). The nearest populations are in

the Santa Rosa Mountains (380 km WNW), on the western edge of

the Sonoran Desert in southeastern California. Bnckvllia

aliactxioidcs (brickellbush. B aiiiula of Munz. 1974) is a Mohave

Desert and Lower Colorado River Valley species thai is known in

Sonora only from steep granitic mountains 99 km east of San Luis

on Highway 2 (21 km WNW). Acacia gresgii (catclaw acacia),

Behhia jiincea (chuckwalla's delight), Brickellia coulleri

(brickellbush), Eriofionumfascicidatum (California buckwheat), E.

wiiKhlii (wild buckwheat), Lycium sp. (wolfberry), and

Pcuccphyllum scluittii (pygmy cedar), identified from the samples,

no longer occur in the Homadays but persist in the Pinacate.

Ephedra nevadensis. Galium slellaliim (desert bedstraw),

Hofmeisiena phiriseta (arrowleaf). Prosopis velurina (velvet mes-

quite), and Vif>uiera cf. dclloidca (desert goldeneye) presently oc-

cur elsewhere in the Homadays. Ephedra nevadensis is occasional

above 400 m on the north slope of the ridge. Plants in the samples

that still occur at site include Ambrosia dumosa, Bursera

micropliylla. Cercidiiim microphylhim. and Trixis californica.

Succulents common in the early Holocene middens were

Carnci^iea i;i^amea. Eerocactiis cylindracciis (California barrel

cactus, F. acamhodes of Munz, 1974), and Opuntia hii;eU)vii. with

lesser amounts of A,i,'<"'(' deserti (desert agave) and Mummillaria

microcarpa (fishhook cactus). Aaave deserti and F. cylindniccK.s no

longer occur at the midden sites but are present elsewhere in the

range. Mammillaria microcarpa and O. higelovii are common to-

day but not as widespread as in the early Holocene. Carnegiea

pjianlea is still present at all midden sites.

Slipa speciosa (desert needlegrass), a widespread C-3 perennial

in the Mohave Desert, was in the 8910 and 9370 yr B.R. samples.

Relict populations survive in the Tinajas Altas Mountains (60 km

NNW) and on Pinacate Peak (28 km SSE). The remainder of the

midden grasses, with the exception of the robust Hihiria rii^ida. are

short-lived annuals and perennials that have been present in the

Sonoran Desert throughout the Holocene (Van Devender et al.,

1990).

The early Holocene assemblages contained more short-lived

herbs than occur at the sites today. Some species now absent from

the Homadays, including the perennial Ari;ythaninia neomexicana

(wild mercury). Mirahilis lyii;clovii (wishbone bush), Lyrocarpu

coulteri (lyrefruit). Verbena sp. (vervain), and the annuals

Amsinckia ressellata (fiddleneck) and Parietaria liespera (pelli-

tory), still occur in the nearby Pinacate. Most of the other herbs

were observed on bajada or dune habitats in the vicinity of the

Homadays. A number of annuals not observed probably appear in

years with adequate rainfall.

Middle Holocene

Three middle Holocene (4330 to 8660 yr B.R) samples provide

records of a desertscmb dominated by Encelia farinosa and Larrea

divaricafa with less Ephedra nevadensis. By 606.5 yr B.R.

Carnegiea gigantea was a codominant. Prosopis velutina increased

until it was abundant in the 44.30 yr B.R sample.

Plants no longer in the Homadays but in the Pinacate include

Acacia greggii. Behbia juncea, Eriogonum fasciciilatum.

Peucephyllum schottii. and Phoradendron californicum (desert

mistletoe). Opuntia ramosissima (diamond cholla) was not .seen in

the area but is widely scattered in lowland habitats in the Lower

Colorado River Valley (Felger, 1980, unpublished). Cercidiiim

floridiim (blue paloverde). Ephedra nevadensis. Hyptis emoryi.

Olnexa lesota. and Prosopis velutina no longer occur at the sites but

remain elsewhere in the range. Trees and shrubs that still occur at

T. R. Van Devender er al.

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Holocene Vegetation of the Homaday Mountains

13

the sites include Ambrosia diimosa. Burserci microphylla. C.

miciophyllum. Eiuelici fciiiiiosci. Fouqiiieria splendens, Larrea

divariccitii, and Tri.yi.s icjlifcnniii.i.

Succulents common during this period were Caniei^iea

gigantea, Mammillaha microcarpa. and Opuntia higelovii.

Ferocactiis cylindraceiis was common in the 8660 yr B.P. sample

but absent or uncommon in all younger samples and the modem

flora. The only fossils of Opuntia kunzci ( Yuma club cholla) and O.

ramosissima were in the 6065 yr B.P. sample. Opiinlia fulsiida

(chainfruit cholla). the unique spiny arborescent Sonoran Desert

succulent thai accumulates chains of fruits, was comtnon in the

4430 yr B.P. sample but does not grow at the site today.

The abundances of desert grasses peaked in the middle Holo-

cene, whereas perennial herbs, with the exception of several species

adapted to the most arid conditions, declined markedly. Persistent

species include Allionia incarnata (windmills). Euphorbia cf.

polycarpa (spurge), Physalis sp. (probably P. crassifolia: ground

cherry), and Spluwrulcea sp. (probably 5, amhigua: globe mallow).

The annuals were similar to those of the early Holocene with

increases in Boerhaavia wrii;htii (spiderling) and the first record for

Kallstrocmia sp. (sutiitner poppy). Dalea mollissima (silk dalea)

and Rafmesquia neomcxicana (desert chicory) are winter-spring

annuals that are widespread in the deserts of the Southwest but

barely enter Sonora in the Gran Desierto. The only locality for D.

niollissinia is 31 km east of San Luis on Highway 2 (140 km

WNW). The only localities for/?, neomcxicana are from 43 ktn west

of Los Vidrios on Highway 2 (17 km NW) and at 1100 m in

Pinacate Pass. Lupinus concinnus is a widespread species in the

Southwest that is found in the Gran Desierto only on coastal sand

dunes. The nearest collections are from the Bates Mountains in

Organ Pipe Cactus National Monument (74 km ENE; Bowers,

1980).

Late Holocene

Four late Holocene (1720 to 2320 yr B.P.) samples record a

desertscrub dominated by Encelia farinosa and OIncya tesota.

Larrea divaricala declined in abundance from the middle Holocene

as Ccrcidium microphyllum modestly increased. Abundances of

Carnegiea gigantea and Prosopis velulina declined after 2320 yr

B.P. Ambrosia deltoidca, Argytliamnia cf. lanccolata (wild mer-

cury), and Jatropha cuneata are coinmon desert shrubs that first

appeared in the late Holocene.

Argytliamnia cf. neomexicana (wild mercury), Ecliinoccreus

sp. (hedgehog cactus), Hymenoclea salsola (cheesebush), and

Lupinus concinnus no longer occur in the Homadays, but except for

L. concinnus. all occur in the Pinacate today. The seeds of

Echinocereus sp. probably represent E. engelmannii. a species

widespread in the Mohave Desert and the Lower Colorado River

Valley. Hymenoclea salsola is a shrub that in the Sonoran Desert

typically grows along the edges of washes rather than on rocky

slopes.

Argytliamnia cf. lanceolala. Fouquieria splendens, Olneva

tesota, and Prosopis velutina no longer occur at the sites but are

present elsewhere in the Homadays. Trees and shrubs that still

occur at the sites indude Ambrosia deltoidea, Bursera microphylla,

Cercidium microphyllum. Encelia fdrino.sa, Jatropha cuneata, and

Larrea divaricata. Except for the decline in Carnegiea gigantea

and the single record of Echinocereus sp., the succulents were

similar to those of the middle Holocene.

The grasses in the middens decline markedly in abundance and

number of taxa from the middle Holocene. The perennial and

annual herbs with a few exceptions were similar to those of the

middle Holocene. Allionia incarnata increased in importance and

was abundant in the 1930 and 1720 yr B.P. samples. Kallstroemia

.sp. increased in the samples and was common in the IS.'iO yr B.P.

sample. Both K calijornica and K grandi flora tiiay be found in the

Pinacate in years with adequate summer or fall rainfall.

Modem Vegetation

The vegetation near the midden rockshelters is a desertscrub

dominated by Encelia farinosa, but the composition of the commu-

nity has shifted markedly in the last 1700 years. Although the

abundances of Carnegiea gigantea, Cercidium microphyllum, and

Fouquieria splendens did not change significantly, several plants

declined or disappeared from the sites. Olneya tesota, an important

desert tree, is the most notable of these. Others 'mc\\xdt Argytliamnia

lanccolata. Echinocereus sp.. Euphorbia polycarpa, Ferocactus

cylindraceus, Opuntia fulgida, Physalis crassifolia, and

Sphaeralcea amhigua. Alli(mia incarnata declined at most sites.

In contrast, a number of shrubs have increased near the

rockshelters, including Ambrosia deltoidea, Bursera microphylla,

.latropha cuneata. and Larrea divaricata. Interestingly, A. dumosa,

Hyptis emoryi, and Trixis California were present at the sites in the

early and middle Holocene, absent in the late Holocene. but have

returned to the sites. A number of easily identifiable plants appar-

ently have moved into this habitat only recently: Argytliamnia

hrandegeei (Brandegee wild mercury), Fagoiiia laevis. Hibiscus

denudatus (desert rose mallow), Horsfordia newbenyi (yellow felt

plant), and Porophyllum gracile (odora).

Quantitative Vegetation Trends

Although plant remains in the Homaday middens document a

Sonoran desertscrub dominated for the last 10.000 years by Encelia

farinosa, Larrea divaricata, and Carnegiea gigantea, the compo-

sition and structure of the community have been remarkably dy-

namic. Species that no longer occur in the Pinacate region were

never common, with only 5.3% in the early Holocene, 5.4% in the

middle Holocene, and 2.1% in the late Holocene. After 8910 yr

B.P., the only species in the samples but not in the region today

were winter-spring annuals. The flora near the midden sites and for

the entire range has become progressively more modem (Table 5).

Interestingly, all samples record richer, more mesic floras than

occur at the sites today. In the last 1720 years, a number of desert

shrubs and succulents have declined while others have increased.

We used Sorenson's index of similarity to establish trends in the

Holocene vegetation. To calculate this index, twice the number of

taxa that two samples share is divided by the sum of the taxa

identified from each sample (Mueller-Dombois and Ellenberg,

1974). The index approaches 1.0 as samples become more similar.

In biological systems differences in sample sizes usually prevent

the index from reaching 1.0. Longer-lived perennials (e.g., trees,

shrubs, subshrubs. succulents, and Hilaria rigida, a large perennial

grass) identified in each midden were compared with the modem

flora near each site to assess progressive inodemization of the flora.

The midden assemblages were also compared with the next oldest

and youngest samples to assess variation through time. Plants

within 30 to 50 m of the five midden rockshelters were compared to

assess variability between sites today.

Similarity indices comparing the midden assemblages with the

site's current floras generally increase through the Holocene as the

flora becomes more modem (Fig. 4). The 10,000 yr B.P. sample

was the most different, with a value of near 0.30. Until 6065 yr B.P.

the values remained less than 0.5 (0.3-0.48, av. 0.41 ). From 4430 to

1720 yr B.P the values increa.sed from 0.53 to 0.70 (av. 0.60), a

little lower than the results of modem comparisons between sites.

The 2320 yr B.P, sample, with a value of 0.70, was most similar to

the modem flora; values for three younger samples were lower. The

14

T. R. Van Devender ei al

Table 5. Distribution of plants identified in packrat middens from the Homaday Mountains. Sonora."

"n. found within 30 m of midden rockshelter: w. could be found near the rocksheller in wet years; H. not found near

rockshelter but present elsewhere in the range; P, no longer in the range but occurs in the nearby Pinacate

Mountains; e. no longer in the Pinacate region.

increase from 0.55 to 1.0 (= modem) indicates substantial change in

the vegetation in the last 1720 years.

Similarity indices comparing midden assemblages to the next

youngest sample provide an indication of internal changes wtihin

the community (Fig. 4). The youngest assemblage was compared to

the modem flora at the site. Except for the 6065 to 4430 yr B.P.

comparison, the values range between 0.58 and 0.72 (av. 0.64).

indicating that species composition of the community changed

steadily for most of the last 1 0.000 years. Increased species turnover

between 6065 and 4430 yr B.P. reflects the dynamic nature of the

middle Holocene.

Similarity indices comparing the perennial plants near the five

midden rockshelters yielded values of 0.43 to 0.91 (av. 0.70).

Although the exposure of the sites varied from south to southwest to

east, differences in similarity in the flora appeared not to be related

to slope. The basic similarities of all sites and the inherent site-to-

site variability in the composition of the desertscrub communities

reflects the hot. dry climate with highly variable precipitation.

Local microhabitat features such as steepness of slope, size of

cliffs, and amount of shade produced by the local bedrock configu-

ration are usually more important than general aspect. The intersite

variability in the modem flora is of a magnitude similar to that

found between midden assemblages through time.

DISCUSSION

Holocene Floristic History

Eaii\ Holocene. — The midden records from the Lower Colo-

rado River Valley help us understand the unusual distributions of

plants in northwestern Sonora. Populations of Arlcmisia

ludoviciana (white sage). Berheris hacmatocaipa (barberry).

Opuntia chlororica (silver dollar cactus). Rhus ciromaricci (skunk

bush). Salvia mohavensis (Mojave sage), and Stipa speciosa occur

only above about 650 to 800 m elevation on Pinacate Peak (Felger.

unpublished) and are restricted to north-facing slopes or cliffs on

the north side of the mountain mass. These populations are relicts of

extensive late Wisconsin and eariy Holocene distributions. Isolated

10.0 -

0.2 0.3

0.4 0.5 0.6

Simi larity

0.7 0.8

Index

0.9 1.0

Figure 4. Sorenson's index of similarity comparing long-lived perennial plants Irom the Homaday Mountains of northwestern Sonora, Me.xico.

Comparison of packrat midden assemblages to the modem flora within 30-50 m of the site (solid line, closed circles) reflects modernization of the flora.

Comparison of midden assemblages to the next youngest sample (open circles, dashed line) illustrates vanability in community composition through time.

The youngest assemblage was compared with the modem flora at site. Data points plotted between radiocarbon dates.

Holoccne Vegetation of the Homaday Mountains

15

populations of Ambrosia accnuhicarpa (burweed) in sandy areas

along Highway 8 between Sonoyta and Puerto Penasco. of Noliiia

hii;dovii (Bigelow beargrass) in the Sierra del Rosario. Sierra los

Alacranes, and Cerro Pinto (Felger. U)8()), and of O^ hasilwis

(beavertail cactus) and Yucca whipplei (Whipple yucca) in the Si-

erra los Alacranes reflect expansions not recorded in the Homaday

samples. Ephedra nevadensis, which is scattered in the lower

ranges and reaches its southern range limits in Sonora in the

Pinacates, was widespread and abundant in the early Holocene.

Ericamena ciincata. E. laricifolia (turpentine bushes), Monar-

di'lUi arizomca. and Salvia pingidfolia (rock sage) were identified

frotii older middens from the Puerto Blanco and Tinajas Altas

mountains but were not in the Homaday samples. The closest

populations today are in the Ajo Mountains in Organ Pipe Cactus

National Monument ( 100 km ENE). If their late Wisconsin ranges

included the Pinacate they were extirpated in the Holocene. Other

species found in Ajo Mountains middens that may have been in

Wisconsin woodlands of Pinacate Peak include Berheris

harrisoniana (Harrison barberry), Ceanotluis gregi;ii (buckbrush),

and Qiwrciis uirhinclla ssp. ajoensis (Ajo oak).

The absence of Opiiiilia acaiuhocurpa (buckhorn cholla) from

the Homaday middens and the Pinacate today is surprising because

the species occurs in middens of various ages from other Sonoran

Desert sites and is widespread in the Sonoran and Mohave deserts

today. Similarly, the absence of Ambrosia ilicifolia (hollyleaf

bursage), a distinctive subshrub endemic to the Lower Colorado

River Valley, frotii the Homaday middens or modem flora is inter-

esting as this species is abundant in early Holocene samples from

the Butler and the Tinajas Altas mountains (60-75 km NNW) and in

the modem flora of the Sierra del Rosario (50 km W) and other

granitic mountains of northwestem Sonora. Apparently their ranges

did not expand as far to the southeast during the early Holocene as

did those of some of their modem associates.

The vegetation and climate of the early Holocene in the

Homadays and other areas of the Sonoran Desert were transitional

between those of the late Wisconsin and more modem times. Ex-

cept for a few species, the vegetation was a desertscrub composed

of plants that still occur in the region. Important Sonoran species

such as Biirsera micropliylla. Caniegiea giganlea. and Cercidium

microphyllum were present. However, the community reflects an

environment more mesic than today's with species now restricted to

washes such as Acacia greggii, Behbia juncea, and Prosopis

velutina growing on exposed slopes. Plants that are now rare and

apparently limited by aridity were more widespread, e.g.. Agave

deserti. Bricketlia atractyloides. B. coidreri. Ephedra nevadensis,

Eriogonum fascicidaliim. E. wrighrii. Galium stellalum, and

Viguiera deltoidca.

Of the annuals in the early Holocene samples (« = 17) 88.2%

grow only in the winter-spring rainy season. Annual grasses in the

middens included one winter-spring obligate (Vidpia sp.) and two

species that flower opportunistically (Aristida adscensionis,

Muhlenhergia microsperma). A few fruits of Boerhaavia wrightii

and Boerhaavia sp. in two samples were the only annuals indicating

warm-season moisture. The five-sided fruits of Boerhaavia sp.

could represent B. erccta or B. spicata. found in the Pinacate. The

establishment of Carnegiea giganlea seedlings is dependent on

occasional periods of adequate summer-fall rainfall (see

Steenbergh and Lowe. 1977). These data suggest the relatively

mesic conditions in the early Holocene were due to an extension of

winter rainfall from the Pacific frontal storms that dominated the

late Wisconsin coupled with a modest increase in summer mon-

soons (Van Devenderet al., 1987). The assertions by Spaulding and

Graumlich (1986) that woodland plants such as Jiiniperus

californica persisted in the early Holocene of the Sonoran Desert as

rainfall shifted from winter to a summer monsoonal maximum by

9000 years ago are not supported. The results also strongly differ

from the simulations of Kutzbach (1987) based on atmospheric

general circulation models for North America. His climatic recon-

struction for 9000 yr B.P. for the Southwest included July tempera-

tures l°C wanner than today's and annual precipitation greater than

in both the late Wisconsin and today due to increased summer

monsoons.

Middle Holocene. — In contrast, many indicators of enhanced

monsoons reported from other Sonoran Desert midden sites (Van

Devender, 1987, 19901 were present in the middle Holocene in the

Homadays. Trees and shrubs now restricted to washes grew on

exposed rocky slopes. Acacia greggii. continuing from the early

Holocene, declined and disappeared but was replaced by Cercidium

floridum and Prosopis velutina. Middle Holocene remains of C.

floridum on slopes have also been found in the Whipple, Butler,

Tinajas Altas, and Waterman mountains. Prosopis velutina, a tree or

shmb widespread in desert-grassland and the portion of the Sonoran

Desert enjoying summer rainfall, increased in abundance until it

dominated the 4430 yr B.P. samples. Phoradendron californicum in

the 6065 and 4430 yr B.P. samples is a parasite on desert legumes

that is limited by aridity at its lower elevational limit.

Bouleloua aristidoides (needle grama), B. harhata (six-weeks

grama), and Erioneuron pulchclluni (fluff grass) first appeared and

became common in the middle Holocene. Grass fossils are typically

scarce in middens, underrepresenting grasses' abundances (Van

Devender et al., 1990). These grasses were probably more common

in the community than their fossils (RA = 3) in the 6065 and 4430

yr B.P. samples indicate. Although these opportunistic grasses may

also flower after winter-spring rains, plants grow faster, reach

larger sizes, and produce more florets after summer or fall rains

(Tevis, 1958).

Although the annual flora in the middle Holocene samples (n =

19) continued to be dominated by winter-spring obligates (78,9%),

there were summer rainfall species, including Boerhaavia wrightii,

Dicoria canescens (bugseed). Kallstroemia sp., and Pedis papposa

(chinchweed). Increases in the abundance of AUionia incarnata

probably signal increases in summer perennial habit rather than as a

winter-spring annual. Peak abundances oi Bouteloua aristidoides.

Carnegiea giganlea. Cercidium floridum, Erioneuron pulchellum.

Hyplis emoryi. and Larrea divaricata were in the 6065 yr B.P.

sample. Today the distributions of plants in the Pinacate are strongly

correlated with rockiness, as discrete communities are found on

different substrates, including rocky slopes and sand dunes (Ezcurra

and Lopez-Portillo, 1987). The presence of D. canescens and other

sand-loving species, including Hilaria rigida. Opuntia

ramosissima. and O. kunzei. on rocky slopes at 6065 yr B.P. may

indicate strong southwesterly winds that blew sand from the low-

lands onto the mountain side. Southerly winds transport sands

northward from the Bahia del Adair in the Gulf of Califomia (Fig.

1 ). These indications are in opposition to the conclusions of

Lancaster et al. (1987), who suggested that the last major accumu-

lation of sand in the Gran Desierto was in the middle Holocene

from high-velocity northerly winds transporting sediments from the

Colorado River. These records suggest that the middle Holocene

was the time of maximum monsoonal development.

Several other aspects of the middle Holocene record are inter-

esting. The 8660 yr B.P. sample shared a few species not found in

the range today {Behbia juncea. Eriogonum fasciculatum) as well

as high abundances of Acacia greggii and Ephedra nevadensis with

the early Holocene samples. In contrast both the 4430 and 2320 yr

B.P. samples had high abundances of Carnegiea gigantea, Olneya

tesota, and Prosopis velutina. Although C. gigantea was present

throughout the Holocene, its abundance peaks in the 6065 to 2320

yr B.P. samples. In the Puerto Blanco Mountains its midden records

are very similar, with its appearance by 10,540 yr B.P., continued

16

T. R. Van Devender ei al.

presence through the Holocene. and dominance from 7970 lo 3440

yr B.P. (Van Devender. 1987). In the Tinajas Ahas Mountains. C.

gi^antea was in six samples from 8970 to 1 230 yr B.P. but common

only at 8970 and 8650 yr B.P. In the Butler Mountains it was present

but scarce in samples dated at 11.250. 11.060. 8160. and 3820 yr

B.P. and is uncommon today. We suspect the few seeds in older

juniper samples are younger contaminants.

Olneya tesota first appeared in the Homaday midden record al

8660 yr B.P. and increased in importance by 4430 yr B.P. It first

appeared at 7560 yr B.P. in the Puerto Blanco Mountains, 7530 yr

B.P in the Butler Mountains, and 8750 yr B.P in the Wellton Hills.

Questionable records of 9900 and 8700 yr B.P. and more reliable

records of 4010 and 1230 yr B.P. are from the Tinajas Altas Moun-

tains. O. tesoui was a dominant in a 3820 yr B.P. sample from the

Butler Mountains. In contrast, it was found only in a 110 yr B.P.

sample at Picacho Peak and not at all in the Whipple Mountains on

the limits of its modem range. Its disappearance from rocky slopes

after 1720 yr B.P. in the Homadays is similar to the Puerto Blanco

Mountains record, where it retreated from the slopes between 1910

and 990 yr B.P

In the Homadays Hyptis enioryi was dominant only in the 6065

yr B.P. sample. It first appeared in the Puerto Blanco and Butler

mountains records at 7560 and 7530 yr B.P. respectively.

Peucephyllum scholtii was found only in the 6065 yr B.P. sample.

Its rarity in the midden record and absence from the Homadays

today is surprising because it is abundant in nearby MacDougal

Crater, the Sierra del Rosario, the Butler and Tinajas Altas moun-

tains, and in the midden record from other areas.

Fouqideria splendens appeared in the 4430 yr B.P. sample but

was never common. In the Puerto Blanco Mountains it was uncom-

mon in samples dated at 3440 and 130 yr B.P. In the Tinajas Altas

Mountains a few thorns were in samples dated at 5080 and 4010 yr

B.P. In the Butler Mountains it was common in a single sample

dated at 740 yr B.P. At Picacho Peak it was found only in a 1 10 yr

B.P. and two modem samples (Cole, 1986). In the Whipple Moun-

tains today, F. splendens is present with Olneya tesola below about

320 m but it apparently did not reach the midden sites at 365-525

m. Because the abundance and geographic, elevational, and eco-

logical ranges of F. splendens are so great and because packrats

readily eat its bark and stems, the poor record is enigmatic.

Late Holocene. — With declines in Lairea divaricatu and in-

creases in Olneya tesola in the late Holocene, the vegetation of the

Homadays became essentially a wetter version of today's

desertscmb. Piosopis veliitina was a dominant in the 2320 yr B.P.

sample before disappearing from the sites. Carnegiea gigantea

declined after 2320 yr B.P. Allionia incarnata continued to be

common and outnumbered Encclia farinosa and O. tesota in the

1930 and 1720 yr B.P. samples. Peak abundances such as these

probably indicate heavy fall rainfall from the huge late summer-fall

storms from the tropical Pacific (chubascos; Pyke. 1972). The

annual flora (/; = 17) continued to be dominated by winter-spring

annuals (82.4%) with few summer species [Boerhaavia wrightii.

Boerhaavia .sp., and possibly Amaranllws sp.). The hot summers

and high evaporation of the Gran Desierto currently favor spring

annuals, which comprise 39 species. However, warm-season annu-

als can occur in huge numbers with enomious biomass after a

chubasco. Unpublished floral surveys of the granitic ranges of

northwestem Sonora suggest that about 25 species of summer

ephemerals appear and nourish when adequate moisture is present.

In the Arizona portion of the .Sonoran Desert, they mostly flower in

summer or fall. The wamier winters of the Gran Desierto and the

coast of the Gulf of California induce many species to flower in

winter and spring as well. Only a few species, e.g., Amaranllws

palmeri (pigweed), B. wrightii. Dicoria canescens, and Tidestromia

lanuginosa (espanta vaqueras), grow only in summer and fall.

The late arrivals of Ambrosia deltoidea. Argxthamnia cf.

lanceolata. and Jatropha cuneata in the Homadays are interesting, i

Ambrosia deltoidea is a dominant subshrub in Arizona LIpland

communities lo the east in Arizona and Sonora. Argxthamnia

lanceolata is a widespread subshrub of rocky slopes in the Sonoran

Desert. It first appeared in the Puerto Blanco Mountains midden

record at 3480 yr B.P

Jatropha cuneata is a succulent-stemmed, frost-sensitive desert

shmb that is widespread on rocky slopes at low elevations in Sonora

and Baja Califomia and in .Arizona from the Tinajas Altas Moun-

tains to the Puerto Blanco Mountains. It was scarce in the 1720 yr

B.P. Homaday midden but is a dominant at the sites today. In the

Puerto Blanco Mountains it was not found in the Holocene middens

but is abundant at the Twin Peaks site today. Jatropha cuneata was

a dominant in a 1230 yr B.P. sample from the Tinajas Altas Moun-

tains.

The shift to a more xeric vegetation after 1720 yr B.P. and the

very xeric nature of the modem vegetation at the Homaday sites

resemble the sequence reported for the Puerto Blanco Mountains

(Van Devender. 1987). In that study desertscrub assemblages indi-

cated summer rainfall somewhat greater than today's from 3480 to

1910 yr B.P Samples dated at 99o''and 980 yr B.P reflected an even

wetter period. Samples dated at 130 and 30 yr B.P. signaled essen-

tially modem community composition and climate, although the

vegetation at the sites today is even more sparse. The Homaday

vegetation was probably more dynamic than our midden results

indicate because samples from the inferred mesic period at about

1000 yr B.P. were missing. Photographic rematches from

MacDougal Crater record massive die-offs of Cercidium

microphxllum and Larrea divaricata due to prolonged drought from

1936 lo 1964 (Turner, 1990). The final "desertification" of these

areas took place apparently in this century and was due to regional

(or global) climatic changes unrelated to such human activities as

grazing and fuelwood cutting.

Historical Biogeography

The midden fossils illuminate the biogeographic affinities in the

flora and document dispersals. During the late Wisconsin and early

Holocene, the present Sonoran Desert supported species from the

Mohave Desert and adjacent California juniper woodland/

desertscmb from Arizona and Califomia south to northwestem

Sonora and northern Baja Califomia. The early Holocene samples

from the Homadays suggest that the influences from these sources

declined along the latitudinal and elevational gradients toward the

Gulf of Califomia. A corridor connecting the floras of Baja Cali-

fomia and Sonora was apparently well developed for widespread

desert species such as Ephedra nevadensis, Eriogonum

fasciculatum, Larrea divaricata, and Viguiera deltoidea. The

paleoclimatic significance of the absence of fossils of Fouqideria

columnaris (boojum tree), Pachycormus discolor (elephant tree),

and \'. laciniata (San Diego goldeneye), apparently indicating a

lack of northward movements of dominants of arid Baja Califomia,

needs additional study.

Typical dominants such as Carnegiea gigantea and Cercidium

microphyllum retreated from the present Arizona Upland during the

glacial and retumed during the Holocene as summer rainfall in-

creased. Unfortunately the Homaday records are not old enough to

document the ice age presence of these species or those of Bur.wra

microphylla or Encelia farinosa. If they were displaced to the

southeast by summer drought and cooler winters, it was probably

not far. Certainly Carnegiea gigantea and E. farinosa expanded into

Arizona very quickly afler 1 1.000 yr B.P.

The di.spersals of other desert species into their modem ranges

had different patterns. Larrea divaricata was an important member

Holocene Vegelalion of the Homaday Mountains

17

of late Wisconsin woodland and desertscrub below 330 m. In the

Holocene, it dispersed rapidl) into the Mohave Desert in Califor-

nia, reaching the Marble Mountains by 7430 yr B.P. (Spaulding,

1990) and Ihc Lucerne Valley by 5880 yr B.P. (King. 1976). In

contrast, L. divarkala. perhaps the Sonoran Desert telraploid chro-

mosomal race, did not appear in the Puerto Blanco Mountains until

3400 yr B.P. However, a recent tandem accelerator mass spectrom-

eter (TAMS) radiocarbon date from the Waterman Mountains west

of Tucson establishes the presence of L. dixaricata in the north-

eastern Sonoran Desert by 6l9.'i yr B.P.

If the two early Holocene records of Cenidiiini micropliyUiim

are not contaminants, thai species was present in the Homadays

4000 years earlier than its arrival in the Puerto Blanco Mountains

(75 km E) by 5240 yr B.P. Ccnidnim flortdiim and OIncya tcsuhi

reached Arizona sites soon after they were recorded in middle

Holocene samples in the Homadays. Biirscra micwphylla was

present throughout the Holocene in the Homadays but reached the

Tinajas Altas Mountains (55 km N) only by 5820 yr B.P

In the Puerto Blanco Mountains, the late Holocene is charac-

terized by the arrival of more subtropical Sonoran Desert species,

including Fciocactus emoiyi (Coville barrel cactus, F. covillei of

Munz, 1974), Scipiiim hilociilarc (Mexican jumping bean), and

Stenocereus thurberi (organ pipe cactus). These species occur in the

Pinacate but apparently did not reach the Homadays. Probably the

present gradient of aridity between the Arizona Upland and Lower

Colorado River Valley subdivisions was established about 4000

years ago. And finally, some species such as Jatropha cuneata

appear to have reached the Homadays and the Arizona sites during

the last 1300 years. Effective invasions appear to be related to

changes in regional climate at various levels of intensity and the

dispersal abilities of the individual species. Jatropha cuneahi with

its large seed may have difficulty dispersing northward during

warm periods and establishing populations large enough to endure

subsequent freezes.

Late-GIacial Deserts

Plant macrofossils in late Wisconsin and early Holocene

middens have revealed expansions of woodland communities into

the modem warm deserts in much of the southwestern United States

(Van Devender et al., 1987). Assemblages of late Wisconsin age

without woodland dominants were found at 240-300 m elevation in

the Picacho Peak area on the California side of the Colorado River

just north of Yuma, Arizona (Cole, 1986). Lanea divaricaia has

been in the area for the last 1 3,000 years and probably for much of

the Pleistocene after its immigration from South America, presum-

ably in a Pleistocene interglacial (Wells and Hunziker, 1976). Four

late Wisconsin (13,380 to 11.160 yr B.P) Picacho Peak samples

contained low levels of Chrysothammis leretifolius. Coleogyne

ramosissima (blackbrush). Ephedra iievadensis, Ferocactus

cylindraceiis. Opunlia acaulhocarpa. Salvia mohavensis. Yucca

brevifolia (Joshua tree), and K whipplei. In the early Holocene E.

nevadensis increased in abundance as Mohave Desert species in-

cluding Chrysothammis tcrclifoUus. Coleogyne ramosissima. S.

mohavensis. Y. brevifolia. and Y. whipplei declined or disappeared.

Encelia farinosa and Peiuephylliim schottii appeared and became

codominants by 10,540 yr B.P. Interestingly, Ambrosia diimosa and

Olneya lesota did not appear in the midden record until a few

hundred years ago.

Other midden studies in the Lower Colorado River Valley help

define the limits of the late-glacial desert (Van Devender et al.,

1987; Van Devender, 1990). In the Whipple Mountains of Califor-

nia to the north along the Colorado River, .luniperus californica

was dominant in late Wisconsin and early Holocene woodlands on

rhyolite at 320-525 m. Finns monophylla was present above 510 m

prior to about 1 1 ,000 yr B.P. Wells and Hunziker ( 1 976) reported J.

osteosperma (LUah juniper) at 16,900 yr B.P. from 258 m elevation

in the Chemchuevi Mountains, 35 km north of the Whipple Moun-

tains. The simultaneous disappearance of./, caiifornua and Yucca

whipplei from the Whipple Mountains sites about 8900 years and

increases in abundances of Larrea divaricata and Peiicepliyllum

schottii mark the end of the early Holocene. Ambrosia dumosa did

not become important in the area until 5020 yr B.P. The lowest

midden site (320 m) supported a woodland dominated by J.

californica and Y. brevifolia with L. divaricaia at 11,015 yr B.P.

(Van Devender. 1990). Codominants in the assemblage were Nolina

bigelovii. Opiintia acanthocarpa. and Salvia mohavensis with

lesser amounts of Atriplcx confertifolia (shadscale). Eriot;onum

fasciculaliim. Ferocactus cylindraceus, and Y. baccata (banana

yucca).

The Wellton Hills midden sites are at 160-180 m on granite near

the Gila River 35 km east of its junction with the Colorado River, 45

km east- southeast of Picacho Peak, and 85 km north-northwest of

the Homadays. A sample dated at 10.750 yr B.P. was dominated by

Encelia frutescens (green brittlebush). Ephedra nevadensis. Larrea

divaricaia. and Salvia mohavensis. Less common species were

Brickellia airactyloidcs. Castela emoryi (Sonoran crucifixion

thorn). Erioj>onum fasciculaliim, Ferocactus cylindraceus. and

Gxmiiosperma glutinosum (tatalencho). At 8750 yr B.P., Ephedra

nevadensis and L. divaricata continued to dominate as Acacia

greggii. Ambrosia dumosa. Olneya lesota. Peucephyllum schottii,

and Prosopis velutina became important. A sample at 8150 yr B.P.

was similar, with dominance by Ephedra nevadensis and L.

divaricaia and most of the associates. Acacia greggii. Olneya

lesota, and Prosopis velutina were absent. Carnegiea giganlea,

Encelia farinosa, Opunlia acanthocarpa. and O. higelovii were not

found.

The Butler Mountains are a small granitic range west of the

Tinajas Altas Mountains on the eastem edge of the Yuma Mesa, the

Arizona extension of the Gran Desierto (Fig. 1; 74 km NNW).

Juniperus californica was a codominant in samples from 240-250

m dated at 11.060 and 10.360 yr B.P. with Ambrosia dumosa.

Encelia farinosa. Ephedra nevadensis. Ericameria laricifolia.

Eriogonum fasciculatuin, Koeberlinia spinosa (allthom). Larrea

divaricata. and Salvia nuiliavensis. Ambrosia ilicifolia. Carnegiea

giganlea. Echinocaclus polycephalus (cottontop barrel cactus),

Ferocactus cylindraceus. Opunlia higelovii. O. ramosissima. and

Slipa speciosa were present in small quantities. Ambrosia ilicifolia

is restricted to desert ranges in the lower Colorado River Valley.

Interestingly, L. divaricata twigs in a sample differing only in the

presence of a few twigs of J. californica yielded a date of 1 1 .250 yr

B.P (Van Devender et al., 1985). A sample dated at 8160 yr B.P

was dominated by Acacia greggii. Ambrosia ilicifolia. Encelia

farinosa. Ephedra nevadensis. L. divaricata. and Peucephyllum

schottii with lesser abundances of Ambrosia dumosa. C. giganlea,

Echinocaclus polycephalus. E. cylindraceus, Krameria erecla (in-

cluding K. parvifolia; range ratany), and Lycium macrodon (wolf-

berry). By 7530 yr B.P. Acacia greggii and Prosopis velutina dis-

appeared. Ephedra nevadensis declined markedly, and Hyptis

emoryi and Olneya tesola appeared in the record.

A full-glacial sample from 330 m in the nearby Tinajas Altas

Mountains records a woodland dominated by .luniperus californica,

Chrvsolhamnus leretifolius. Larrea divaricata, Yucca brevifolia,

and Y. whipplei with lesser abundances of Agave deserti. Encelia

farinosa. Ephedra nevadensis. Ericameria laricifolia. Eriogonum

fasciculalum. Monardella arizonica. Nolina higelovii. Prosopis

velutina. and Salvia mohavensis. A TAMS date of 1 8,700 yr B.P. on

twigs from the sample is the oldest record for L. divaricaia. the

widespread dominant in the warm deserts of North America. Higher

elevations in the Tinajas .'Mtas supported more mesic woodlands.

18

T. R. Van Devcnder er al.

The lowest elevational record for Piniis monophylla was from 460

m at 1 1 .040 yr B.P. Leaves of Ambrosia diimosa from a sample from

580 m elevation yielded an age of 10.600 vr B.P. (Van Devender et

al.. 1985).

The Puerto Blanco Mountains are on the edge of the Arizona

Upland in Organ Pipe Cactus National Monument (75 km E). A

14,120 yr B.P. sample from 565 m on rhyolite was dominated by

Juniperus californica, Opiintia whipplei (Whipple cholla), and

Yucca hrevifolia, with significant amounts of Agave deserli.

Chrysothamnus teretifoUus. Ericameria laricifolia. and Salvia

mohavensis (Van Devender, 1987). Eriogoiuim wrightii,

Monardetla arizonica. Opuntia chlorotica ( silver dollar cactus ). and

Yucca whipplei were present in small quantities. Four early Holo-

cene samples ( 10.540 to 9070 yr B.P.) record a shift to a Sonoran

desertscrub similar to the contemporaneous Homaday vegetation.

i.e., dominance by Encelia faiinosa. rarity of J. californica. and

lesser abundances oi Acacia greggii. Carnegiea giganlea. Ephedra

nevadensis, and Prosopis velutina. Interestingly, Larrea divaricala

did not appear in the Puerto Blanco samples until .^400 yr B.P.

Set in this regional framework, the Homaday samples provide

additional perspective. The single twig of Juniperus cf. californica

in the 9370 yr B.P. sample dominated by Encelia farinosa. Ephedra

nevadensis, and Larrea divaricata indicates that it may have been

near its lower elevational limits at 260 m. In light of the magnitude

of vegetation change reflected in contemporaneus samples from the

Picacho Peak area and the Butler Mountains, the late Wisconsin

vegetation of that of the Homadays was probably a slightly more

mesic version of the early Holocene with a little more /. californica.

Chrysolhamnus teretifoUus. and Eriogonum fasciculatum. Nolina

higelovii. Salvia mohavensis. and Yucca whipplei may have reached

the area as well. From the top of Pinacate Peak at 1 290 m to the Gulf

of California, a pinyon-juniper woodland with Pinus monophylla

probably grew above 610 m, a juniper woodland/chaparral with

J. californica and associates extended down to perhaps 280 m. and

Sonoran desertscrub with additional Mohave desert elements oc-

curred at lower elevations. The paleowoodland on the north slope

was probably better developed and supported more mesic and

northern plants than did the south slope.

Another factor contributing to the area of the ice age desert was

sea level lowering of 120±60 m at the Wisconsin glacial maximum

(18,000 yr B.P.) as water from the oceans was incorporated into

continental and montane glaciers (Bloom, 198.3). A lowering of this

magnitude would expose significant areas at the head of the Gulf of

California, increasing the area of the Gran Desierto markedly. By

10,000 yr B.P. sea levels on the west coast of the United States had

risen to within about 50 m of today's.

Evolution and Community Stability

Axelrod ( 1979) placed the origin of the Sonoran Desert and the

major evolutionary radiation of its unique plants in the latest Mio-

cene, 5 to 8 million years ago, reflecting the culmination of general

trends toward aridity that began about 15 million years ago as the

Rocky Mountains and Sierra Madre Occidental were uplifted.

About 14 million years ago rifting opened the proto- Gulf of Cali-

fornia and isolated several large islands from mainland Mexico

(Murphy. 1983). By the end of the Miocene the land mass had

moved northward to meet California and form the Baja California

Peninsula. About 3 to 4 million years ago the Gulf of California

extended to the San Gorgonio Pass area east of Los Angeles,

increasing the isolation of Baja California. In response to these

events the Baja California Peninsula has been a secondary arena for

the evolution of the Sonoran Desert biota. During the 15-20 glacials

of the 1.8 million years of the Pleistocene (Imbrie and Imbrie,

1979), temperate woodlands were probably widespread, with the

ranges of most subtropical Sonoran Desert species contracted into

southern refugia.

In the 1920s and 1930s, Henry A. Gleason and Forrest Shreve, a

pioneering desert ecologist with the Carnegie Desert Botanical

Laboratory on Tuniamoc Hill, championed the individualistic

theory of plant communities (see Bowers, 1988). They felt that each

species had its own distribution, physiological tolerances, and

evolutionary history, and that communities were dynamic associa-

tions of them. The palynologist Margaret D. Davis (1986) has

expressed a similar concept from a paleoclimatic perspective, i.e.,

community composition varies continuously as species respond

differently to climatic fluctuations on scales from years to millen-

nia, and rarely if ever reaches equilibrium. The plant macrofossils

preserved in packrat middens from the Homadays and other ranges

and studies by later generations of Tuniamoc Hill desert ecologists

provide elegant support for this concept from the arid core of the

Sonoran Desert. Interestingly, today all of the species in the early

Holocene samples from the Homadays can be found in association

either within the region today or at the chaparral/desertscmb eco-

tone on the western edge of the Sonoran Desert in southeastern

Califomia or northem Baja California. Late Wisconsin and early

Holocene assemblages from higher areas in the Sonoran Desert

yielded numerous anomalous associations of species whose ranges

no longer overlap (Van Devender, 1990).

ACKNOWLEDGMENTS

Laurence J. Toolin identified the grass fossils. Donald J. Pinkava

and Bruce Parfitt examined an unknown cactus stem. Peter L.

Kresan provided the photograph for Figure 2. Charles T. Mason and

Rebecca K. Van Devender have helped with the thousands of

voucher specimens deposited into The University of Arizona Her-

barium. The careful reviews and editing of J. Piatt Bradbury, Arthur

H. Harris, Philip Unitt, and Robert S. Thompson improved the

manuscript. Funds were provided by the Southwest Parks and

Monuments Association through Organ Pipe Cactus National

Monument. Fernando Chiang and Carol Madeheim translated the

Resumen. Jean Morgan proces.sed the manuscript. Dana Domer

drafted the figures.

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