Holocene Vegetation and Climate in the Puerto Blanc0 Mountains, Southwestern Arizona

Home , Encelia farinosa, New Water Mountains, Olneya

QUATERNARY RESEARCH 21, 5 J-12 (1987)

THOMAS R. VAN DEVENDER Arizona-Sonora Desert Museum, 2021 N. Kinney Rd., Tucson, AZ 85743 Received December 9. 1985

Plant macrofossils from 21 pack rat (Neotoma sp.) middens at 535-605 m from the Puerto Blanc0 Mountains, southwestern Arizona, provide an excellent history of vegetation and climate for the last 14,120 yr B.P. in the Sonoran Desert. A late Wisconsin juniper-Joshua tree woodland gave way to a transitional early Holocene desertscrub with sparse Juniperus californica (California juniper) by 10,540 yr B.P. Important Sonoran Desert plants including Carnegiea giganrea (saguaro) and Encelia farinosa (brittle bush) were dominants. Riparian trees such as Acacia greggii (catclaw acacia), Prosopis velutina (velvet mesquite), and Cerdicium j7oridum (blue palo Verde) grew on dry, south-facing slopes in a middle Holocene Sonoran desertscrub in a warm, wet summer climate with frequent winter freezes. Modern subtropical Sonoran desertscrub formed about 4000 yr B.P. as summer rainfall and winter freezes declined. Cercidium microphyllum (foothills palo Verde), Sapium biloculare (Mexican jumping bean), Olneya tesofa (ironwood) and Sten- ocereus thurberi (organ pipe cactus) became dominant as riparian trees retreated to wash habitats. The inferences of a latest Wisconsin/early Holocene summer monsoonal maximum by J. E. Kutz- bath (1983), Modeling of Holocene climates. In “Late-Quaternary Environments of the United States,” Vol. 2, “The Holocene” (H. E. Wright, Ed.), pp. 271-277. Univ. of Minnesota Press, Minneapolis) are not supported for the Southwest. Apparently the persistence of late Wisconsin circulation patterns offset any increases in insolation. c 1987 University of Washington.

Plant remains preserved in ancient pack rocks out of a variety of local plant mate- rat (Neotoma) middens allow excellent re- rials, including thorny branches and cactus constructions of the local history of vegeta- joints. Middens are areas near entrances to tion and climate in the North American internal passages where fecal pellets, plant deserts for 30,000 yr of the last glacial/in- fragments, and general debris are deposited terglacial cycle (Spaulding, et al., 1983; during house cleaning (Finley, 1958). In dry Van Devender and Spaulding, 1979). In the rock shelters where the house is small or late Wisconsin prior to about 11,000 yr ago, absent, the midden can become cemented most of the present warm deserts sup- with urine and preserved for tens of thou- ported woodlands dominated by pinyons, sands of years. Pack rats are very efficient junipers, and shrub oaks. In Arizona, samplers of the plants within about 30-50 pinyon-juniper-oak woodlands dominated m of the shelter. Radiocarbon dating allows by Pinus monophylla (singleleaf pinyon) the midden assemblages to be placed in grew at 1555-610 m. In the modern Son- time. Series of radiocarbon-dated midden oran Desert, juniper woodland and creo- assemblages from a single cave or several sote bush desertscrub with Juniperus cali- caves in a small local area provide detailed fornica (California juniper) grew as low as records of the local vegetation history. 305 and 240 m, respectively (Van Devender In this study, plant macrofossils were an- et al., 1985). alyzed in a chronological series of pack rat Pack rat middens are hard, dark, shiny midden assemblages from the heart of the organic deposits that are built by rodents in Sonoran Desert in southwestern Arizona. the genus Neotoma. In open areas, they This sequence not only provides a detailed build their houses near shrubs or large Holocene vegetation history for a unique

51 0033-5894187 $3.00 Copyright 0 1987 by the University of Washington. 52 THOMAS R. VAN DEVENDER subtropical desert, but allows paleoclimatic Hill, 1974). Using an elevational lapse rate reconstruction in an area dominated by calculated from nearby weather stations, summer monsoonal rainfall and limited by the top of Mount Ajo at 1465 m receives an severe winter freezes. The data will help estimated 457-510 mm/yr. evaluate recent paleoclimatic models based on changes in the earth’s oribtal param- METHODS AND RESULTS eters that suggest that summer monsoon In the present study, 21 pack rat midden was best developed in the Southwest in the samples were collected from rhyolitic rock- latest Wisconsin and the early Holocene shelters at 535-605 m from three areas (Kutzbach, 1983; Spaulding and Graum- (Ajo Loop, Cholla Pass, Twin Peaks) in the lich, 1985). The study area is well suited to Puerto Blanc0 Mountains within 3 km of evaluating whether the middle Holocene the Organ Pipe Cactus National Monument was a period of Altithermal drought Visitor Center (Fig. 2). Most of the samples (Antevs, 1955) or of increased summer were collected from xeric, south-facing rainfall (Martin, 1963). slopes. The subtropical vegetation on these slopes is exceptionally sensitive to fluctua- ENVIRONMENTAL SETTING tions in rainfall and to grazing. The Puerto Blanc0 Mountains are a small Detailed plant lists and collections were desert range reaching 960 m elevation in made within 30-50 m of each rockshelter Organ Pipe Cactus National Monument, and a composite flora was prepared for the Pima County, Arizona (31”58-59’N, entire study area. Voucher specimens of 112”47-48’W; Fig. 1). A suite of subtro- unusual plants were deposited in the her- pical Sonoran Desert plants including Sten- baria at the University of Arizona and ocereus thurberi (organ pipe cactus), Lo- Organ Pipe Cactus National Monument. phocereus schottii (senita cactus), Sapium Pack rat midden samples were washed in biloculare (Mexican jumping bean), Ja- water, oven-dried, and sorted. Weights tropha cinerea, and J. cuneata (limber after washing ranged from 63 to 377 g. bushes) reach the United States in this Plant macrofossils were identified using unique desert area (Bowers, 1980). The reference collections in the University of massive Ajo Mountains, 9 km northeast of Arizona Herbarium and Laboratory of Pa- the midden sites, rise to 1465 m elevation leoenvironmental Studies. Internal relative and support desert-grassland and sparse abundances were assigned to each plant chaparral. The present vegetation near the taxa in an assemblage on a scale of 1-5. In sites is a xeric Sonoran desertscrub domi- this scale a single specimen is a 1, the most nated by Encelia farinosa (brittle bush), common taxa a 5, with other abundances Cercidium microphyllum (foothills palo ranked between. Although the number of Verde), Sapium biloculare, and Steno- plants in ranks 2-5 varies depending on the cereus thurberi (Fig. 2). The area is at the total number of identified specimens, the xeric lower edge of the Arizona Upland relative abundances between samples are Subdivision of the Sonoran Desert (Shreve, readily compared. 1964; Turner and Brown, 1982). Twenty-one radiocarbon dates on mate- The climate of Organ Pipe Cactus Na- rials from the pack rat middens yielded tional Monument is characteristic of the ages from 14,120 to 30 yr BP with fairly northeastern Sonoran Desert with bisea- continuous coverage from the late Wis- sonal rainfall from Pacific winter frontal consin to present (Table 1). The only indi- storms and the summer monsoon, hot vidual species dated were twigs of Juni- summers, and infrequent winter freezes. perus californica and Encelia farinosa. The Total annual rainfall at the Visitor Center at latter date was obtained using the Univer- 510 m elevation is 233 mm/yr (Sellers and sity of Arizona tandem accelerator mass I 1 I 1 1 116 112* 1100 / NEVADA 114*

; Organ Pipe Cactus I- -“&& i National Monument I

CALIFORNIA ARIZONA

----SONORAN DESERT BOUNDARY A PACKAAT MIDDENS

FIG. I. Map showing areas discussed in text. Boundary of Sonoran Desert after Shreve (1964). 54 THOMAS R. VAN DEVENDER HOLOCENE VEGETATION IN ARIZONA 55

TABLE I. RADIOCARBON DATES OF PACK RAT MIDDENS FROM THE PUERTO BLANCO MOUNTAINS. ORGAN PIPE CACTUS NATIONAL MONUMENT, PIMA COUNTY, ARIZONA Elevation Radiocarbon Sample Cm) date (yr B.P.) Lab No. Material dated

Twin Peaks #l 565 14,120 + 260 A-3986 Jmiperus cal~fornicu twigs Cholla Pass #lA 605 10,540 2 250 A-4276 Neotomu fecal pellets Twin Peaks #7 565 9,860 t 180 A-4212 Neotomu fecal pellets Twin Peaks #5 560 9,720 2 460 AA-625” Encelia furinosu twigs Cholla Pass #lB 605 9,070 2 170 A-4278 Neotomu fecal pellets Cholla Pass #lC 605 8.790 2 210 A-4277 Neoromu fecal pellets Ajo Loop # 1D 550 7,970 2 130 A-398 1 Neotomu fecal pellets Twin Peaks #6 580 7,580 t 100 A-3980 Neotomu fecal pellets Ajo Loop #I G 550 7.560 t 170 A-3985 Neotomu fecal pellets Ajo Loop # 1A 550 5.240 -c 90 A-4213 Neotomu fecal pellets Ajo Loop #5B 535 3.480 t 80 A-4233 Neoromu fecal pellets Cholla Pass #2 590 3.440 t 90 A-4279 Neotomu fecal pellets Ajo Loop #6C 550 3,400 t 100 A-4235 Neotomu fecal pellets Ajo Loop #6B 550 3.220 r 100 A-4234 Neotomu fecal pellets Twin Peaks #2 580 2,340 r 70 A-3973 Neotomu fecal pellets Twin Peaks #3 s50 2.160 k 60 A-3982 Midden debris Twin Peaks #4 555 1.910 k 50 A-3998 Neotomu fecal pellets Ajo Loop #3A 550 990 2 50 A-3972 Neotomu fecal pellets Ajo Loop #4 550 980 2 70 A-4214 Neotomu fecal pellets Ajo Loop #2C 550 130 2 50 A-397 1 Neotomu fecal pellets Ajo Loop #2B 550 30 IT 50 A-3970 Neotomu fecal pellets -. a Date from tandem accelerator mass spectrometer. spectrometer. The remainder of the dates and herbs (29.1%). Chronological sum- were for pack rat fecal pellets or midden maries of important trees and shrubs, suc- debris. Careful stratigraphic collection and culents, and grasses are presented in removal of the weathering rind minimizes Figures 3-5. Distributions of plants were any contamination problems for these determined through personal observations, types of radiocarbon samples. notes and collections, surveys of the Uni- The chronology proposed for the South- versity of Arizona Herbarium, an atlas of west by Van Devender and Spaulding Sonoran Desert plant distributions (1979) with modifications by Van Devender (Hastings et al., 1972), a local flora et al. (1985) and Van Devender et al. (in (Bowers, 1980), and regional floras press) is followed in this paper. The divi- (Kearney and Peebles, 1964; Munz, 1974: sions are late Wisconsin, 22,000-11,000 yr Wiggins, 1980). B.P.; early Holocene, ll,OOO-ca. 8900 yr VEGETATION HISTORY B.P.; middle Holocene, ca. 8900-ca. 4000 yr B.P. : late Holocene, ca. 4000 yr B.P. to Late Wisconsin present. The 14,120 yr B.P. sample yielded a xeric The pack rat midden assemblages con- juniper-Joshua tree woodland assemblage tained remains of a total of 103 plant taxa dominated by Juniperus culifornicu and with 11-34 (X = 21.8) taxa per assemblage Yucca brevifolia (Joshua tree). Associates (Table 2). The plants identified in the included Salvia mohavensis (Mohave samples included woody trees and shrubs sage), Yucca btlhipplei (Whipple yucca). (25.2%). subshrubs (14.6%), stem and ro- Opuntia whipplei (Whipple cholla), sette succulents (16..5%), grasses (14.6%), Opuntia chlorotica (silver dollar cactus). TABLE 2. PLANTS IDENTIFIED FROM PACK RAT MIDDENS FROM THE PUERTO BLANCO MOUNTAINS, PIMA COUNTY, ARIZONA

Age (10” yr B.P.P

Species Common name 14.1 10.5 9.9 9.7 9.1 8.8 8.0 7.6 7.6 5.2 3 5 3.4 3.4 3.2 2.3 2.2 1.9 1.0 1.0 0.1 0.0 Trees and shrubs& Chrysothamnus Rabbit brush 3e teretifolius Ericameria ktricifolia lhrpentine bush 3A Monardella arizonica IA Salvia mohavensis Mohave sage 3e le Juniperus californica California juniper 5e le 3e ?le Condalia 2r globosalwarnockii Eriogonum California buckwheat 2A fasciculatum Lycium sp. Wolfberry 2s 2” Cercidium jloridum Blue palo Verde 2r sr 4r 3r 2r Acacia greggii Catclaw acacia 3r 2r 3r 3r 2r 2r 2r 2r Ephedra nevadensis Mormon tea 2P IP 4P 3” Pros& velutina Velvet mesquite lr Ir 2r 2r 2r 2r Encelia farinosa Brittle bush 5” 5” 5n 5” 5n 4n 5n 3n 5n 4n 4n 4n 5n 3n 3n 3n 4n 4n 3n Salvio pinguifolia Rock sage 2A HymenocIea salsolo Cheese weed lr Celtis pallida Desert hackberry 2r Ir 2r Ir 2r Hyptis emoryi Desert lavender 2” 2” 2” I” 2” 2s 2s 3n 2” 2” 2” 2n Olneya tesota Ironwood 2f 3f 4f 3f 4f 5f 5f 5f Lycium cf. Wolfberry 2s 5” 2s 2s 3s 4s 3n 3” 3” 2s 2s berlandieri Horsfordia sp. Felt plant 2” tn 2” 2n 2” I” 3” 2” Cercidium Foothills palo Verde 4” 3n 5n 5n 5n 4n 4n 4n 5n 5” 5” 5” microphyllum Jatropha Limber bush 2P cordiophyllo Fouquieria splendens Ocotillo I” IS Sapium biloculare Mexican jumping bean ?I” 3s 5” 4” 5” 2” 3” 5” 4” 5n 5” Croton .sonorae Encinilla tP IP 2” 2P Larrea divaricata Creosote bush ?I” 2” 2” 2n 3” 3” 2” 2”

N = 26 Total 5 7 4 6 4 2 8 8 7 8 5 8 10 8 8 5 9 9 9 8 7 Subshrubsb Gutierrezia sp. Snake weed 2A Eriogonum wrightii Wild buckwheat 2P 2P 2P 2P 3P 2P 2P 2P Viguiero deltoidea Desert goldeneye 2m 3m 2m 2n Im Sphaeralcea sp. Globe mallow 2r 2P IP Ir Bebbia juncea Chuckwalla’s delight 2r 2s 2s HOLOCENE VEGETATION IN ARIZONA 57 THOMAS R. VAN DEVENDER Dctturcc rnrteloides Indian apple Ir Cryptantha pteroca~rl Wingnut nievitaa IP lm Plantago it7srtloris Indian wheat lP Is Ercphorbia sp. Spllrge 3n 2m 2s In Is 2s Is 2s In Is Boerhaavin sp. Spiderling l-P 7f 2f 2f Phoradendron Desert mistletoe Ir 2r 2r lr 2r cnlifornicum Aliionin inmntutcr Windmills lP Orobanche cooperi Broom rape IP Tragia nepetaefoliu Noseburn ?r lr Cryptantha sp. Nievitas In Kallstrormia sp. Summer poppy Ir 2Y Dyssodia concinno Fetid marigold 2f Chorizanthe corrugcrta Corrugated spiny herb le Perityle emoryi Rock daisy 2n 2Y Pectocaryyo Comb bur heterocflrpo Antarmthus sp. Careless weed

N = 30 Total 4 4 5 4 9 3 4 2 4 3 2 8 8 4 5 3 6 8 8 5 3 Total = 103 Total 17 IX 14 21 23 11 26 22 20 17 14 31 29 25 21 16 27 34 28 20 17

u Ages in thousands of years for each mldden follow Table 1. b Relative abundances: I = rare. 2 = uncommon. 3 = common. 4 = very common. 5 = abundant. ? = probable contaminant. Distribution codes: A = restricted to higher elevations in the Ajo Mountains; e = extralocal. no longer occur\ in Monument; f = on flats below: m = mesic microhabitat on nearby north- or east-facing slope: n = nearby. within 30 m of rockshelter: P = restricted to higher elevations in the Puerto Bianco. Bates, and Ajo mountains; p = elsewhere in the Puerto Bianco Mountains; r = restricted to rlparian habitats along wabhr\ below zites: \ = elsewhere on south-facing slope, u = uncommon or rare. scattered in Monument: y = probably at site in wetter year. 60 THOMAS R. VAN DEVENDER HOLOCENE VEGETATION IN ARIZONA

% “6 oOO,. OS Puerto WISCONSIN HOLOCENE , EARLY j MIDDLE I LATE Blanc0 LATE Juniper- Brittle Bush- Saguaro- ! Cotclow-Mesquite-Blue PoloVerda Foothills PoloVerde-lronwood Mountains IUO Tree ?d Junioer Desertscrub : Desertscrub Organ Pipe Cactus Desertscrub

Plams brlstlegrass ti Six weeks needle gromc Threeown Clttteseed muhly SIX weeks gromo Fluff grass Coitontop *Little barley *Desert needlegross Six weeks threeawn Cane beardgrass Panic gross Hall panic grass Southwestern cupgros! 3ed gromo

FIG. 5. Temporal distribution of grasses in pack rat middens from the Puerto Blanc0 Mountains. Pima County, Arizona. Time scale = 14C yr B.P.; ages in italics are radiocarbon dates (Table I). L. J. Toolin analyst. HOLOCENE VEGETATION IN ARIZONA 63 and Monardella arizonica. None of the zona (545 km N), was formerly called Y. woody trees and shrubs or succulents in newberryi. The nearest Y. whipplei to the the present Sonoran desertscrub commu- midden site is a tiny relict population at 380 nity at the site were in the midden assem- m in the Sierra 10s Alacranes, Sonora (130 blage. Opuntia chlorotica is an aborescent km W; T. L. Burgess personal communica- prickly pear found at 490-915 m elevation tion, 1985). Otherwise, the nearest Y. in the Puerto Blanc0 and Ajo mountains whipplei is in the Sierra Juarez, Baja Cali- (Bowers, 1980). Monardella arizonica is a fornia. small, shrubby mint restricted in the Mon- Opuntia whipplei is the northwestern ument to elevations above 915 m in the Ajo representative of a species group that in- Mountains (Bowers, 1980). cludes 0. spinosior (cane cholla) and 0. The remainder of the important woody imbricuta (tree cholla). It is found from plants in the late Wisconsin midden assem- northwestern New Mexico across the Col- blage no longer occur in the area. The orado Plateau to Utah and Nevada and nearest Arizona population of Juniperus south into the western Arizona portion of californica is in the Arrastra Mountain area the Mohave Desert. The nearest Arizona on the Mohave-Yavapai County line (280 populations are near Oracle on the north km NNW of the study area). The nearest side of the Santa Catalina Mountains, Pinal population to the west is in the Sierra County (195 km ENE), and near Roosevelt Juarez of northern Baja California, Mexico Dam, Gila County (240 km NE; Benson, (260 km W; Wiggins, 1980). 1982). Today, 0. spinosior lives at 460-610 Yucca brevifolia is the arborescent yucca m elevation in desert-grassland in the that dominates many areas in the Mohave northern Ajo Mountains (19 km NE; Desert in Utah, Nevada, California, and Bowers, 1980). Arizona (Fig. 2). The nearest Arizona pop- In contrast, the herbs and grasses in the ulation is north of Aguila in Yavapai midden samples do not live far away today. County (225 km NNW). In California, the In wet years, Phacelia sp. (caterpillar southernmost population is in the Cotton- weed) and Lotus sp. (deer vetch) would wood Mountains, Riverside County (340 probably grow at the site. km NW; Rowlands, 1978). Salvia mohavensis is a shrubby mint Early Holocene found in the Mohave Desert from southern Four samples in the Puerto Blanc0 Nevada and southeastern California to Mountains midden series record the early west-central Arizona (Munz, 1974). The Holocene vegetation. The assemblages are nearest Salvia mohavensis to the study dominated by Encelia farinosa and Acacia area is a relict population on Sierra Pina- greggii (catclaw acacia) in association with cate in northwestern Sonora (50 km SW; G. Ephedra nevadensis (Mormon tea), Erio- Starr, personal communication, 1984). To gonum wrightii (wild buckwheat), and Pro- the west, it occurs in the Sierra Juarez, sopis velutina (velvet mesquite). Juniper-us Baja California (Wiggins, 1980). The californica was present at low levels in nearest Arizona population is in the Sierra samples dated at 10,540 and 9070 yr B.P. Estrella near Phoenix, Maricopa County from 605 m at the Cholla Pass site. Samples (225 km NNE). dated at 9860 and 9720 yr B.P. from Yucca whipplei is a common, widespread 560-565 m on Twin Peaks lacked juniper. A rosette succulent in the chaparral commu- single leaf of Salvia mohavensis was in the nities of California and Baja California 10,540 yr B.P. sample. (Wiggins, 1980). An isolated population of Important succulents in the early Holo- Y. whipplei in the northwestern Mohave cene assemblages include Opuntia chloro- Desert in the western Grand Canyon, Ari- tica, persisting from the late Wisconsin. 64 THOMAS R. VAN DEVENDER and Carnegiea gigantea (saguaro), Opuntia gantea continued to dominate the slopes in bigelovii (teddy bear cholla), Ferocactus association with Acacia greggii, Prosopis acanthodes (California barrel cactus), and velutina, and both species of Ferocactus. F. cf. wislkenii (fishhook barrel cactus). The riparian Cercidium floridurn reappears Carnegiea gigantea is the stately, arbo- on south-facing slopes between 7970 and rescent cactus whose northeastern range 5240 yr B.P. Hyptis emoryi (desert lav- limit sets the boundary of the Sonoran ender), a common, widespread subtropical Desert itself (Shreve, 1964; Turner and desert shrub, appeared in the 7580 yr B.P. Brown, 1982). Its appearance in the 10,540 sample. Olneya tesota (ironwood) ap- yr B.P. sample suggests that it dispersed pearing in the 7560 yr B.P. sample is a large into the Organ Pipe area from its ice-age leguminous tree that is characteristic of rel- refugium soon after the end of the last gla- atively flat desert areas in much of the ciation (ca. 11,000 yr B.P.). F. acanthodes Lower Colorado River Valley. Opuntia is a widespread barrel cactus in the low- acanthocarpa (staghorn cholla) is in sev- lands of the Mohave and Sonoran deserts eral middle Holocene samples. that is uncommon in the Puerto Blanc0 The 5240 yr B.P. sample records the ar- Mountains above the sites (Bowers, 1980). rival of Cercidium microphyllum (foothills F. wislizenii is the barrel cactus character- palo Verde), the dominant tree in the istic of the Arizona Upland desertscrub, modern Arizona Upland communities. The desert-grassland and Chihuahuan desert- only long-lived perennials in the middle scrub east to near El Paso, Texas. Holocene samples that presently live at the Encelia farinosa, Carnegiea gigantea, sites are Encelia farinosa, Cercidium mi- and Opuntia bigelovii are the only long- crophyllum, Hyptis emoryi, and Carnegiea lived plants in the midden assemblages that gigantea. now grow near the sites. Acacia greggii and Prosopis velutina no longer grow on Late Holocene rocky slopes and are restricted to riparian The 5240 yr B.P. sample with abundant wash habitats. Cercidium jloridum (blue Cercidium microphyllum and riparian wash palo Verde) found in the 9720 yr B.P. trees on rocky slopes was transitional be- sample is also restricted to washes. Salvia tween middle and late Holocene Sonoran pinguifolia (rock sage) found in the 7970 yr Desert communities. In general the late B.P. sample is presently rare above 915 m Holocene assemblages recorded a shift to a in the Ajo Mountains (Bowers, 1980). The warmer, drier desertscrub community as single seed of Sapium biloculare in the many plants (e.g., Acacia greggii, Celtis 9720 yr B.P. sample is probably a contami- pallida, Cercidium jloridum, Hymenoclea nant . salsola, and Prosopis velutina) retreated from exposed rocky slopes into riparian Middle Holocene wash habitats. Encelia farinosa, Cercidium The final disappearance of woodland microphyllum, and Olneya tesota were plants such as junipers and oaks from the dominant in association with Carnegiea gi- desert lowlands marks the end of the early gantea, Hyptis emoryi, Opuntia acantho- Holocene. At the Cholla Pass site Cali- carpa, and Opuntia bigelovii. Other impor- fornia juniper disappeared between 9070 tant Sonoran Desert plants appeared in the and 8790 yr B.P. Five middle Holocene record by 3400 yr ago including the modern samples between 8790 and 5240 yr B.P. dominants Sapium biloculare and Larrea record Sonoran plants that presently live divaricata. Jatropha cardiophylla (limber within a few kilometers of the rock shelters bush) is a succulent-stemmed shrub of the but in a desertscrub community with very Arizona Upland and more subtropical veg- different composition and structure than etation to the south that was found in the today. Encelia farinosa and Carnegiea gi- 3440 yr BP. sample. Since its appearance HOLOCENE VEGETATION IN ARIZONA 65 in the midden record, Stenocereus thurberi the Holocene, Carnegiea gigantea is now was consistently present and at times domi- restricted to the flats or ridge crests and is nant. The two columnar cacti (S. thurberi, very rare on open slopes. Many Steno- Carnegiea gigantea) have been desertscrub cereus thurberi on the slopes are dead. associates in the late Holocene although Ferocactus covillei is rare in the area their abundances appear to have been inde- today; a few living and dead plants are pendent. Ferocactus covillei (Coville barrel seen. Opuntia acanthocarpa is now much cactus) is a subtropical species that appar- more common on the flats than on the ently replaced F. acanthodes and F. wisli- rocky slopes. Young plants were not seen zenii in the late Holocene. for any of these succulents. Lycium berlan- Within the late Holocene, there appear dieri (wolfberry) and Croton sonorae (en- to have been some interesting vegetation cinilla) are much less common today than changes. Olneya tesota dominated xeric previously. A few plants in the late Holo- south-facing slopes until after 1910 yr B.P. cene samples including the grasses men- when it retreated to the flats below. tioned above and Chorizanthe corrugata Opuntia cf. fulgida (chain fruit cholla) grew (corrugated spiny herb; 2340 yr B.P.) ap- on slopes until this century but is now re- parently no longer occur in the Monument. stricted to flats. Ambrosia deltoidea (trian- On the other hand, Cercidium micro- gleleaf bursage), a common dominant in phyllum, Stenocereus thurberi, Sapium bi- Arizona Upland desertscrub communities, loculare, Horsfordia newberryi, and Larreu appeared only in the 3480 yr B.P. sample divaricata are presently at their peaks in and is now scattered at the base of the hills. abundance. Several shrubs including Aca- Ambrosia dumosa (white bursage), the lypha pringlei (Sonoran three-seeded mer- dominant of lowland desertscrub communi- cury), Aloysia wrightii (oreganillo), Fa- ties in the Lower Colorado River Valley, is gonia cali’ornica, and Jatropha cuneata recorded at 2340 and 990 yr B.P. In the grow near the midden sites but were not Monument today it is mostly restricted to found in the middens. sandy flats along the international border Livestock grazing in Organ Pipe Cactus and is rare on rocky slopes. National Monument was fairly intense from The samples dated at 980 and 990 yr B.P. the late 1930s until 1979 when the last cattle record a wet interval in the late Holocene. and burros were removed. Some recent Species richness was much greater than vegetation changes seen in the midden today (28-34 versus 10 taxa) with more record may be due to grazing impact. Palat- woody trees and shrubs (10 to 6) and suc- able herbs such as Abutilon incanum (ln- culents (7 to 1). Prosopis velutinu, Opuntia dian mallow), Lotus salsuginosus (deer leptocaulis (Christmas cactus), and 0. vetch), Lupinus sparsiflorus (blue bonnet), phaeacantha (variable prickly pear) are Phacelia distans (caterpillar weed), Phx- now restricted to shady microhabitats in salis crassifolius (ground cherry), and the wash below the site. The abundance of Sphaeralcea ambigua (globe mallow) are perennial and ephemeral herbs (nine) and well represented in the middens but rare or grasses (seven) suggest reliable rainfall in absent from the area today. The seed banks both seasons. Three grasses in one sample of many ephemeral and short-lived peren- (Bouteloua trifida, Eriochloa lemmoni, and nial herbs and grasses may still be recover- Panicum cf. ha&i) no longer occur in the ing from grazing depletions. Exposed Monument. rocky slopes may be slower than flats or washes to recover because of stony, Historical Period shallow soils. On the other hand, Allionia The differences between the modern veg- incarnata (windmills), Amsinckin tessellata etation and very young midden samples are (fiddleneck). Kallstroemia grandiflora surprising. After being present for most of (summer poppy), and Plantago insularis 66 THOMAS R. VAN DEVENDER

(Indian wheat) are widespread ephemerals californica was abundant and Yucca brevi- readily sampled by pack rats that have folia absent. A similar midden assemblage never been common on rocky slopes in the with reduced P. monophylla dated 32,000 study area. The decline on rocky slopes of yr B.P., extending the pinyon-juniper-oak spiny perennials such as Opuntia acantho- woodland to the middle Wisconsin. carpa, 0. bigelovii, and Fouquieria Three Alamo Canyon middens record splendens (ocotillo) are probably not re- early Holocene oak-juniper woodland lated to grazing. Declines in adult Carne- from 10,580 to 8130 yr B.P. dominated by giea gigantea, Ferocactus covillei, and Quercus ajoensis with Juniperus califor- Stenocereus thurberi are probably not the nica and J. cf. erythrocarpa (redberry ju- result of grazing, while the establishment of niper). Both Q. ajoensis and J. erythro- young plants may be directly related. The carpa still occur locally in more mesic local vegetation is clearly recovering from areas in the Ajo Mountains. Whereas ju- grazing impacts and recent catastrophic cli- nipers grew at the site until after 8130 yr matic events. B.P., their decline in abundance sometime after 9910 yr B.P. could correspond to the COMPARISON WITH OTHER AREAS disappearance of J. californica about 8900 Ajo Mountains yr ago in the Puerto Blanc0 Mountains. In- Four pack rat middens from 975 m on deed, J. californica may have become ex- Montezuma’s Head in the Ajo Mountains tinct in both areas at the same time. east of the midden study areas record a rel- Although the midden site is near the atively stable pinyon-juniper-oak wood- present upper elevational limit of Carne- land dominated by Pinus monophylla (sin- giea gigantea, its seeds were common in gleleaf pinyon), Juniperus spp. (junipers), the Alamo Canyon samples. Three tandem and Quercus ajoensis (Ajo oak) from accelerator radiocarbon dates on the seeds 21,840 to 13,500 yr BP (Van Devender and yielded results of 8080-9240 yr B.P. (Van Spaulding, 1979). On reexamination the ju- Devender et al., 1985). Considering the niper material was determined to represent 10,540 yr B.P. appearance in the Puerto J. scopulorum (Rocky Mountain juniper) Blanc0 Mountains, C. gigantea apparently and a second species with toothed leaf arrived in the area simultaneously over a margins. Quercus ajoensis is an Ajo Moun- 310-m elevational range. However, C. gi- tains endemic considered a relict of Q. gantea no longer grows at the midden site, ajoensoides, a late Miocene species from while Stenocereus thurberi is nearby. Oregon (Tucker and Mullet-, 1956). More The late Wisconsin woodland at 565 m recently it has been considered as a sub- elevation in the Puerto Blanc0 Mountains species of Q. turbinella (shrub live oak), clearly represents the lower, drier end of the widespread chaparral dominant (Felger the vegetation gradient from the more mas- and Lowe, 1970). Other important plants in sive Ajo Mountains. the Montezuma’s Head samples include Artemisia tridentata-type (big sagebrush; Lower Colorado River Valley Artemisia nova or Artemisia tridentata), In the late Wisconsin, California juniper Atriplex confertifolia (shadscale), and woodland with Pinus monophylla has been Yucca brevifolia. found as low as 460 m in the Tinajas Altas A midden dated at 14,500 yr B.P. from Mountains, 615 m in the New Water Moun- 915 m elevation in Alamo Canyon in the tains of southwestern Arizona, and 510 m central Ajo Mountains was also dominated in the Whipple Mountains of southeastern by Pinus monophylla, Juniperus scopu- California (Van Devender and Spaulding, lorum, and Quercus ajoensis with Arte- 1979; Van Devender et al., 1985). Middens misia tridentata-type. However, Juniperus containing California juniper-Joshua tree HOLOCENE VEGETATION IN ARIZONA 67 assemblages similar to the late Wisconsin The final late Holocene transition to the Puerto Blanc0 Mountains sample at 565 m relatively modern vegetation and climate have been found from 550 m in the Kofa about 4000 yr ago in the Puerto Blanc0 Mountains, Arizona, and 320 m in the Mountains is similar to the final establish- Whipple Mountains. Nolina bigelovii (Big- ment of modern desertscrub communities elow beargrass), an important desert suc- in the Tinajas Altas Mountains, and the Pi- culent in these middens, was absent from cache Peak area of southeastern California the Puerto Blanc0 Mountains sample. (Cole, 1986). The late Wisconsin vegetation of the mountainous lowlands (below 610 m) in PALEOCLIMATES Organ Pipe Cactus National Monument Many of the 103 taxa identified from the was apparently a southeastward extension middens are identified to the species level. of the woodland that extended from the allowing their present distributions and ec- modern Mohave Desert throughout the ological adaptations to be used in recon- Lower Colorado River Valley, except for structing paleoclimates. The assemblages the lowest elevations in the Picacho Peak include a variety of life forms including area, California (Cole, 1986). Most of the freeze-limited subtropical desert trees, species coexist today in Arizona around shrubs and succulents, and short-lived the base of the Hualapai Mountains, ephemerals that only grow in the winter- Mohave County (280 km NNW), or in spring or summer rainy seasons. Few southeastern California in the Providence- plants in the flora live more than 100 yr. New York Mountains area (450 km NW; Response times to climatic changes are rel- Thorne et al., 1981) and Joshua Tree Na- atively short because of catastrophic mor- tional Monument (340 km NW; Rowlands, tality in droughts or freezes and relatively 1978). Many of the plants, with the excep- short distances to seed source areas in tions of Yucca brevifolia and Opuntia Sonora. Thus the plants in midden assem- whipplei, extend into Baja California on the blages provide independent, sensitive drier, eastern slopes of the Sierra Juarez proxies for reconstructing different facets (Wiggins, 1980). Salvia mohavensis and of Holocene paleoclimates (Bryson, 1985). Yucca whipplei have small relict populations in the Sierra Pinacate area of north- Late Wisconsin western Sonora. The late Wisconsin California juniper- After about 11,000 yr ago in the early Joshua tree woodland on Twin Peaks sug- Holocene, a xeric California juniper wood- gests a paleoclimate similar to areas in the land persisted in the lowlands (275-610 m) Mohave Desert today. The driest areas that of the Lower Colorado River Valley for support Yucca brevifolia receive a precipi- several thousand years (Van Devender, tation of at least 110 mm/yr in California 1977). The final disappearance of juniper and 170 mm/yr in Arizona (Rowlands, and the transition to desert climate and 1978). Mixed communities of Y. brevifoliu vegetation of the middle Holocene was and Juniperus californica probably receive found to be about 8900 yr B.P. in the about 280 mm/yr rainfall, mostly from Tinajas Altas Mountains (Van Devender et winter Pacific frontal storms. This would al., in press), after 8910 yr B.P. in the represent a minimum increase in annual Whipple Mountains, and 7870 yr B.P. in the precipitation for the study area of only New Water Mountains, Arizona. The 16%, but a major shift in seasonality. The Puerto Blanc0 Mountains transition at few ephemerals (2) in the sample are about 8900 yr B.P. correlates well with the winter-spring obligates. The late Wis- Tinajas Altas and Whip,ple mountains consin precipitation inferred from the records. pinyon-juniper-oak woodland with the 68 THOMAS R. VAN DEVENDER mesic J. scopulorum at 915 m on an open, herbs (11) and grasses (3) identified in early west-facing slope on Montezuma’s Head Holocene samples are winter- spring rain- would be at least 510-560 mmyr based on fall obligates. The grasses are opportunistic comparison with weather records from C-4 species that can also have limited fall known J. scopulorum populations below or spring activity. the Mogollon Rim (Sellers and Hill, 1974). Encelia farinosa, the community domi- This suggests an increase of 55-70% over nant, is frost sensitive but regrows rapidly today. Extrapolation to higher elevations after a freeze. Carnegiea gigantea is ap- using modern lapse rates suggests that parently limited today on its northern range Mount Ajo probably had a precipitation of by freezing temperatures during a single at least 775-865 mm/yr and easily sup- midday (e.g., more than 32 hr; Shreve, ported larger conifers such as Pseudotsuga 1911; Turnage and Hinckley, 1938). The menziesii (Douglas fir) or Pinus ponderosa continued presence and abundances of E. (Ponderosa pine). The difference in precipi- farinosa and C. gigantea suggest that hard tation between the Puerto Blanc0 and Ajo freezes have not been very frequent at any mountains midden sites was greater in the time in the Holocene. late Wisconsin than today, suggesting a steeper elevational gradient. Orographic Middle Holocene uplift of warm, moist air probably resulted The middle Holocene climatic regime in summer rainfall in the Ajo Mountains. was essentially modern, although the fre- Significant summer rainfall was not present quencies of events were quite different in any areas further west in the late Wis- than today. Early Holocene rainfall levels consin. continued, with a greater shift to sum- As for late Wisconsin temperatures, the mer rainfall and fewer serious summer summers were probably much cooler, with droughts. Only 37.5% of the ephemeral the July mean as much as 8-11°C lower; herbs (4) and grasses (4) in the middle Ho- conditions similar to today’s winter-spring locene samples are winter-spring rainfall existed throughout the year. The midden obligates. Today, desertscrub communities fossils do not give a strong signal about the with Encelia farinosa, Carnegiea gigantea, winter temperatures because most of the Acacia greggii, Cercidium jloridum, and species were probably in the lower portions Prosopis velutina grow on open slopes at of their elevational ranges and not limited the upper margin of the Arizona Upland by winter freezes. The absence of Sonoran above more typical palo Verde-saguaro Desert plants was probably due as much to communities. Good examples can be seen very cool summers and little summer rain- at 915-1065 m below Molino Basin in the fall as to winter freezes. Santa Catalina Mountains, Pima County, and north of Magdalena, Sonora. Indeed, a Early Holocene band of similar vegetation marks the The early Holocene climate of the Puerto northern periphery of the Sonoran Desert Blanc0 Mountains was transitional be- in Arizona. Apparently the winter freezes tween glacial and modern climates. A few were frequent and severe enough to pre- California juniper lingered until about 8900 vent the expansion of more subtropical yr BP in the Cholla Pass area. The pres- plants but not to eliminate E. farinosa and ence of Acacia greggii and Prosopis velu- Carnegiea gigantea. tina on south-facing slopes suggests that A tooth of Notiosorex crawfordi (desert rainfall increased to perhaps 330 mm/yr, or shrew) in the 7580 yr B.P. sample is a 30% more than today, as summers warmed, fauna1 record with paleoclimatic implica- with a greater percentage falling in the tions. This tiny insectivore is widespread in summer. A total of 7 1.4% of the ephemeral the Southwest, but so secretive it is seldom HOLOCENE VEGETATION IN ARIZONA 69 collected. Mostly it inhabits grasslands at through a series of Holocene Sonoran des- elevations well above the midden study ertscrub communities, culminating in the area. The only extant population in Organ modern subtropical brittle bush-foothills Pipe is a small relict in the marsh at Quito- palo Verde-Mexican jumping bean-organ baquito Springs (Y. Petryszyn, personal pipe cactus association about 4000 yr ago. communication, 1984). The presence of N. Dispersal distances for Sonoran Desert crawfordi suggests that summers were less plants from glacial refugia in Sonora were harsh than today, with fewer droughts. probably 100 km or less, with relatively rapid response times following climatic Late Holocene changes. The dispersal and isolation of The late Holocene climate of the Puerto subtropical plants into Arizona, such as Blanc0 Mountains, with its essentially Bursera microphylla (elephant tree) in the modern regime, began after 5240 yr B.P. Waterman Mountains, Stenocereus thur- and probably about 4000 yr B.P. Decreased beri in the Picacho Mountains, and Justiciu rainfall with greater frequencies of summer candicans (chuparosa) in the Santa Cata- drought forced the retreat of Acacia lina Mountains, are probably late Holocene greggii, Cercidium floridurn, and Prosopis events. Indeed a tandem accelerator mass velutina from open slopes. Today they sur- spectrometer radiocarbon date for a B. mi- vive in riparian communities along large crophylla seed records the arrival of this washes (Fig. 2). A total of 54.5% of the species in the Tinajas Altas Mountains by ephemeral herbs (17) and grasses (5) are 5820 yr B.P. (Van Devender et al., 1985). winter-spring rainfall obligates. Appar- The present vegetation in the Sonoran ently reduced frequency of hard freezes al- Desert is apparently the most xeric associa- lowed the expansion of subtropical desert- tion with lowest species richness seen in scrub plants, including Stenocereus thur- the present interglaciation. The overall des- beri and Sapium biloculare. The expansion ertscrub community composition has of Olneya tesota onto xeric south-facing changed continuously as temperature and slopes from the nearby flats between 5240 rainfall fluctuated. and 1910 yr B.P. suggests that summer Larrea divaricata is an important, wide- rainfall was somewhat greater than today. spread dominant in a myriad of desertscrub The samples at 990 and 980 yr B.P. reflect a communities throughout the warm deserts wet period that may have been of short du- of North America. After a basic diploid ration. Only the 130 and 30 yr B.P. samples stock migrated from Argentina to the Chi- reflect the modern climate very well. The huahuan Desert during some earlier inter- present vegetation is even more sparse glaciation, creosote bush dispersed west- than the youngest samples, reflecting ward giving rise to the tetraploid Sonoran droughts of the 1970s. the hard freeze of Desert and hexaploid Mohave Desert chro- November 1978, and grazing impacts. mosomal races (Wells and Hunziker. 1976). Pack rat midden sequences from south- DISCUSSION central New Mexico record its arrival in Organ Pipe Cactus National Monument the northern Chihuahuan Desert by 4340 yr was established in 1937 to protect a unique B.P. in the San Andres Mountains (Van De- subtropical desertscrub vegetation in the vender and Toolin, 1983) and by 3300 yr United States that is more typical of the B.P. in the Sacramento Mountains (Van Sonoran Desert in Sonora, Mexico. The Devender et al., 1984). In contrast, new pack rat midden series from the Puerto tandem accelerator mass spectrometer Blanc0 Mountains records a series of plant (TAMS) radiocarbon dates of L. divaricclta communities from a late Wisconsin Cali- twigs from pack rat middens document its fornia juniper- Joshua tree woodland presence in late Wisconsin woodland and 70 THOMAS R. VAN DEVENDER desertscrub communities in the Lower Col- Mexico and Trans-Pecos Texas favored orado River Valley. Creosote bush twigs desert-grassland over Chihuahuan desert- from near Picacho Peak, California, and the scrub (Van Devender et al., 1984). Butler Mountains, Arizona, yielded ages of Kutzbach (1983) presented a model of 12,730 and 11,250 yr B.P., respectively Holocene climates for the entire globe (Cole, 1986; Van Devender et al., 1985). A using the possible effects of orbital varia- TAMS date of 18,700 yr B .P. from a Cali- tions and their consequent changes in solar fornia juniper-Joshua tree sample at 330 radiation on climate. He concluded that m in the Tinajas Altas Mountains docu- 10,000 to 9000 yr B.P. was a time of max- ments its presence in the full-glacial flora imum solar insolation in summer, with (Van Devender et al., in press). In this con- winter temperatures lower than today. Pe- text, the late Holocene appearance of L. riods of global warmth are reflected by in- divaricata in the Puerto Blanc0 Mountains tensification of the subtropical high pres- is very interesting. I suggest that these sure cells and summer monsoon circulation plants were Sonoran Desert tetraploids ar- patterns. The Puerto Blanc0 Mountains riving with other subtropical plants from vegetation record in the eastern portion of their ice-age refugia in Sonora. The older the Sonoran Desert, with its dependence Lower Colorado River Valley records were on summer rainfall and limitation by winter probably Mohave Desert hexaploids. If so, freezes, presents an opportunity to eval- the connection between the creosote bush uate the conclusions of the Kutzbach desertscrub communities of the eastern model for the Southwest. Early Holocene Sonoran Desert and the northwestern Chi- summers were apparently cool enough to huahuan Desert in southeastern Arizona support California juniper in low desert was probably after 5240 yr B.P. Unfortu- habitats until at least 9070 yr B.P. Only nately, the chromosomal races are not 28.6% of the ephemeral plants in early Ho- readily discernible from the midden mate- locene samples are typical summer mon- rials. soon species and none are obligates. Al- The conclusion that the most subtropical though summer precipitation increased climate of the last It ,000 yr was in the late from virtually none in the late Wisconsin to Holocene is interesting for several reasons. perhaps 30% in the early Holocene, peak The middle Holocene has traditionally summer monsoonal activity at 60% was been considered to be the “thermal max- after 8900 yr B.P. in the middle Holocene. imum” and has been variously called the Monsoonal activity continued to be high, Xerothermic (Sears, 1942), the Hypsi- although more irregular, throughout the thermal (Deevey and Flint, 1957), and the late Holocene. The early Holocene was Altithermal (Antevs, 1955). The Puerto clearly a transitional period from the glacial Blanc0 middens demonstrate decisively climate of the late Wisconsin and by no that the middle Holocene climate was means warmer than today. warmer and much wetter than today, as in- Spaulding and Graumlich (1985) at- ferred by Martin (1963), and not the intense tempted to account for early Holocene ju- drought proposed for the Altithermal. niper woodlands in the Lower Colorado However, the warm, moist climate did not River Valley as due to very hot summer favor the most subtropical plants. Appar- temperatures and a summer monsoonal ently the frequency of winter freezes was rainfall maximum. This hypothesis is un- great enough to prevent the expansion of likely because Juniperus calz~ornica is the the most cold-sensitive plants until after juniper most strongly correlated with 5000 yr B.P. and has been lower subse- winter rainfall of any in the woodlands quently. Similar middle Holocene climates bounding the warm deserts of North at higher elevations in south-central New America. It is abundant in dry chaparral HOLOCENE VEGETATION IN ARIZONA 71 and juniper-Joshua tree woodlands in the stimulating discussions of the ecological and climatic western Mohave Desert where there is significance of the fossils. Jean Morgan typed the manuscript. Helen A. Wilson drafted the figures. little or no summer rainfall. Moreover, the woodland vegetation of the early Holocene became increasingly more mesic and better REFERENCES developed west of the Puerto Blanc0 Antevs, E. (1955). Geological-climatic dating in the Mountains at lower elevations toward the West. American Antiquity 20, 317-355. source area of winter rainfall. Remnant Benson, L. (1982). “The Cacti of the United States woodlands in the early Holocene would and Canada.” Stanford Univ. Press, Stanford. have fared better to the southeast where Bowers, J. E. (1980). 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