Flora and Vegetation of the Tucson Mountains, Pima County, Arizona

Authors: Renée J. Rondeau (1,2,3), Thomas R. Van Devender (2),
C. David Bertelsen (2,3), Philip D. Jenkins (3), Rebecca K. Van Devender
(3), and Mark A. Dimmitt(2)

1 = Herbarium, Biology Dept., Colorado State University
2 = Arizona & Sonora Desert Museum
3 = Herbarium, Department of Plant Sciences, University of Arizona

Table of Contents

ABSTRACT
INTRODUCTION
RESULTS

• Flora
• Cactaceae
• Rare & Endangered Species
• Undescribed Species & Additions to Arizona
• Species Richness
• Vegetation
• Creosotebush-Bursage Association
• Paloverde-Saguaro-Ironwood Association
• Paloverde-Saguaro Association
• Jojoba- Mixed Shrub Association
• Desert Grassland
• Desert Riparian Scrub
• Vegetation Dynamics

ACKNOWLEDGEMENTS
LITERATURE CITED

Go to Tucson Mountains Plant List

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The Tucson Mountains are a small (about 40,000 hectares) desert range inthe Arizona Upland subdivision of the Sonoran Desert in Pima County, southernArizona. The dominant vegetation types are desertscrub and desert grassland.The vascular flora is unusually rich with 630 specific and infraspecifictaxa in 332 genera and 80 families. Fourteen percent are introduced exotics.Half were found in less than five sites; 175 (28%) were rare. Life formsinclude trees (1%), shrubs (9%), subshrubs (7%), succulents (6%), and herbs(73%), including grasses (20%), herbaceous perennials (5%), perennial/annualherbs (3%), and annuals (45%). The annuals grow in response to precipitationin the winter & spring (61%), summer & fall (33%), or both (6%).A disproportionate number of 25 species found in the Tucson Mountains priorto 1950 but not recollected in the present study were winter annuals.

INTRODUCTION

The Tucson Mountains are a small, isolated desert range west of Tucsonin southern Arizona. Tucson has been continuously inhabited since 1687when the Jesuit missionary Padre Eusebio F. Kino visited the Indian villageof Chuk Son on the Santa Cruz River at the base of Sentinel Peak. However,the earliest plant collections from the Tucson Mountains that we are awareof were in 1884 by Cyrus G. Pringle, a prolific nineteenth century plantcollector & explorer from Vermont, who stayed briefly at Fort Lowell,an army camp established to protect Anglo and Mexican settlers from raidingbands of Apaches. For the most part Pringle and other early plant collectorssuch as John G. Lemmon and Samuel B. Parish were quickly lured into theforested Santa Catalina and Huachuca mountains in search of new botanicaltreasures. However in April and May, Pringle collected a specimen fromthe "Sierra Tucson" that was to be described by Asa Gray of HarvardUniversity as Metastelma arizonicum (now Cynachum arizonicum).

In 1903, Daniel T. MacDougal established the Carnegie Desert BotanicalLaboratory on Tumamoc Hill in the Tucson Mountains. The site was selectedbecause of the unique, interesting vegetation dominated by the picturesqueCarnegiea gigantea (saguaro), Cercidium microphyllum (foothills paloverde),and a diverse array of associates. Nineteen permanent plots establishedon the grounds are the longest & monitored vegetation plots in theworld (White, 1985; Goldberg and Turner, 1986). Until the early 1940's,the Desert Laboratory was a haven for resident and visiting botanists includingFrederick Clements, Howard S. Gentry, Robert R. Humphrey, and Volney M.Spalding (Bowers, 1988). The pioneering studies of Forrest Shreve, directorand resident ecologist, helped to define the deserts of North America andto understand the distributions and adaptations of the plants in them.He established an herbarium of about 30,000 specimens from many areas fromthe southwestern United States and northern Mexico, but not many from theTucson Mountains. In contrast, John J. Thornber, professor of botany andcurator of the herbarium at the University of Arizona for 43 years, collectedvoluminously on Tumamoc Hill and lower areas in the Tucson Mountains (Thornber,1909).

In the 1920's Pima County set aside part of the range as Tucson MountainPark. In 1952, the Arizona & Sonora Desert Museum was established withinTucson Mountain Park to exhibit and interpret the natural history of theSonoran Desert region. Today the Museum is an internationally recognizednatural history museum with over 600,000 visitors each year and extensiveenvironmental education programs. In 1961, much of the remainder of theTucson Mountains were protected in Saguaro National Monument. Some of thedensest saguaro "forests" anywhere are protected in these twoparks. In 1990, the Sonoran Desert Station was established by the Universityof Arizona for the long & term study of insects.

Thornber's (1909) summary of the plants of the Desert Laboratory domainwas one of the earliest local floras in Arizona, then a Territory. Withthe completion of Arizona Flora by Thomas H. Kearney and Robert Peeblesin 1951 and its second edition with supplement in 1960, the stage was setfor the study of floras of additional local mountain ranges or preserves.Between 1959 and 1980, local floras were the subject of 16 journal articlesand bulletins and seven unpublished masters theses at Arizona State University,Northern Arizona University, and the University of Arizona (Bowers, 1982).In 1985 Bowers and Turner revised and updated Thornber's flora of TumamocHill. The history of invasions of exotic plants onto the Desert Laboratoryproperty has been especially well documented (Burgess et al., 1991). Theonly major flora in the Tucson area is for the Rincon Mountains east ofTucson in Saguaro National Monument (Bowers and McLaughlin, 1987).

The Tucson Mountains, located in the ecological transition between theSonoran Desert and higher biotic communities including desert grassland,chaparral, and montane woodlands and forests, are an excellent floristicstudy area. Few areas have the rich historical background and early collectionsto build on. Here we present an annotated checklist of the vascular plantspecies of the Tucson Mountains, descriptions of the vegetation and habitats,and evidence of changes in species distributions this century.

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RESULTS

Flora. The vascular flora of the Tucson Mountains contains 607 speciesand 23 infraspecific taxa in 332 genera and 80 families. Only modest numbersof exotic species (14%) are present, notably in the Gramineae (33%), Cruciferae(10%) Compositae (9%), and Leguminosae (9%). The largest families are Compositae(17%), Gramineae (15%) and Leguminosae (8%). Ten families make up 62% ofthe flora while 30 families are represented by a single species. The generawith the most species are Euphorbia (16), Opuntia (12 species plus twovarieties and eight hybrids), and Bouteloua (10).

Herbaceous plants are the most speciose life form in most tropical andtemperate floras (Hammel, 1990). Life forms in the Tucson Mountains florainclude herbs (74%), shrubs (9%), subshrubs (7%), succulents (5%), andtrees (1%). The herbs are largely represented by grasses (20%) and composites(17%). Annuals or ephemerals, the commonest life form (45%), include winter& spring (61%), summer & fall (33%), and opportunistic species(6%). A few species (3%) are perennial/annual herbs that can flower andfruit the first year in open, exposed spots but persist as herbaceous perennialsin favorable sites.

The majority of the taxa (83%) in the flora are common, uncommon, orrare with few very common (13%) or abundant (4%) species. Most taxa havelocal distributions (51%; less than five locations), with fewer scattered(21%) and widespread (28%). Most taxa (26%) were both local and rare; nospecies were widespread and rare. Widespread species are generally verycommon to common, scattered species common to uncommon, and local speciesuncommon or rare.

Cactaceae. Cacti are conspicuous in the desertsof North America, with the greatest number of species in the United Statesin Texas, New Mexico, Arizona, and southeastern California (Benson, 1982).Twenty & nine percent of 68 native Arizona species and varieties ofcacti are found in the Tucson Mountains. Saguaro, prickly pear (Opuntia,subgenus Platyopuntia), and cholla are conspicuous visual dominants inthe vegetation. The Cactaceae with 21 species is the sixth largest familyand Opuntia (12 species) the second largest genus in the Tucson Mountainflora. Relict populations of claret cup cactus (Echinocereus triglochidiatus)and fishhook pincushion (Mammillaria viridiflora) are found above 640 melevation. Whipple cholla (Opuntia whipplei) was identified from an ancientpackrat (Neotoma sp.) midden from Picture Rocks Pass radiocarbon datedat 21,000 yr B.P. (radiocarbon years before 1950). The southernmost localityfor it today is in desert grassland near Oracle in Pinal County 55 km tothe east & northeast (Benson, 1982).

Arizona, especially the desert mountain ranges of southern Arizona,has far more chollas (Opuntia, subgenus Cylindropuntia) than other states.The chollas in the Tucson Mountains are diverse (seven species) and remarkablypromiscuous with nine hybrids among them (Baker et al., unpubl. data).Some of the hybrids have formal names including Opuntia xkelvinensis (O.fulgida X O. spinosior), O. xvivipara (O. arbuscula X O. cf. versicolor),and O. xtetracantha (O. leptocaulis X O. spinosior). Another hybrid O.xcongesta (O. acanthocarpa X O. leptocaulis) is likely to be found in therange. In most cases both parent species occur in the same area as thehybrids. An isolated stand of O. acanthocarpa X O. spinosior at 1425 melevation on the top of Wasson Peak 480 m above the highest cane chollais a notable exception. A single O. imbricata X O. versicolor near SpeedwayBoulevard below Gates Pass appears to reflect hybridization between anornamental tree cholla (O. imbricata) no longer present and local staghorncholla (O. versicolor) sometime after Las Lomas Estates was establishedas a dude ranch in 1929.

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Hybrids between the small Christmas cactus (O. leptocaulis) and theshrubby cane cholla (O. spinosior) and staghorn cholla are very distinctwith intermediate plant size and shape, stem diameter, flowers, fruits,and seeds. Opuntia xkelvinensis, a distinctive plant with bronze stemsscattered on the lower western bajada in Saguaro National Monument, appearsto be a single clone. "Hybrid swarms" are in the Gates Pass (O.acanthocarpa X O. versicolor) and Camino de Oeste & Sweetwater Drive(O. spinosior X O. versicolor) areas. Individuals in hybrid populationsare often difficult to identify due to the bewildering range of parentalcharacter combinations. The dense "swarm" on Camino de Oestemay have formed since the middle 1950's when an intense drought severelyimpacted the local desertscrub.

Another common hybrid in the Tucson Mountains is Ferocactus cylindraceusx F. wislizenii. Fishhook barrel cactus (F. wislizenii) is common and widespread,often on bajadas, while California barrel cactus (F. cylindraceus) is scatteredon steep rocky slopes in the northern part of the Tucson Mountain range.The hybrid has a wider distribution in the range than the California barrelcactus.

Rare and Endangered Species. The Tucson Mountainssupports three federally listed threatened or candidate species: ParishIndian mallow (Abutilon parishii), Pringle lipfern (Cheilanthes pringlei),and Tumamoc globeberry (Tumamoca macdougalii). The rarest of these threespecies is the Parish Indian mallow, an herbaceous perennial thought tobe endemic to Arizona. Prior to 1978 it was only known from Santa Catalinaand Mazatzal mountains collections prior to 1940, and was misplaced byKearney and Peebles (1960) in the synonymy of Palmer Indian mallow (A.palmeri). It is now known from about ten areas in Arizona from Little ShippWash near Bagdad to Peck Canyon in the Atascosa Mountains, and three sitesin Sonora as far south as Nacapule Canyon near Guaymas. It is most commonin the Santa Catalina Mountains. In the Tucson Mountains, a few plantshave been found in six locations on rocky slopes and in steep rocky drainages.

Pringle lipfern is a subtropical fern reaching its northern limit insouthern Arizona. It is common in Sonora as far south as the Alamos area.In the Tucson Mountains it is very common on rhyolitic slopes, cliff faces,and under boulders. Most populations are in the southwestern part of thestudy area in the Cat Mountain & Golden Gate Mountain area, althoughsmall populations were found in Safford Canyon, near Sus Picnic Area, andin Rattlesnake Pass.

Tumamoc globeberry is officially listed as Endangered by the U. S. Fishand Wildlife Service. The type specimen was collected from Tumamoc Hillnear the Desert Laboratory by Daniel T. MacDougal in 1908. Tumamoc globeberryis an herbaceous vine with a fasciculated underground tuber. It is difficultto see twined inside shrubs except in late summer when its fruit turn orange& red. In the Tucson Mountains it has been found in the Tumamoc Hillregion, on Oxbow Road on the east slope, and in the western portion ofSaguaro National Monument.

Undescribed Species and Additions to Arizona. Two species of special interest were found in the Tucson Mountains. Tucsonpigweed (Amaranthus sp. nov.) looks similar to Amaranthus palmeri, althoughit is monoecious and not dioecious. The first specimen was collected inSeptember 1949 by Robert R. Humphrey on Black Mountain near the San XavierMission on the Tohono O'odham (Papago) Indian Reservation (just south ofthe Tucson Mountains), but identified as A. retroflexus by Thomas H. Kearney.It is a summer & fall annual scattered and common on rocky slopes indesertscrub and desert grassland in the Tucson Mountains. It has also beenfound in the Ajo, Santa Catalina, and Waterman mountains, on Ragged Top,and near Quijotoa on the Reservation. The species is currently being describedby James R. Henrickson, California State University at Los Angeles.

The Yavapai lipfern (Cheilanthes yavapensis Windham) is similar to andeasily confused with the beaded lipfern (Cheilanthes wootonii). The specieswas recently described by Michael D. Windham at the University of Utahbased on Yavapai County specimens (Windham, 1993). In the Tucson Mountains,it is commonon on Radio Towers Peak.

Species Richness. The Tucson Mountainsare a floristically rich area due to several factors, especially geographiclocation, elevational range, and habitat diversity. Biseasonal rainfall,relatively mild winters, and a range of microclimates and habitats helpto support a diverse flora. Largely, different plants respond to winterand summer precipitation. The many winter & spring annuals in the florareflect strong ties with the Mohave Desert. The combination of mild winterswith infrequent, modest freezes of short duration, the hot, dry arid foresummer,and summer rainfall favors cacti and subtropical desertscrub species suchas ironwood.

Although there are several different substrates found throughout therange, most species are not edaphically restricted. Plants distributionsare more a reflection of coarseness and chemical composition of soils.Soils become progressively finer with more sand and clay from bedrock tobajada to flats. Granite weathers rapidly into gruss forming "plantfriendly" soils. Widespread Chihuahuan Desert species such as helechillo(Notholaena cochisensis), hierba del corazon (Dalea pogonathera), milkwort(Polygala macradenia), mariola (Parthenium incanum), oreja del perro (Tiquiliacanescens), and red grama (Bouteloua trifida) are restricted to or mostcommon on limestone and calcareous caliche soils in the Tucson Mountains.

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The Tucson Mountains are in a complex transition zone between severalphytogeographic provinces and biotic communities. The range is locatedin the Arizona Upland, a subdivision of the Sonoran Desert characterizedby diverse floras rich in trees and succulents with strong ties to Sinaloanthornscrub in Sonora. Bundleflower (Desmanthus covillei), Calliandra schottii,gatuño (Mimosa distachya var. laxiflora), malva peluda or tuchi(Malvastrum bicuspidatum), and Sonoran bursage (Ambrosia cordifolia) arewidespread subtropical desert shrubs near the northeastern limits of rangein the Tucson Mountains.

Lowland creosotebush (Larrea divaricata) desertscrub typical of theLower Colorado River Valley subdivision extends up the Gila River Valleyinto the Casa Grande area and back southeastward into Avra Valley on thewest side of the Tucson Mountains. Elements of this xeric flora includeCalifornia barrel cactus, smallseed sandmat (Euphorbia polycarpa), whitebursage (Ambrosia dumosa), whitehair nievitas (Cryptantha maritima), andyellow felt plant (Horsfordia newberryi).

The upper portions of the range support local areas of desert grassland.Some species such as black grama (Bouteloua eriopoda), cane beardgrass(Bothriochloa barbinodis), coyote melon (Apodanthera undulata), curly mesquite(Hilaria belangeri), Dalea wrightii, Evolvulus nuttalianus, gordolobo (Gnaphaliumwrightii), hairy grama (Bouteloua hirsuta), Indian mallow (Abutilon parvulum),plains lovegrass (Eragrostis intermedia), scarlet morning glory (Ipomoeacristulata), sideoats grama (Bouteloua curtipendula), sotol (Dasylirionwheeleri), tobosa (Hilaria mutica), twoleaf desert senna (Senna bauhinioides),and wait & a & minute bush (Mimosa aculeaticarpa var. biuncifera)are widespread in desert grassland in the southwestern United States. Shindagger(Agave schottii), which occurs in dense stands on rocky slopes above 1070m north of Gates Pass and in the Wasson Peak area, is an important dominantin desert grassland throughout southeastern Arizona. Surprisingly, othercommon associates including burro grass (Scleropogon breviflorus), canecholla, and soaptree yucca (Yucca elata), are restricted to elevationsbelow desert grassland in the Tucson Mountains. Other common plants indesert grassland in the Tucson Mountains such as desert rock pea (Lotusrigidus), pancake prickly pear (Opuntia chlorotica), turpentine bush (Ericamerialaricifolia), and wild buckwheat (Eriogonum wrightii) are more importantin central Arizona desert grasslands zoned between paloverde & saguarodesertscrub and chaparral.

Relict chaparral plants in the area include Arizona rosewood (Vauqueliniaarizonica), banana yucca (Yucca baccata), shrub live oak (Quercus turbinella),and skunk bush (Rhus aromatica). Brickell & bush (Brickellia californica)is important in both chaparral and woodland. Relatively few plants in theTucson Mountains are woodland and forest species that are not also importantin desert grassland or chaparral. Claret cup cactus, red four o'clock (Oxybaphuscoccineus), and Texas mulberry (Morus microphylla) are notable exceptions.

The flora of the Tucson Mountains can be compared to those of OrganPipe Cactus National Monument (Bowers, 1980; Pinkava et al., 1991) andthe White Tank Mountains Regional Park (Keil, 1973) . All have similartemperatures, elevation ranges, and vegetation but are located west ofTucson. The Agua Dulce, Ajo, Growler, and Puerto Blanco mountains, andQuitobaquito Springs are in the Organ Pipe study area. The White Tank Mountainsare a desert range west of Phoenix. Organ Pipe Cactus National Monumentsupports 550 species of plants (70.9% shared with the Tucson Mountains)and the White Tank Mountain Regional Park 332 (81.0% shared). Shreve (1964)showed that the number of obligate winter & spring annuals increasesfrom east to west across the Sonoran Desert toward the winter & rainfallMohave Desert. The percentages of winter & spring annuals in thesefloras (73%, 75%) are greater than in the Tucson Mountains (61%). However,the Tucson Mountains with 164 has more winter & spring annuals thanthe other areas (127 and 119). The lower percentage of winter & springannuals in the Tucson Mountains actually reflects greater numbers of summer

& fall annuals (105). Thus, the Tucson Mountains have a much largernumber of annuals (269) than the others areas (94 and 110).

Vegetation. The Sonoran Desert has a greaterdiversity of life forms and plant communities than the other North Americandeserts (Shreve, 1964). Major subdivisions of the Sonoran Desert, mostlyreflecting vegetation and climate, were defined by Shreve (1964) and refinedby Turner and Brown (1982). Tucson is located in the Arizona Upland subdivisionin the northeastern Sonoran Desert. The hot, dry Lower Colorado River Valleysubdivision is mostly in the lowlands of southeastern California, southwesternArizona and the Gran Desierto of northeastern Baja California and northwesternSonora. It also extends northeastward in the Gila River Valley to aboutCasa Grande and then southeastward into the Avra Valley west of Tucson.

On Turner's (1974) vegetation map of the Tucson area, creosotebush andpaloverde & saguaro desertscrub, and a woody phase of desert grasslandformed the vegetation of the Tucson Mountains. Here, the plant communitiesand associations of the Tucson Mountains are described based on our observationsfollowing the hierarchical classification of the biotic communities andplant associations of Brown and Lowe (1974) .

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Creosotebush & Bursage Association. TheCreosotebush & Bursage association is typically found at 650 to 800m elevations on fine & grained soils on the least rocky, most levelterrain. This fairly simple association is composed mainly of shrubs andsubshrubs with trees and stem succulents generally lacking or at most scattered.The dominant shrub is creosotebush, which often grows in uniform standsof even stature and equal spacing. Triangleleaf bursage (Ambrosia deltoidea)is a common subshrub, although white bursage is also present in the northwesternpart of the study area. On the Santa Cruz River floodplain along SilverbellRoad, allscale (Atriplex polycarpa) and fourwing saltbushes (Atriplex canescensand A. linearis) are dominant with creosotebush.

Paloverde & Saguaro & Ironwood Association. This association is mostly on alluvial soils on the gentle bajadas at 650& 900 m elevation on the western slopes of the Tucson Mountains. Itis a structurally diverse community characterized by foothills paloverde(Cercidium microphyllum), saguaro, and ironwood (Olneya tesota) in a denseunderstory of subshrubs and smaller cacti. The subshrub layer is oftena monotypic stand of triangleleaf bursage, although white bursage is commonon sandy soils in the northwestern corner of the study area. Creosotebushand stem succulents including buckhorn cholla, chainfruit cholla (Opuntiafulgida), pencil cholla (O. arbuscula), and variable prickly pear (O. phaeacantha)generally make up the midstory.

Paloverde & Saguaro Association. Thisis the prevalent association throughout the Tucson Mountains from 650 to1450 m elevation. The association is structurally and floristically diverse,species & rich, and best developed on well & drained soils of rockyslopes and middle and upper bajadas. It forms a mosaic of trees, shrubs,subshrubs, cacti, and grasses. The primary species are foothills paloverdeand saguaro. Shrubs and subshrubs are more varied than the trees, oftenin a multilayered understory of 5 to 15 species. Important shrubs includecreosotebush, jojoba (Simmondsia chinensis), limberbush (Jatropha cardiophylla),and ocotillo (Fouquieria splendens). The understory may be dominated bytriangleleaf bursage on moderate slopes or brittlebush (Encelia farinosa)on rockier slopes. Sonoran bursage is a dominant subshrub in the GatesPass area. Other common subshrubs are white desert zinnia (Zinnia acerosa),ratany (Krameria grayi), and paper flower (Psilostrophe cooperi). The cactireach their greatest diversity in this association, especially buckhorn,chainfruit, staghorn, and teddy bear (O. bigelovii) chollas, Christmascactus, variable prickly pear, fishhook barrel cactus, fishhook pincushion(Mammillaria grahamii), and hedgehog cactus (Echinocereus fasciculatus).

Jojoba Mixed & Scrub Association. Thejojoba mixed & scrub association is most often found on rocky northslopes at 800 to 1,430 m, but best developed in the upper part of the range.The dominant shrub is jojoba which forms almost pure stands on higher east& and north & facing slopes. In areas at lower elevations and onhotter west & or south & facing slopes triangleleaf bursage isa common associate. Other associates include buckhorn cholla, desert zinnia,fishhook barrel cactus, ocotillo, pancake prickly pear, wild buckwheat,and wolfberry. [Renee & & jojoba extends down into the other communities;this discussion should be limited to the jojoba dominated areas. Are saguaro,foothills paloverde, or ironwood in the community or stragglers from lowercommunities. Are any of the relicts & & & Arizona rosewood,shrub live oak, etc. mostly in this association?

Desert Grassland. Desert grassland is anunevenly distributed, complex mosaic in the upper parts of the Tucson Mountains.Even the best areas on south & facing slopes at 1,125 to 1430 m inthe Wasson Peak area, are patchy and include many desertscrub plants. Thephytogeographic components were discussed above. Summer & rainfallperennial grasses include Arizona cottontop (Digitaria californica), blackgrama, curly mesquite, green sprangletop (Leptochloa dubia), plains lovegrass,hairy, slender (Bouteloua repens), and sideoats grama. Tanglehead (Heteropogoncontortus), a widespread tropical grass, is unusually common for desertgrassland. Local patches are conspicuous because the mature foliage turnsa distinctive orange & gold color. Other important plants in desertgrassland include banana yucca, shindagger, sotol, turpentine bush, velvetmesquite, and wild buckwheat.

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Desert Riparian Scrub. Riparian vegetationhas been defined as that which occurs in or adjacent to drainageways and/orfloodplains, and which differs in species and/or life forms from that ofthe immediately surrounding vegetation (Lowe, 1961; Szaro, 1989). The onlyperennial water in the Tucson Mountains are small areas where bedrock forceswater to the surface in Gatuño, King, Safford, and Sweetwater canyons.A few water & loving species such as centaury (Centaurium calycosum)and monkey flowers (Mimulus guttatus, M. nasutus) were found in these areas.A few small Fremont cottonwoods (Populus fremontii) are in rocky canyonssouth of Camino del Cerro and in Javelina Wash. An abandoned stock tanknear the Tohono O'odham Indian saguaro harvest camp near Sandario Roadin Saguaro National Monument supports a dense stand of Johnson grass (Sorghumhalepense) and disturbance plants.

Although the term riparian has been used to imply vegetation associatedwith perennial water such as a stream or cienega, relatively mesic, linearcommunities along drainages are integral parts of deserts. A typical riparianzone restricted to the edges of washes passing through foothills paloverde& saguaro desertscrub contains canyon ragweed (Ambrosia ambrosioides),catclaw acacia (A. greggii), desert hackberry (Celtis pallida), desertlavender (Hyptis emoryi), velvet mesqite (Prosopis velutina), whitethornacacia (Acacia constricta), and wolfberry (Lycium berlandieri). Baby bonnets(Coursetia glandulosa) is a common large shrub in King Canyon and an unnamedwash by Trails End Road. Blue paloverde (Cercidium floridum) is scatteredand cheesebush (Hymenoclea salsola) is common along washes in the lowerbajada.

Vegetation Dynamics. Plant remains in ancientpackrat middens have documented widespread expansions of woodland and chaparraltrees into the deserts of North America from 45,000 to 11,000 years agoin the Wisconsin glacial period (Van Devender et al., 1987). In the SonoranDesert, a pinyon & juniper & oak woodland dominated by singleleafpinyon (Pinus monophylla), Utah juniper (Juniperus osteosperma), and shrublive oak descended down to about 550 m elevation into areas presently occupiedby Arizona Upland desertscrub (Van Devender, 1990). A more xeric woodland/chaparraldominated by California juniper (J. californica) and Joshua tree (Yuccabrevifolia) extended down to about 300 m elevation in the Lower ColoradoRiver Valley. Few, if any, Sonoran desertscrub dominants persisted in theseareas during glacial periods when climates differed from today in greaterwinter and greatly reduced summer rainfall, mild winters with few freezes,and cooler summer temperatures. Although the present interglacial, theHolocene, began about 11,000 years ago, more xeric woodland and chaparralplants lingered in desert elevations until about 8,900 years ago. A cool,mesic Sonoran desertscrub and a relatively modern biseasonal climatic regimedeveloped at that time (Van Devender, 1990).

Packrat middens have been found in several areas in the Tucson Mountains(Van Devender, 1990; Van Devender and Rondeau, unpubl. data). Samples radiocarbondated to 12,430 yr B.P. from 865 m elevation near Golden Gate Mountainand 21,000 yr B.P. from Gates Pass yielded singleleaf pinyon & Utahjuniper woodland assemblages containing a big sagebrush (Artemisia tridentata

& type), an oak (Quercus sp.), desert agave (Agave deserti), pancakeprickly pear, sotol, trnebushes. A radiocarbon date documents the survival of redberry juniper until7,985 yr B.P. at 700 m elevation on a shady, north & facing cliff baseon Rillito Peak. This date likely marks the final restriction of chaparraland woodland plants to relict areas in the Tucson Mountains (Brown, 1978).Relict populations of rock sage (Salvia pinguifolia), shrub live oak, skunkbush, and Texas mulberry are restricted to north slopes on the higher peaks.Relictual species found at middle to high elevations are Arizona rosewood,banana yucca, California barrel cactus, claret cup cactus, cuneate turpentinebush (Ericameria cuneata), and pancake prickly pear.

The isolated stand of Opuntia acanthocarpa X O. spinosior at 1425 mon the top of Wasson Peak may provide some historical insight. Althoughcane cholla is a desert grassland species occurring as high as 1980 m elevation(Benson, 1982), it is restricted below 945 m in the Tucson Mountains. Itsgenetic signature in the Wasson Peak hybrids indicates that it was muchmore widespread in the past. Packrat midden studies from desert grasslandareas in Texas and New Mexico concluded that 4,000 to 8,900 years ago (middleHolocene) summer monsoons and grasslands were better developed than today(Van Devender et al., 1987). During this period it is likely that desertgrassland in the Tucson Mountains was better developed and more widespreadthan at present. Modern climate and vegetation were likely establishedabout 4,000 years ago (Van Devender, 1990).

The unusual time depth in the Tucson Mountains flora provides evidencethat the flora and vegetation are still in flux. A total of 26 speciesthat were present or common prior to 1950 have not been reported since(Rondeau et al., 1991). Considering that most were annuals (75%, 85% winter& spring species), the extirpations were likely due to decreasing winterrainfall through this century (Sellers and Hill, 1974).

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ACKNOWLEDGEMENTS

The floristic survey was the subject of Rondeau's Master of Science thesis(1991) in the Department of Plant Sciences, University of Arizona, underthe direction of Steven P. McLaughlin, Charles T. Mason, and James O'Leary.Charles T. Mason provided access to the University of Arizona Herbariumand use of a computer. Steven P. McLaughlin provided his expertise on localfloras. John and Charlotte Reeder identified or verified most of the grasseswe collected over the years, seeminly never tiring of our numerous collectionsof Aristida. Marc A. Baker helped sort out the chollas and their confusingarray of hybrids. Sam Friedman helped with the field work and collectedmany interesting specimens from around his home on Anklam Road. GordonRodda helped with computer analyses as well as reviewing the manuscriptnumerous times. Many employees of Saguaro National Monument helped withlogistics, permits, and suggestions, among them are William Paleck, MegWeesner, and Elizabeth Belantoni. John F. Wiens, collected two speciesnew to the flora and commented on the manuscript. Cathy Stefens helpedwith the section on geology of the Tucson Mountains. The research was supportedby grants from the Roy Chapman Andrews Fund of the Arizona & SonoraDesert Museum and the Southwest Parks and Monuments Association. Figure1 was drafted by Dana Dorner.

LITERATURE CITED

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Content last updated: 24 February 2000