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
- Rare & Endangered Species
- Undescribed Species & Additions to Arizona
- Species Richness
- Creosotebush-Bursage Association
- Paloverde-Saguaro-Ironwood Association
- Paloverde-Saguaro Association
- Jojoba- Mixed Shrub Association
- Desert Grassland
- Desert Riparian Scrub
- Vegetation Dynamics
The Tucson Mountains are a small (about 40,000 hectares) desert range in the Arizona Upland subdivision of the Sonoran Desert in Pima County, southern Arizona. The dominant vegetation types are desertscrub and desert grassland.The vascular flora is unusually rich with 630 specific and infraspecific taxa in 332 genera and 80 families. Fourteen percent are introduced exotics.Half were found in less than five sites; 175 (28%) were rare. Life forms include trees (1%), shrubs (9%), subshrubs (7%), succulents (6%), and herbs(73%), including grasses (20%), herbaceous perennials (5%), perennial/annual herbs (3%), and annuals (45%). The annuals grow in response to precipitation in the winter & spring (61%), summer & fall (33%), or both (6%).A disproportionate number of 25 species found in the Tucson Mountains prior to 1950 but not recollected in the present study were winter annuals.
The Tucson Mountains are a small, isolated desert range west of Tucson in southern Arizona. Tucson has been continuously inhabited since 1687 when the Jesuit missionary Padre Eusebio F. Kino visited the Indian village of 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 aware of were in 1884 by Cyrus G. Pringle, a prolific nineteenth century plant collector & explorer from Vermont, who stayed briefly at Fort Lowell,an army camp established to protect Anglo and Mexican settlers from raiding bands of Apaches. For the most part Pringle and other early plant collectors such as John G. Lemmon and Samuel B. Parish were quickly lured into the forested Santa Catalina and Huachuca mountains in search of new botanical treasures. However in April and May, Pringle collected a specimen from the "Sierra Tucson" that was to be described by Asa Gray of Harvard University as Metastelma arizonicum (now Cynachum arizonicum).
In 1903, Daniel T. MacDougal established the Carnegie Desert Botanical Laboratory on Tumamoc Hill in the Tucson Mountains. The site was selected because of the unique, interesting vegetation dominated by the picturesque Carnegiea gigantea (saguaro), Cercidium microphyllum (foothills paloverde),and a diverse array of associates. Nineteen permanent plots established on the grounds are the longest & monitored vegetation plots in the world (White, 1985; Goldberg and Turner, 1986). Until the early 1940's,the Desert Laboratory was a haven for resident and visiting botanists including Frederick Clements, Howard S. Gentry, Robert R. Humphrey, and Volney M.Spalding (Bowers, 1988). The pioneering studies of Forrest Shreve, director and resident ecologist, helped to define the deserts of North America and to understand the distributions and adaptations of the plants in them.He established an herbarium of about 30,000 specimens from many areas from the southwestern United States and northern Mexico, but not many from the Tucson Mountains. In contrast, John J. Thornber, professor of botany and curator of the herbarium at the University of Arizona for 43 years, collected voluminously 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 Mountain Park. In 1952, the Arizona & Sonora Desert Museum was established within Tucson Mountain Park to exhibit and interpret the natural history of the Sonoran Desert region. Today the Museum is an internationally recognized natural history museum with over 600,000 visitors each year and extensive environmental education programs. In 1961, much of the remainder of the Tucson Mountains were protected in Saguaro National Monument. Some of the densest saguaro "forests" anywhere are protected in these two parks. In 1990, the Sonoran Desert Station was established by the University of Arizona for the long & term study of insects.
Thornber's (1909) summary of the plants of the Desert Laboratory domain was one of the earliest local floras in Arizona, then a Territory. With the completion of Arizona Flora by Thomas H. Kearney and Robert Peebles in 1951 and its second edition with supplement in 1960, the stage was set for the study of floras of additional local mountain ranges or preserves.Between 1959 and 1980, local floras were the subject of 16 journal articles and 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 Tumamoc Hill. The history of invasions of exotic plants onto the Desert Laboratory property has been especially well documented (Burgess et al., 1991). The only major flora in the Tucson area is for the Rincon Mountains east of Tucson in Saguaro National Monument (Bowers and McLaughlin, 1987).
The Tucson Mountains, located in the ecological transition between the Sonoran Desert and higher biotic communities including desert grassland,chaparral, and montane woodlands and forests, are an excellent floristic study area. Few areas have the rich historical background and early collections to build on. Here we present an annotated checklist of the vascular plant species of the Tucson Mountains, descriptions of the vegetation and habitats,and evidence of changes in species distributions this century.
Flora. The vascular flora of the Tucson Mountains contains 607 species and 23 infraspecific taxa in 332 genera and 80 families. Only modest numbers of 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 genera with the most species are Euphorbia (16), Opuntia (12 species plus two varieties and eight hybrids), and Bouteloua (10).
Herbaceous plants are the most speciose life form in most tropical and temperate floras (Hammel, 1990). Life forms in the Tucson Mountains flora include herbs (74%), shrubs (9%), subshrubs (7%), succulents (5%), and trees (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 and fruit the first year in open, exposed spots but persist as herbaceous perennials in favorable sites.
The majority of the taxa (83%) in the flora are common, uncommon, or rare with few very common (13%) or abundant (4%) species. Most taxa have local distributions (51%; less than five locations), with fewer scattered(21%) and widespread (28%). Most taxa (26%) were both local and rare; no species were widespread and rare. Widespread species are generally very common to common, scattered species common to uncommon, and local species uncommon or rare.
Cactaceae. Cacti are conspicuous in the desertsof North America, with the greatest number of species in the United States in Texas, New Mexico, Arizona, and southeastern California (Benson, 1982).Twenty & nine percent of 68 native Arizona species and varieties of cacti are found in the Tucson Mountains. Saguaro, prickly pear (Opuntia,subgenus Platyopuntia), and cholla are conspicuous visual dominants in the vegetation. The Cactaceae with 21 species is the sixth largest family and Opuntia (12 species) the second largest genus in the Tucson Mountain flora. Relict populations of claret cup cactus (Echinocereus triglochidiatus) and fishhook pincushion (Mammillaria viridiflora) are found above 640 m elevation. Whipple cholla (Opuntia whipplei) was identified from an ancient packrat (Neotoma sp.) midden from Picture Rocks Pass radiocarbon dated at 21,000 yr B.P. (radiocarbon years before 1950). The southernmost locality for it today is in desert grassland near Oracle in Pinal County 55 km to the 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 remarkably promiscuous 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 Speedway Boulevard below Gates Pass appears to reflect hybridization between an ornamental tree cholla (O. imbricata) no longer present and local staghorn cholla (O. versicolor) sometime after Las Lomas Estates was established as a dude ranch in 1929.
Hybrids between the small Christmas cactus (O. leptocaulis) and the shrubby cane cholla (O. spinosior) and staghorn cholla are very distinct with intermediate plant size and shape, stem diameter, flowers, fruits,and seeds. Opuntia xkelvinensis, a distinctive plant with bronze stems scattered on the lower western bajada in Saguaro National Monument, appears to 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 populations are often difficult to identify due to the bewildering range of parental character combinations. The dense "swarm" on Camino de Oeste may have formed since the middle 1950's when an intense drought severely impacted 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 scattered on steep rocky slopes in the northern part of the Tucson Mountain range.The hybrid has a wider distribution in the range than the California barrel cactus.
Rare and Endangered Species. The Tucson Mountains supports three federally listed threatened or candidate species: Parish Indian mallow (Abutilon parishii), Pringle lipfern (Cheilanthes pringlei),and Tumamoc globeberry (Tumamoca macdougalii). The rarest of these three species is the Parish Indian mallow, an herbaceous perennial thought to be endemic to Arizona. Prior to 1978 it was only known from Santa Catalinaand Mazatzal mountains collections prior to 1940, and was misplaced by Kearney and Peebles (1960) in the synonymy of Palmer Indian mallow (A.palmeri). It is now known from about ten areas in Arizona from Little Shipp Wash near Bagdad to Peck Canyon in the Atascosa Mountains, and three sites in Sonora as far south as Nacapule Canyon near Guaymas. It is most common in the Santa Catalina Mountains. In the Tucson Mountains, a few plants have been found in six locations on rocky slopes and in steep rocky drainages.
Pringle lipfern is a subtropical fern reaching its northern limit in southern 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 the study area in the Cat Mountain & Golden Gate Mountain area, although small populations were found in Safford Canyon, near Sus Picnic Area, and in Rattlesnake Pass.
Tumamoc globeberry is officially listed as Endangered by the U. S. Fish and Wildlife Service. The type specimen was collected from Tumamoc Hill near the Desert Laboratory by Daniel T. MacDougal in 1908. Tumamoc globeberry is an herbaceous vine with a fasciculated underground tuber. It is difficult to 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 Hill region, on Oxbow Road on the east slope, and in the western portion of Saguaro National Monument.
Undescribed Species and Additions to Arizona. Two species of special interest were found in the Tucson Mountains. Tucson pigweed (Amaranthus sp. nov.) looks similar to Amaranthus palmeri, although it is monoecious and not dioecious. The first specimen was collected in September 1949 by Robert R. Humphrey on Black Mountain near the San Xavier Mission on the Tohono O'odham (Papago) Indian Reservation (just south of the Tucson Mountains), but identified as A. retroflexus by Thomas H. Kearney. It is a summer & fall annual scattered and common on rocky slopes in desertscrub and desert grassland in the Tucson Mountains. It has also been found in the Ajo, Santa Catalina, and Waterman mountains, on Ragged Top,and near Quijotoa on the Reservation. The species is currently being described by James R. Henrickson, California State University at Los Angeles.
The Yavapai lipfern (Cheilanthes yavapensis Windham) is similar to and easily confused with the beaded lipfern (Cheilanthes wootonii). The species was recently described by Michael D. Windham at the University of Utah based on Yavapai County specimens (Windham, 1993). In the Tucson Mountains, it is common on on Radio Towers Peak.
Species Richness. The Tucson Mountains are a floristically rich area due to several factors, especially geographic location, elevational range, and habitat diversity. Biseasonal rainfall,relatively mild winters, and a range of microclimates and habitats help to support a diverse flora. Largely, different plants respond to winter and summer precipitation. The many winter & spring annuals in the flora reflect strong ties with the Mohave Desert. The combination of mild winters with infrequent, modest freezes of short duration, the hot, dry arid foresummer,and summer rainfall favors cacti and subtropical desertscrub species such as ironwood.
Although there are several different substrates found throughout the range, most species are not edaphically restricted. Plants distributions are 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 most common on limestone and calcareous caliche soils in the Tucson Mountains.
The Tucson Mountains are in a complex transition zone between several phytogeographic provinces and biotic communities. The range is located in the Arizona Upland, a subdivision of the Sonoran Desert characterized by diverse floras rich in trees and succulents with strong ties to Sinaloan thornscrub in Sonora. Bundleflower (Desmanthus covillei), Calliandra schottii,gatuño (Mimosa distachya var. laxiflora), malva peluda or tuchi (Malvastrum bicuspidatum), and Sonoran bursage (Ambrosia cordifolia) are widespread subtropical desert shrubs near the northeastern limits of range in the Tucson Mountains.
Lowland creosote bush (Larrea divaricata) desertscrub typical of the Lower Colorado River Valley subdivision extends up the Gila River Valley into the Casa Grande area and back southeastward into Avra Valley on the west side of the Tucson Mountains. Elements of this xeric flora include California barrel cactus, smallseed sandmat (Euphorbia polycarpa), white bursage (Ambrosia dumosa), whitehair nievitas (Cryptantha maritima), and yellow 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 1070 m north of Gates Pass and in the Wasson Peak area, is an important dominant in desert grassland throughout southeastern Arizona. Surprisingly, other common associates including burro grass (Scleropogon breviflorus), canecholla, and soaptree yucca (Yucca elata), are restricted to elevations below desert grassland in the Tucson Mountains. Other common plants in desert 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 important in central Arizona desert grasslands zoned between paloverde & saguaro desertscrub and chaparral.
Relict chaparral plants in the area include Arizona rosewood (Vauquelinia arizonica), 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 the Tucson Mountains are woodland and forest species that are not also important in 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 Organ Pipe Cactus National Monument (Bowers, 1980; Pinkava et al., 1991) and the White Tank Mountains Regional Park (Keil, 1973) . All have similar temperatures, elevation ranges, and vegetation but are located west of Tucson. The Agua Dulce, Ajo, Growler, and Puerto Blanco mountains, and Quitobaquito Springs are in the Organ Pipe study area. The White Tank Mountains are a desert range west of Phoenix. Organ Pipe Cactus National Monument supports 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 increases from east to west across the Sonoran Desert toward the winter & rainfall Mohave Desert. The percentages of winter & spring annuals in these floras (73%, 75%) are greater than in the Tucson Mountains (61%). However,the Tucson Mountains with 164 has more winter & spring annuals than the other areas (127 and 119). The lower percentage of winter & spring annuals in the Tucson Mountains actually reflects greater numbers of summer
& fall annuals (105). Thus, the Tucson Mountains have a much larger number of annuals (269) than the others areas (94 and 110).
Vegetation. The Sonoran Desert has a greater diversity of life forms and plant communities than the other North American deserts (Shreve, 1964). Major subdivisions of the Sonoran Desert, mostly reflecting vegetation and climate, were defined by Shreve (1964) and refined by Turner and Brown (1982). Tucson is located in the Arizona Upland subdivision in the northeastern Sonoran Desert. The hot, dry Lower Colorado River Valley subdivision is mostly in the lowlands of southeastern California, southwestern Arizona and the Gran Desierto of northeastern Baja California and northwestern Sonora. It also extends northeastward in the Gila River Valley to about Casa Grande and then southeastward into the Avra Valley west of Tucson.
On Turner's (1974) vegetation map of the Tucson area, creosote bush and paloverde & saguaro desertscrub, and a woody phase of desert grassland formed the vegetation of the Tucson Mountains. Here, the plant communities and associations of the Tucson Mountains are described based on our observations following the hierarchical classification of the biotic communities and plant associations of Brown and Lowe (1974) .
Creosote bush & Bursage Association. The Creosote bush & Bursage association is typically found at 650 to 800 m elevations on fine & grained soils on the least rocky, most level terrain. 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 creosote bush, which often grows in uniform stands of 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 Silverbell Road, all scale (Atriplex polycarpa) and four wing saltbushes (Atriplex canescensand A. linearis) are dominant with creosote bush.
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 denseunder story of subshrubs and smaller cacti. The subshrub layer is often a monotypic stand of triangleleaf bursage, although white bursage is common on sandy soils in the northwestern corner of the study area. Creosote bush and 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. This is the prevalent association throughout the Tucson Mountains from 650 to 1450 m elevation. The association is structurally and floristically diverse,species & rich, and best developed on well & drained soils of rocky slopes and middle and upper bajadas. It forms a mosaic of trees, shrubs, subshrubs, cacti, and grasses. The primary species are foothills paloverde and saguaro. Shrubs and subshrubs are more varied than the trees, often in a multi-layered understory of 5 to 15 species. Important shrubs include creosote bush, jojoba (Simmondsia chinensis), limberbush (Jatropha cardiophylla), and ocotillo (Fouquieria splendens). The understory may be dominated by triangleleaf bursage on moderate slopes or brittlebush (Encelia farinosa) on rockier slopes. Sonoran bursage is a dominant subshrub in the Gates Pass area. Other common subshrubs are white desert zinnia (Zinnia acerosa),ratany (Krameria grayi), and paper flower (Psilostrophe cooperi). The cacti reach their greatest diversity in this association, especially buckhorn, chainfruit, staghorn, and teddy bear (O. bigelovii) chollas, Christmas cactus, variable prickly pear, fishhook barrel cactus, fishhook pincushion (Mammillaria grahamii), and hedgehog cactus (Echinocereus fasciculatus).
Jojoba Mixed & Scrub Association. The jojoba mixed & scrub association is most often found on rocky north slopes 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 on hotter west & or south & facing slopes triangleleaf bursage is a 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 lower communities. Are any of the relicts & & & Arizona rosewood,shrub live oak, etc. mostly in this association?
Desert Grassland. Desert grassland is an unevenly 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 in the Wasson Peak area, are patchy and include many desertscrub plants. The phytogeographic components were discussed above. Summer & rainfall perennial grasses include Arizona cottontop (Digitaria californica), black grama, curly mesquite, green sprangletop (Leptochloa dubia), plains lovegrass, hairy, slender (Bouteloua repens), and sideoats grama. Tanglehead (Heteropogon contortus), a widespread tropical grass, is unusually common for desert grassland. Local patches are conspicuous because the mature foliage turnsa distinctive orange & gold color. Other important plants in desert grassland include banana yucca, shindagger, sotol, turpentine bush, velvet mesquite, and wild buckwheat.
Desert Riparian Scrub. Riparian vegetation has been defined as that which occurs in or adjacent to drainage ways and/or floodplains, and which differs in species and/or life forms from that of the immediately surrounding vegetation (Lowe, 1961; Szaro, 1989). The only perennial water in the Tucson Mountains are small areas where bedrock forces water 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 canyons south of Camino del Cerro and in Javelina Wash. An abandoned stock tank near the Tohono O'odham Indian saguaro harvest camp near Sandario Road in Saguaro National Monument supports a dense stand of Johnson grass (Sorghumhalepense) and disturbance plants.
Although the term riparian has been used to imply vegetation associated with perennial water such as a stream or cienega, relatively mesic, linear communities along drainages are integral parts of deserts. A typical riparian zone 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 mesquite (Prosopis velutina), white-thorn acacia (Acacia constricta), and wolfberry (Lycium berlandieri). Baby bonnets (Coursetia glandulosa) is a common large shrub in King Canyon and an unnamed wash by Trails End Road. Blue paloverde (Cercidium floridum) is scattered and cheesebush (Hymenoclea salsola) is common along washes in the lower bajada.
Vegetation Dynamics. Plant remains in ancient packrat middens have documented widespread expansions of woodland and chaparral trees into the deserts of North America from 45,000 to 11,000 years ago in the Wisconsin glacial period (Van Devender et al., 1987). In the Sonoran Desert, a pinyon & juniper & oak woodland dominated by single-leaf pinyon (Pinus monophylla), Utah juniper (Juniperus osteosperma), and shrub live oak descended down to about 550 m elevation into areas presently occupied by Arizona Upland desertscrub (Van Devender, 1990). A more xeric woodland/chaparral dominated by California juniper (J. californica) and Joshua tree (Yucca brevifolia) extended down to about 300 m elevation in the Lower Colorado River Valley. Few, if any, Sonoran desertscrub dominants persisted in these areas during glacial periods when climates differed from today in greater winter and greatly reduced summer rainfall, mild winters with few freezes,and cooler summer temperatures. Although the present interglacial, the Holocene, began about 11,000 years ago, more xeric woodland and chaparral plants lingered in desert elevations until about 8,900 years ago. A cool, mesic Sonoran desertscrub and a relatively modern bi seasonal climatic regime developed 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 radio carbon dated to 12,430 yr B.P. from 865 m elevation near Golden Gate Mountainand 21,000 yr B.P. from Gates Pass yielded single leaf pinyon & Utah juniper 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 until 7,985 yr B.P. at 700 m elevation on a shady, north & facing cliff base on Rillito Peak. This date likely marks the final restriction of chaparral and 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 turpentine bush (Ericameria cuneata), and pancake prickly pear.
The isolated stand of Opuntia acanthocarpa X O. spinosior at 1425 m on the top of Wasson Peak may provide some historical insight. Although cane 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. Its genetic signature in the Wasson Peak hybrids indicates that it was much more widespread in the past. Packrat midden studies from desert grassland areas in Texas and New Mexico concluded that 4,000 to 8,900 years ago (middle Holocene) summer monsoons and grasslands were better developed than today (Van Devender et al., 1987). During this period it is likely that desert grassland in the Tucson Mountains was better developed and more widespread than at present. Modern climate and vegetation were likely established about 4,000 years ago (Van Devender, 1990).
The unusual time depth in the Tucson Mountains flora provides evidence that the flora and vegetation are still in flux. A total of 26 species that 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 winter rainfall through this century (Sellers and Hill, 1974).
The floristic survey was the subject of Rondeau's Master of Science thesis(1991) in the Department of Plant Sciences, University of Arizona, under the direction of Steven P. McLaughlin, Charles T. Mason, and James O'Leary.Charles T. Mason provided access to the University of Arizona Herbarium and use of a computer. Steven P. McLaughlin provided his expertise on local floras. John and Charlotte Reeder identified or verified most of the grasses we collected over the years, seemingly never tiring of our numerous collections of Aristida. Marc A. Baker helped sort out the chollas and their confusing array of hybrids. Sam Friedman helped with the field work and collected many interesting specimens from around his home on Anklam Road. Gordon Rodda helped with computer analyses as well as reviewing the manuscript numerous times. Many employees of Saguaro National Monument helped with logistics, permits, and suggestions, among them are William Paleck, Meg Weesner, and Elizabeth Belantoni. John F. Wiens, collected two species new to the flora and commented on the manuscript. Cathy Stefens helped with the section on geology of the Tucson Mountains. The research was supported by grants from the Roy Chapman Andrews Fund of the Arizona & Sonora Desert Museum and the Southwest Parks and Monuments Association. Figure 1 was drafted by Dana Dorner.
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Content last updated: 24 February 2000