Late Quaternary Plants and Vertebrates from Picacho Peak, Arizona

Abstract

Eighty-two plant and 26 small vertebrate taxa were identified from four radiocarbon dated woodrat (Neotoma sp.) midden samples from a rock crevice at 655 m elevation on Picacho Peak, Pinal Co., Arizona. A late Wisconsin (11,100 to 13,170 YBP) pinyon-juniper-oak woodland dominated by Pinus monophylla (singleleaf pinyon) and Juniperus cf. osteosperma (Utah juniper) and J. erythrocarpa/monosperma (redberry/oneseed juniper) shifted to a more xeric juniper-oak woodland/chaparral in the early Holocene (9,420 to 10,280 YBP). Extralocal species that no longer live on Picacho Peak were 29.9% in the late Wisconsin and 20.1% in the early Holocene with the greatest changes in longlived trees, shrubs, and succulents. The biogeographic and taxonomic significance of Scaphiopus hammondi (western spadefoot) and the ecological adaptability of Phrynosoma platyrhinos (desert horned lizard), Micranthe whitneyi (elf owl), and Neotoma cf. albigula (white-throated woodrat) are discussed. All taxa in the vertebrate fauna are widespread in desertscrub and desert-grassland, still occur in the area, and enter woodlands only at the upper or northern edges of their ranges. The typical inhabitation of woodlands by these vertebrates and anomalous associations in the flora probably reflect the expansion of equable climates that now characterize ecotonal habitats during the late Wisconsin glacial. Analyses of plant remains preserved in ancient woodrat (Neotoma sp.) middens are providing excellent developmental sequences for local floras and vegetation for the last 20,000 to 40,000 years on rocky slopes in the southwestern United States and northern Mexico (Van Devender et al., 1987a). During the last glacial (the Wisconsin) period, woodlands expanded into the modern Sonoran Desert in southwestern Arizona and adjacent California (111* to 113*W). Pinyon-juniper-oak woodland dominated by Pinus monophylla (singleleaf pinyon) was at 600 to 1,555 m elevation with a xeric Juniperus californica (California juniper) woodland down to about 300 m, and Larrea divaricata (creosotebush) desertscrub at lower elevations along the Colorado River and around the head of the Gulf of California in Sonora and Baja California (Van Devender, 1990). Animal remains are much less common than plants in woodrat middens but provide insight into faunal responses to changing climate and vegetation (Van Devender and Mead, 1978; Van Devender et al., 1979; Mead and Phillips, 1981; Mead et al., 1983). Here we report the pla and vertebrate remains in woodrat middens from Picacho Peak, Arizona. MATERIALS AND METHODS-Picacho Peak was an isolated butte in Picacho Peak State Park rising from 550 m elevation at the base to 1,030 m at the top (Briscoe, 1967; Shafiquallah et al., 1976; Yeend, 1976). The nearest mountains of larger size were the Santa Catalina Mountains just north of Tucson (55 km SE) that rose to 2,790 m elevation. The lowest elevation near Picacho Peak was 520 m along the Santa Cruz River (3 km SW). The local climate was characterized by infrequent winter freezes, hot summers, and biseasonal rainfall from Pacific winter frontal storms and the subtropical summer monsoon. Mean temperatures for Eloy (15 km NW) at 475 m were 10.8*C for January and 32.3*C for July with extremes of 10.6*C and 47.2*C (Sellers and Hill, 1974). Precipitation was 215 mm/year with 40.6% falling from June to September. Picacho Peak was in the relatively well-watered Arizona Upland Subdivision of the northeastern Sonoran Desert (Shreve, 1964; Turner and Brown, 1982). The vegetation on rocky slopes and bajadas was dominated by Cercidium microphyllum (foothill paloverde), Olneya tesota (ironwood), and Carnegiea gigantea (saguaro) in This content downloaded from 207.46.13.149 on Mon, 03 Oct 2016 06:10:19 UTC All use subject to http://about.jstor.org/terms September 1991 Van Devender et al.-Late Quaternary plants and vertebrates 303