A 22,000-year record of vegetation change in the north-central Sonoran Desert

Abstract

Plant macrofossils from eleven packrat ( Neotoma sp.) middens provide a history of vegetation change over the last 22,140 years at Eagle Eye Mountain, near the northern border of the Sonoran Desert in Arizona. This site is located in the Arizona Upland subdivision. Departures and arrivals of various perennial plant species during the Holocene were similar to other records from more southerly sites and support earlier generalizations of Van Devender [Van Devender, T.R., 1990. Late Quaternary vegetation and climate of the Sonoran Desert, United States and Mexico. In: Betancourt, J.L., Van Devender, T.R., Martin, P.S. (Eds.), Packrat Middens: The Last 40,000 Years of Biotic Change. University of Arizona Press, Tucson, pp. 134–165.] regarding the tempo and mode of late Quaternary changes in the Arizona Upland. Evergreen woodland containing Pinus monophylla (singleleaf pinyon), Juniperus osteosperma (Utah juniper), and Quercus turbinella (shrub live oak) occupied the site during full glacial times. The latter two species, but not P. monophylla, persisted into the early Holocene and were joined by a limited number of desertscrub species, including Acacia greggii (catclaw), Carnegiea gigantea (saguaro), and several other succulents. With the arrival of many other species during the middle Holocene, a desertscrub community that differed considerably from the modern vegetation developed. Acacia greggii and Cercidium floridum (blue paloverde) were the principal large woody species on exposed hillslopes. Assemblages similar in composition to the modern desertscrub did not appear until approximately 4000 years ago, when the more xerophytic Cercidium microphyllum (foothills paloverde) arrived and C. floridum disappeared from hillslopes. The delayed arrival of C. microphyllum in comparison to other Sonoran Desertscrub species with similar climatic tolerances is attributed to the limited dispersal distance of its seeds by seed-eating heteromyid rodents. At 4540 and 6425 yr B.P., the cold-intolerant shrub Encelia farinosa was present on northeasterly aspect exposures where it is absent today, suggesting slightly warmer winter conditions during the middle Holocene. The presence of the tree Cercidium floridum and the perennial, C-4 grass Setaria leucopila on exposed slopes during the middle Holocene indicates substantially moister conditions during that time than in the late Holocene or today. At sites on the xeric limits of the modern distributions of both species, average annual precipitation is from 50% to 100% greater than that presently received at Eagle Eye Mountain. In particular, the responses of S. leucopila and cacti during the middle Holocene indicate substantial increases in warm-season precipitation.