Comparison of pollen and macrofossils in packrat ( Neotama) middens: A chronological sequence from the Waterman mountains of southern Arizona, U.S.A.

Abstract

Pollen percentages in moss polsters and soil samples were compared with percent plant cover along 30-m transects in desertscrub vegetation, Waterman Mountains, Arizona, in the northeastern Sonoran Desert. These data were used to interpret pollen assemblages and associated plant macrofossils from 14 packrat ( Neotoma sp.) middens radiocarbon dated from 1320 to 22,450 yr B.P. A macrofossil: pollen index ( MPI) was used to compare the presence and relative abundances of pollen and macrofossils in midden samples. In general, pollen and macrofossils from Sonoran Desert middens reflect similar vegetation signals. Calculation of Sørensen's Similarity Index ( SI) comparing the presence of midden pollen and macrofossils indicates greater similarities for Sonoran desertscrub than pinyon-juniper woodland. SI values from the Waterman series are greater for all age samples, compared to samples from other desert locations, perhaps reflecting greater species richness and summer rainfall in the northeastern Sonoran Desert. The pollen and macrofossil assemblages reflect a middle and late Wisconsin (22,450-ca. 11,500 yr B.P.) pinyon-juniper woodland with sagebrush and Joshua tree on limestone slopes. A transitional early Holocene juniper woodland/chaparral was replaced by 8900 yr B.P. by a mesic Sonoran desertscrub of saguaro, catclaw acacia, blue paloverde and velvet mesquite. The presence of the latter three on exposed rocky locations, now restricted mostly to riparian washes, indicates moister conditions during the middle Holocene than today. Essentially modern vegetation developed by ca. 4000 yr B.P. Changes over the last ca. 1300 years suggest that modern climate is the most xeric in the entire record.