Packrat middens and the Holocene palaeohistory of Colorado piñon (Pinus edulis) in western Colorado

• Published in: Journal of biogeography 2015-03, Vol.42 (3), p.565-574
• Main Authors: Emslie, Steven D., Coats, Larry, Oleksy, Eva
• Format: Article
• Language: English
• Subjects: Biogeographical reconstruction; climatic impacts; dispersal; Holocene vegetation history; migration history; piñon-juniper woodlands; radiocarbon dating; Responses to environment; Uncompahgre Plateau; Upper Gunnison Basin
• ISSN: 0305-0270; 1365-2699
• DOI: 10.1111/jbi.12431

Aim: Our aim was to determine the age and dispersal history of Colorado pinon (Pinus edulis) in western Colorado during the early to late Holocene using radiocarbon-dated needles and nutshells from packrat (Neotoma spp.) middens. Location: The Uncompahgre Plateau (UP) comprises more than 600,000 ha of public and private lands in west-central Colorado, USA. Elevations within UP range from 1400 to 3140 m and it is characterized by numerous deep canyons and flat-topped mesas. The Upper Gunnison Basin (UGB) encompasses a 243,000-ha area enclosed within the southern Rocky Mountains in south-western Colorado, USA. It spans 2200—4300 m in elevation, with no outlet apart from the Black Canyon of the Gunnison lower than 2650 m. Methods: Middens were sampled by breaking off small sections of solidified deposits located in caves and crevices that were then disaggregated in water with repeated rinsing to separate all the plant remains. Single conifer needles and nutshells from 28 middens were radiocarbon dated to determine the age and timing of dispersal into western Colorado. Results: Thirty-two midden samples were collected, of which 28 produced identifiable plant remains that were radiocarbon dated. Pinon needles and/or nutshells were identified and radiocarbon dated from 20 of these middens from seven localities in UP. The results indicated that pinon did not become established in UP until c. 6000 yr BP, which contrasts with the nearby UGB, where pinon charcoal has been identified from seven archaeological sites with dates ranging from 8000 to 3000 yr BP. Main conclusions: Our data are consistent with a model of late Pleistocene/early Holocene piñon migration into the Colorado Plateau from Arizona and New Mexico before becoming established in western Colorado. The timing of the piñon migration into south-central Colorado along the Rio Grande corridor from New Mexico remains uncertain. While pinon-juniper woodlands currently dominate the lower elevations of UP, pinon had disappeared from UGB by c. 3000 yr BP and remains absent there today except for a few isolated trees.