May 2004
Glacial-Interglacial Vegetation Change in the USA-Mexico Borderlands
Camille Holmgren, University of Arizona, Department of
Geosciences
M. Cristina Peñalba, Universidad Nacional
Autónoma de México, Instituto de Ecología
Kate Aasen Rylander and Julio L. Betancourt, Desert
Laboratory, USGS
A new packrat midden chronology from Playas Valley, southwestern New Mexico reveals glacial-interglacial changes in vegetation in the USA-Mexico Borderlands. The Borderlands, where the states of Arizona and New Mexico intersect with each other and the Mexican states of Chihuahua and Sonora, now contain ephemeral playas, but held pluvial lakes during the Pleistocene. Nine middens in the Playas Valley allow comparisons of two time intervals: 16,000-10,000 and 4,000-0 14C yr B.P.
Plant macrofossil abundance (top graph) and pollen percentage (bottom graph)
through time for select species from Playas Valley, NM. Stippled area
indicates period from which no middens were found. (Click on the image
to see a larger version)
Plant macrofossils and pollen assemblages from middens indicate vegetation along pluvial lake margins consisted of open pinyon-juniper communities dominated by Pinus edulis, Juniperus scopulorum, Juniperus cf. coahuilensis and a rich understory of C4 annuals and grasses. Although both lake and pinyon-juniper expansion across the lowlands have been attributed to greater winter precipitation, the summer-flowering understory, characteristic of modern desert grassland in the Borderlands, indicates at least moderate summer precipitation during the late glacial. P. edulis and J. scopulorum disappeared or were rare in the midden record by 10,670 14C yr B.P., marking the transition to a warmer, drier climate. The disappearance of pinyon and change to more xeric oak-juniper communities is contemporaneous with other midden sites in the northern Chihuahuan Desert and may have occurred abruptly during the "Clovis-aged Drought" when the water table at nearby Murray Springs dropped to unusually low levels just before 10,900 14C yr B.P.
The late Holocene is marked by the arrival of Chihuahuan Desert scrub elements and few departures as the vegetation gradually became modern in character. Larrea tridentata appears as late as 2190 14C yr B.P. based on macrofossils, but may have been present as early as 4095 14C yr B.P. based on pollen. Fouquieria splendens, one of the dominant desert species present at the site today, makes its first appearance only in the last millennium.
Holmgren, C.A., Peñalba, M.C., Rylander, K.A., and Betancourt, J.L. 2003. A 16,000 14C yr B.P. packrat midden series from the USA-Mexico Borderlands. Quaternary Research v. 60, p. 319-329.
For more information contact Camille Holmgren at holmgren@geo.arizona.edu.