Inferences about winter temperatures and summer rains from the late Quaternary record of C4 perennial grasses and C3 desert shrubs in the northern Chihuahuan Desert

Late Quaternary histories of two North American desert biomesC4 grasslands and C3 shrublandsare poorly known despite their sensitivity and potential value in reconstructing summer rains and winter temperatures. Plant macrofossil assemblages from packrat midden series in the northern Chihuahuan Desert show that C4 grasses and annuals typical of desert grassland persisted near their present northern limits throughout the last glacialinterglacial cycle. By contrast, key C3 desert shrubs appeared somewhat abruptly after 5000 cal yr B.P. Bioclimatic envelopes for select C4 and C3 species are mapped to interpret the glacialinterglacial persistence of desert grassland and the mid-to-late Holocene expansion of desert shrublands. The envelopes suggest relatively warm Pleistocene temperatures with moist summers allowed for persistence of C4 grasses, whereas winters were probably too cold (or too wet) for C3 desert shrubs. Contrary to climate model results, core processes associated with the North American Monsoon and moisture transport to the northern Chihuahuan Desert remained intact throughout the last glacial-interglacial cycle. Midlatitude effects, however, truncated midsummer (July-August) moisture transport north of 35oN. The sudden expansion of desert shrublands after 5000 cal yr B.P. may be a threshold response to warmer winters associated with increasing boreal winter insolation, and El Niño-Southern Oscillation variability.