Vegetation history and paleoclimates of the coastal lowlands of Sonora, Mexico — pollen records from packrat middens

Fossil pollen from packrat (Neotoma sp.) middens indicate that elements of the modern Sonoran Desert, such as Bursera microphylla (elephant tree), have been established within the Sierra Bacha of Sonora, Mexico, for the last 10,000 years. However, Fouquieria columnaris (boojum tree) grew on xeric east-facing slopes prior to c. 9000 years ago, indicating locally stronger upwelling or greater effective precipitation over modern conditions. Nearmodern environments were established sometime prior to c. 5400 years ago. The lack of correspondence between pollen-based vegetation reconstructions from middens and nearby marine sediment cores probably reflects different source areas for the pollen.