A modern pollen rain study was performed in a 300 km-long altitudinal transect (~ 28° N latitude) from 300 to 2300 m elevation. The higher elevation modern communities: epithermal oak-pines, pine-oak forest, pine forest, and mixed conifer forest were easy to distinguish from their pollen content. In contrast, lower elevation subtropical communities: thornscrub and tropical deciduous forest were difficult to separate, because they share many pollen taxa. Nevertheless we identify high frequencies of Bursera laxiflora as an important component of the tropical deciduous forest. Additionally, fossil pollen was analyzed at three sites located between 1700 and 1950 m altitude at ~ 28° latitude north in the Sierra Madre Occidental of northwestern Mexico. The sites were in pine-oak (Pinus-Quercus), pine, and mixed-conifer forests respectively. Shifts in the altitudinal distribution of vegetation belts were recorded for the last 12,849 cal yr BP, and climate changes were inferred. The lowest site (pine-oak forest) was surrounded by pine forest between 12,849 and 11,900 cal yr BP, suggesting a cold and relatively dry Younger Dryas period. The early Holocene was also cold but wetter, with mixed conifer forest with Abies (fir) growing at the same site, at 1700 m elevation, 300 m lower than today. After 9200 cal yr BP, a change to warmer/drier conditions caused fir migration to higher elevations and the expansion of Quercus at 1700 m. At 5600 cal yr BP Abies was growing above 1800 m and Picea (spruce) that is absent today, was recorded at 1950 m elevation. Fir and spruce disappeared from the 1950 m site and reached their present distribution (scattered, above 2000 m) after 1000 cal yr BP; we infer an episodic Holocene migration rate to higher elevations for Abies of 23.8 m/1000 cal yr and for Picea of 39.2 m/1000 cal yr. The late Holocene reflects frequent climate oscillations, with variations in the representation of forest trees. A tendency towards an openness of the forest is recorded for the last 2000 yrs, possibly reflecting human activities along with short-term climate change.
Bibliographical noteFunding Information:
Financial support was provided by the European Union in the frame of the MOTIF project (EVK2-CT-2002-00153), by the French (INSU/CNRS) PNEDC through the ECHO project, both to J.G., and by PhD and MSc fellowships from the “Consejo Nacional de Ciencia y Tecnología” (CONACYT, Mexico) to C.I.O.R. Field work was financed in part by the France-Mexico program ECOS M98U02 (SEP-CONACYT-ANUIES/ECOS France), and by the Instituto de Ecología – UNAM, where M.C.P. was a researcher. Modern pollen rain samples were treated at the chemical laboratory of the Estación Regional Noroeste of UNAM at Hermosillo, Sonora, Mexico. We thank Jacques-Louis de Beaulieu, Frédéric Médail, Martín Ortiz, Francisco Paz-Moreno, Jesús R. Vidal Solano, Martha Gómez, Ramsés Rodríguez and Daniel Morales, for assistance in the field. Guillaume Buchet provided laboratory assistance and help in pollen identification. Thomas Van Devender and Ana Lilia Reina introduced us to the cienegas of the Sierra Madre Occidental. Thomas Van Devender and Frédéric Médail helped with the vegetation data from Yepachic and Las Taunas sites. We thank Annie Vincens, Thomas Van Devender, Sarah Metcalfe and Owen Davis and anonymous reviewers for constructive reviews. Thanks also to the Pima Indian communities of El Kípor and Yepáchic, particularly to Luis Coyote, Luz Lao Aguilar and Alfonso Castellanos, for help in the fieldwork and hospitality. Fossil and modern pollen data were entered in the North American Pollen Database.
- Sierra Madre Occidental
- migration rate
- pollen analysis
- vegetation dynamics