Holocene Biomes and Climate Reconstruction of Northwestern Mexico Based on High Resolution Pollen Data

New paleovegetation and paleoclimate data from the Sierra Madre Occidental (SMO) in northwestern Mexico are presented. This work involves three main studies: 1) the fossil pollen analysis of five Holocene peat bogs located at different altitudes (1500 to 2000 m) inside the states of Sonora and Chihuahua; 2) the modern pollen analysis across an altitudinal transect (28° latitude north) from the Sonoran desert towards the highlands of the temperate SMO; and 3) the climatic and biomes reconstruction using Plant Functional Types and Biomization methods. For this last study a modern pollen dataset of 630 sites across the NW Mexico and the SW United States was compiled from different sources (North American Pollen Dataset, Latin-American Pollen Dataset, personal data, and different scientific papers). For the biomization method (Prentice et al., 1996); we have modified the pollen-PFT and PFT-Biomes assignation of Thompson and Anderson (2000) for a better representation of the modern vegetation of NW Mexico. The biomes reconstructed from the modern pollen sites let us to validate the reconstruction method and then his application to the fossil sites. The preliminary results of biome reconstruction from the pollen fossil records shows during the early Holocene that a Cool conifer forest was well distributed at 1700 m, and possibly lower elevation, at the SMO while today this biome is present only at altitudes higher than 2000 m in the Chihuahua state, the annual temperature reconstructed were at less of 2°C colder than today, but annual precipitation was 300 mm/yr lower than the actual (800 mm/yr). The middle Holocene at 6000 yr BP is marked by the installation of the warm mixed forest biome at 1700 m elevation, similar to present vegetation in the region. While at higher elevations (1900 m) the cool conifer forest was still present at the middle Holocene. The increasing in temperature characterize the period from 6000 to 4000 yr BP being more marked at 6000 yr BP with 3°C warmer than today, and an important increasing in summer precipitation is recorded of 200 mm/yr greater than the actual. That suggests that monsoon-like conditions begun to be more important since the middle Holocene in NW Mexico with increasing in summer rains. The last 4000 yr shows the same Biome reconstructed warm mixed forest, but important oscillations in the climatic parameters are recorded, this can reflect: 1) greater oscillations in climate at short term during the late Holocene; 2) the anthropization in the region reflected in the pollen rain or 3) the insufficiency of modern pollen analogues for the Mexican region.