A review of Holocene climate records on the Tibetan Plateau based on pollen records

We present a recently published pollen record from the Hongyuan peatland in the Zoige Basin that reveals the long-term dynamics of an alpine wetland ecosystem on the eastern margin of the Tibetan Plateau over the last13500 years.Changes in pollen assemblages and influx rates suggest that local vegetation has experienced three distinct stages at the site,from alpine coniferous forest-meadow（13500-11500 cal a BP）,through alpine coniferous forest（11500;000 cal a BP）,back to alpine coniferous forest-meadow（3000 cal a BP—present）.This record reflects an ecosystem response along a transition zone where the South Asian and East Asian monsoon systems may have had different paleoclimatic influences.This review focuses on a comparison of this record with other published pollen records across the Tibetan Plateau.At each site it is possible to identify an optimal growth period during the Holocene.The age model of each site is carefully scrutinized to assure that the assumptions apphed at every site are consistent and that there is sufficient age control for a detailed comparison.It is shown that the chronology of the Hongyuan site is by far the most tightly controlled among all of the published records.The Holocene optimum is broadly similar over a large portion of the Tibetan Plateau,however a pattern of regional,temporal and spatial variability is clear.Most of the records show optimal growth conditions around 6 cal ka BP,but the initiation and duration of this optimum is quite different from site to site.The optimum starts anywhere from the beginning of the Holocene to about 8 cal ka BP.Nearly all of the records show relatively dry conditions after about 4 cal ka BP.The differences in these records could result from variable climatic influences at each site,or they might stem from local growth conditions that interfere with an accurate regional pollen picture.Inadequate age control could also contribute to the observed differences.The Hongyuan record demonstrates the usefulness of high temporal resolution in reconstructing Holocene climate from pollen records.