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The focus of this paper is on the reconstruction of the mid-Holocene climatic changes based on the pollen records contained in a 6.2 m thick section in the southern part of the Tengger Desert, northwestern China. To understand the modern analogues of the mid-Holocene changes, modern pollen spectra across different vegetation zones from the Qilian Mountains to the lowland Tengger Desert were studied first. Four pollen combinations were identified, corresponding to four different modern ecological environments: (1) a Picea-Artemisia combination in a spruce forest and its vicinity; (2) an Artemisia-Chenopodiaceae-Gramineae-Salix combination in a mixture belt of montane grassland and poplar woodland; (3) a Chenopodiaceae-Artemisia-Nitraria combination in a desert steppe belt; and (4) an Elaeagnus-Artemisia-Chenopodiaceae-Nitraria combination in a desert riparian forest. Our study also shows that a relatively low percentage of Picea pollen (30–10%) seems to be associated with water transportation (alluvial deposits) and a very low percentage of Picea pollen (10–5%) appears to be related to long-distance air transportation. With references to the modern relationships between pollen assemblages and associated bioclimates, the pollen sequence covering the period between 8500 and 3000 cal. BP was used to infer the climatic change history. The results show that a warmer and drier climate dominated between 8500 and 7950 cal. BP. Subsequent decreases in Artemisia and warm desert forbs/shrubs taxa and contemporaneous fluctuating increases in montane coniferous and aquatics elements from 7950 to 7400 cal. BP reflect a fluctuating increase in humidity and/or a fluctuating decrease in temperature. The following period from 7400 to 5650 cal. BP was generally warm with the Holocene Climate Optimum occurring between 7400 and 6500 cal. BP. A remarkably cool spell followed between 6500 and 6300 cal. BP that was then succeeded by a warmer and more humid period between 6300 and 5650 cal. BP. The period between 5650 and 4450 cal. BP was characterized by drastic oscillations in climatic conditions with three humid and probably cold spells intervened by three relatively dry and warm spells. The period between 4450 and 3500 cal. BP was cold and humid, followed by a wet spell as indicated by a very high percentage of Typha pollen. [ABSTRACT FROM AUTHOR]

The Artemisia/Chenopodiaceae (A/C) ratio is assumed to be a useful index for reconstructing moisture changes in arid and semi-arid regions. Thorough modern pollen studies are still lacking to understand the reliability and limitation of A/C ratio as a moisture indicator, however. Here we review how well this ratio can be applied in arid and semi-arid China on the basis of new surface pollen data, previous data synthesis and other publications. Results indicate that variance in the A/C ratio can permit identification of modern vegetation types and that the A/C ratio generally has a positive relationship with annual precipitation. However, soil salinity, vegetation community composition, human activity and sample provenance (e.g. soil and lake sediments) will affect the values of the A/C ratio in different vegetation zones and therefore the A/C ratio is not comparable in different regions. We argue that the A/C ratio can only be used to reconstruct vegetation types and climate change in regions with precipitation <450–500 mm, and in steppe, steppe desert and desert areas. Careful studies should be undertaken to understand the modern pollen–vegetation–climate relationships in various regions before using the A/C ratio to interpret vegetation and climate. [ABSTRACT FROM AUTHOR]

Pollen, microscopic charcoal and radiocarbon data are used to document changes in vegetation dynamics during the late Holocene from Namelok Swamp in the Amboseli Basin (Kenya). The data reveal changes in savanna vegetation composition driven by an interaction of climate change, anthropogenic and herbivore activities. The abundance of Celtis, Podocarpus and Syzygium reflects a relatively moist climate from around 3000 to 2400 cal. yr BP. Increased abundance of Acacia, Amaranthaceae/Chenopodiaceae and Poaceae suggest a drier and/or warmer climate from 2150 to around 1675 cal. yr BP. The expansion of Syzygium within the catchment and decrease in Amaranthaceae/Chenopodiaceae reflect a relatively wet phase from around 1675 to about 550 cal. yr BP - superimposed on this is a large increase in Poaceae from 1400 to 800 cal. yr BP indicative of a drier environment. The dominance of Amaranthaceae/Chenopodiaceae and Poaceae with an associated decrease in Syzygium from 550 cal. yr BP is thought to correspond to a drier climate. The uppermost samples, dating to the last 150 years, record a large increase in Acacia, Amaranthaceae/Chenopodiaceae and Poaceae with decrease in Syzygium and are attributed to recent land-use changes associated with increased sedentary settlement. The increased presence of Cannabis sativa, Cereal and Ricinus communis pollen, combined with charcoal in the sediment record, particularly from 2500 but more constantly from 1600 cal. yr BP, indicate a long history of human-ecosystem interaction in the Amboseli Basin that has implications for future management of the area. [ABSTRACT FROM AUTHOR]

Pollen, stomates and macrofossils were analysed from a 3.63 m sediment core at Jan Lake, east-central Alaska, in order to improve spatial resolution of patterns of vegetation history in this region. The chronology was based on 19 AMS 14C dates on plant macrofossils and concentrated pollen. Zone Jan-I (12 400–11 600 yr BP) records a unique herb-tundra pollen assemblage, particularly high in Chenopodiaceae, adding supporting evidence for a spatial mosaic of lateglacial vegetation in east-central Alaska. Unexpected pollen of Picea, Betula and Alnus in lateglacial sediments appears to arise from reworking of older pollen-bearing deposits, according to a radiocarbon date of 22 180 yr BPon a pollen sample. Shrub Betula tundra dominates zone Jan-II (11 600–10 000 yr BP), with indications of climatic amelioration. Tree species (Populus, Picea glauca, Betula papyrifera) began to arrive in zone Jan-IIIa (9200–6600 yr BP), and the modern boreal forest, including Alnus and Picea mariana, became established during zone Jan-IIIb (6600–0 yr BP). Fossil stomates appear to be an effective indicator of initial local occurrence of Picea at Jan Lake. Picea arrival at Jan Lake is within 300 years of its arrival at other AMS-dated sites across eastern and central Alaska. A migration route into Alaska through the drainages of the Tanana or Yukon Rivers is hypothesized. Evidence is found for a slight mid-Holocene Picea decline at Jan Lake, intermediate in magnitude between Picea pollen records showing no decline to the east and a larger decline to the west in Alaska. [ABSTRACT FROM AUTHOR]