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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]

This paper examines the mid- to late-Holocene (c. 6850-1160 cal. yr BP) environmental history of Sierra de Gádor in southern Spain. The local vegetation dynamics are reconstructed through the palaeoecological record obtained from a lacustrine deposit situated at 1530 m. Various hypotheses are considered to explain the vegetation dynamics apparent in the palaeoecological evidence, including climatic change, fire occurrence and human activity. Although the vegetation in this region is sensitive to climatic change, threshold events driven by ecological factors are also apparent. Climatic events include a thermo-mesophytic optimum with abundance of deciduous trees and maximum lake water level found from c. 6850 to 5500 cal. yr BP. In contrast, changes in the frequency of major fire episodes appear to have shaped interspecific relationships and vegetation change, especially from c. 4200 cal. yr BP onwards. Biotic properties of the ecosystem such as the inertia of established tree populations, interconnected with competition adjustments, appear also to have played a role. Over the last two millennia, overgrazing, combined with natural and/or human-set fires, appears to have pushed mountain forests over a threshold leading to the spread of grassland, thorny scrub, junipers and nitrophilous communities. [ABSTRACT FROM AUTHOR]

In September, 2006, we found the remains of timber-sized spruce trees (Picea engelmannii) on the floors of melting ice patches at altitudes of 3465-3480 m in the Mummy Range of north-central Colorado. The ice patches occupy northeast-facing recesses in which windblown snow, scoured from a tundra upland to the southwest, accumulates deeply. We hypothesize that the upland was timbered during the early to middle Holocene. Dense forest vegetation intercepted snow, preventing it from blowing to the recesses, and allowing trees to become established there. Neoglacial cooling led to gradual deforestation of the upland, renewed transport and deposition of snow by wind, and death of the ice-patch trees. Radiocarbon dates show that the trees died between 3860 ± 15 and 3780 ± 20 14C yr BP (c. 4200 cal. yr BP). The trunks show decay similar to that of historic log structures built above timberline during the late nineteenth and early twentieth centuries, suggesting that they have been protected by ice for all but a small portion of the past 4200 years. A series of warm summers and dry winters led to their recent emergence. The study illustrates the importance of monitoring glaciers and ice patches for floral, faunal and archaeological remains whenever severe melting occurs. [ABSTRACT FROM AUTHOR]

A 267-cm sediment core spanning the past c. 15200 cal. yr was recovered from Mumbo Lake, in the Trinity Mountains of northwestern California's Klamath Region. Plant macrofossils and pollen detail local and extra-local vegetation history, while high-resolution charcoal analysis provides details on local fire history. For the first c. 3000 years, climate was colder and drier than present, and supported an open, subalpine parkland vegetation, with low fire frequencies and fuel biomass. From c. 12100 to 9800 cal. yr BP increasing moisture and soil development led to a woodland community with three new pine species invading the basin. Fire frequencies remained low, but individual fires may have been more intense because of increased fuel loads. Between c. 9800 and 7200 cal. yr BP, climate warmed and dried considerably, allowing for the expansion of oak and other chaparral species. Fire frequencies increased in the early Holocene, but low charcoal accumulation rates suggest a frequent, relatively low-intensity fire regime. From c. 7200 to 3800 cal. yr BP, the climate became cooler and moister again. Many conifer species appeared for the first time, although chaparral species maintained a strong presence. The fire record shows a dramatic increase in charcoal accumulation rates as well as an increase in fire frequency. From c. 3800 cal. yr BP to present, more conifer species enter the record, and abundance of chaparral species gradually diminishes to present levels. [ABSTRACT FROM AUTHOR]

We examined the variations in vegetation, organic carbon isotopic compositions and biomarker distributions spanning a well-constrained climatic shift in a Sub-Boreal Dutch raised bog. The macrofossils record a vegetation shift from a predominance of degraded Ericaceae rootlets to a predominance of well-preserved Sphagnum species, first S. cuspidatum, followed by S. imbricatum. This shift, as well as evidence from other macrofossils and fungal spores, indicates that the local climatic conditions changed from relatively dry to relatively wet, with the wettest local conditions occurring at the base of the transition. At this boundary, a positive shift occurs in the δ[sup 13]C values of bulk peat (c. 4‰) and compounds derived from specific peat-forming plants (1 to 2‰). This is attributed to a combination of factors, including selective preservation of certain compound classes, changes in precipitation and its effect on the water table, and plant growth rates. The changes in peat preservation also indicate that redox changes occurred across the climatic change event. Indeed, biomarker transformations thought to reflect past redox conditions or bog water acidity - the 1,2,9-trimethyl1,2,3,4-tetrahydropicene to ursolic acid ratio, diacid to hydroxy acid ratio, and 17β,21β(H)-homohopane to total homohopane ratio - all exhibited shifts spanning the climatic change transition. Although some of these trends indicate the need for more fundamental research on biomarker transformations in peat, some, such as the 17β,21β(H)-homohopane to total homohopane ratio, show promise as indicators of past bog water conditions that could complement macrofossil analyses. [ABSTRACT FROM AUTHOR]

Changes in pollen and stomata assemblages in sediment cores recovered from tundra and forest-tundra lakes in alpine regions of northeastern British Columbia reflect vegetation and inferred climatic change throughout the Holocene. Pollen and stomata records are presented from two lakes, BC2 located in the alpine-tundra zone and Dead Spruce Lake at the present elevation of subalpine treeline. The pollen and stomata records from BC2 indicate that an ephemeral shrub and herb assemblage was rapidly replaced by an aspen (Populus)-spruce (Picea)-birch (Betula) woodland at ~10600 cal. yr BP. The occurrence of stomata suggests that treeline was at least 235 m higher than present from ~10 600 until ~7500 cal. yr BP and temperatures were at least 1.4°C warmer than at present. Analyses from Dead Spruce Lake indicate that the density of trees was also higher between ~9000 and 4800 cal. yr BP. High concentrations of macroscopic charcoal between ~9800 and 8000 cal. yr BP suggest that conditions were also drier at this time. Changes in the position of treeline during the early to mid-Holocene appear to track closely shifts in climate, while forest development at our forest-tundra site reflects a combination of both heightened summer insolation and increased fire activity. The fossil stomata record from BC2 suggests that a rapid cooling event at ~8200 cal. yr BP may have been the trigger that resulted in the downslope movement of treeline to its present elevation. Increased Picea percentages, pollen-accumulation rates (PARs) and concentration of stomata between ~3600 and 2700 cal. yr BP provide evidence for a late-Holocene increase in forest-tundra density in response to warmer temperatures. [ABSTRACT FROM AUTHOR]

Pollen stratigraphy of a core 270 cm long from Lake Dalgoto at 2310 m in the Northern Pirin Mountains, southern Bulgaria, was treated by optimal partitioning and compared to a broken-stick model to reveal statistically significant pollen zones. The vegetational reconstructions presented here are based on pollen percentages and pollen influx, on comparisons of modern and fossil pollen spectra, and on macrofossil dates from other sites in the mountains. During the Younger Dryas (11000-10200 [sup 14]C yr BP), an open xerophytic herb vegetation with Artemisia and Chenopodiaceae was widely developed around the lake. Deciduous trees growing at lower elevations contributed to the pollen rain deposited at the higher-elevation sampling sites. Specifically, from 10200 to 8500 yr BP, Quercus, Ulmus, Tilia and Betula expanded rapidly at low and intermediate elevations, and between 8500 and 6500 yr BP they extended to higher elevations close to the upper forest limit, which was formed by Betula pendula at about 1900 m. Coniferous species were limited in the region at this time. After 6500 yr BP, the expansion of conifers (Pinus peuce, P. sylvestris, P. mugo, Abies alba) at high elevations forced the deciduous trees downward. Between 6500 and 3000 yr BP, the forest limit at 2200 m was formed by P. peuce, and A. alba had its maximum vertical range up to 1900 m. Later the abundance and vertical range of P. peuce and A. alba were reduced. After 3000 yr BP, Picea expanded. [ABSTRACT FROM AUTHOR]

In Australia mediterranean-type climates occur in southwestern Australia and in near coastal South Australia and adjacent western Victoria. These regions support species-rich sclerophyllous heaths and shrublands with the biodiversity particularly high in the southwest. Palynological records show that around the time of the Last Glacial Maximum the climate was sufficiently severe that the vegetation of the southeastern regions was of a different character compared to today with grasses and Asteraceae more common everywhere. In the southwest the differences between Last Glacial Maximum and the Holocene were less pronounced and the tree and shrub genera Casuarina and Allocasuarina were more prominent, and heathland and shrubland remaining the main vegetation type. Most of the records from the Last Glacial Maximum verify that common taxa in mediterranean-type climate environments today were present through that period in low numbers. The modern character of the vegetation developed around two key periods: the Pleistocene/Holocene boundary, and, to a much smaller degree, from the mid-Holocene, with the changes somewhat less in the southwest. The mid-Holocene period was marked by slightly wetter and warmer conditions than present across all of southeastern Australia, yet most palynological records show little if any response in terrestrial vegetation in relation to this climate. The most significant Holocene changes appear to be a gradual decline in the importance of Casuarina, and changes in the abundance of Corymbia calophylla in the southwest. Many of the palynological records also have fossil charcoal histories and it is clear that, while there has been some regional variation, fire has been an important environmental feature of the regions throughout the Holocene. However, there are few indications that fire had any major role in vegetation change. What is clear is that European settlement, from about 150–180 years ago, wrought the greatest changes on vegetation, and possibly because of the parallel climate and environmental conditions the main invasive species have come from other mediterranean-type climate regions. [ABSTRACT FROM AUTHOR]