Late Quaternary vegetation, fire and climate history reconstructed from two cores at Cerro Toledo, Podocarpus National Park, southeastern Ecuadorian Andes

The last ca. 20,000 yr of palaeoenvironmental conditions in Podocarpus National Park in the southeastern Ecuadorian Andes have been reconstructed from two pollen records from Cerro Toledo (04°22'28.6"S, 79°06'41.5"W) at 3150 m and 3110 m elevation. Páramo vegetation with high proportions of Plantago rigida characterised the last glacial maximum (LGM), reflecting cold and wet conditions. The upper forest line was at markedly lower elevations than present. After ca. 16,200 cal yr BP, páramo vegetation decreased slightly while mountain rainforest developed, suggesting rising temperatures. The trend of increasing temperatures and mountain rainforest expansion continued until ca. 8500 cal yr BP, while highest temperatures probably occurred from 9300 to 8500 cal yr BP. From ca. 8500 cal yr BP, páramo vegetation re-expanded with dominance of Poaceae, suggesting a change to cooler conditions. During the late Holocene after ca. 1800 cal yr BP, a decrease in páramo indicates a change to warmer conditions. Anthropogenic impact near the study site is indicated for times after 2300 cal yr BP. The regional environmental history indicates that through time the eastern Andean Cordillera in South Ecuador was influenced by eastern Amazonian climates rather than western Pacific climates.     

Holocene vegetation, fire and climate history of the Sawtooth Range, central Idaho, USA

The paucity of low- and middle-elevation paleoecologic records in the Northern Rocky Mountains limits our ability to assess current environmental change in light of past conditions. A 10,500-yr-long vegetation, fire and climate history from Lower Decker Lake in the Sawtooth Range provides information from a new region. Initial forests dominated by pine and Douglas-fir were replaced by open Douglas-fir forest at 8420 cal yr BP, marking the onset of warmer conditions than present. Presence of closed Douglas-fir forest between 6000 and 2650 cal yr BP suggests heightened summer drought in the middle Holocene. Closed lodgepole pine forest developed at 2650 cal yr BP and fires became more frequent after 1450 cal yr BP. This shift from Douglas-fir to lodgepole pine forest was probably facilitated by a combination of cooler summers, cold winters, and more severe fires than before. Five drought episodes, including those at 8200 cal yr BP and during the Medieval Climate Anomaly, were registered by brief intervals of lodgepole pine decline, an increase in fire activity, and mistletoe infestation. The importance of a Holocene perspective when assessing the historical range of variability is illustrated by the striking difference between the modern forest and that which existed 3000 yr ago.     

Pre-Columbian landscape impact and agriculture in the Monumental Mound region of the Llanos de Moxos, lowland Bolivia

We present a multiproxy study of land use by a pre-Columbian earth mounds culture in the Bolivian Amazon. The Monumental Mounds Region (MMR) is an archaeological sub-region characterized by hundreds of pre-Columbian habitation mounds associated with a complex network of canals and causeways, and situated in the forest–savanna mosaic of the Llanos de Moxos. Pollen, phytolith, and charcoal analyses were performed on a sediment core from a large lake (14 km²), Laguna San José (14°56.97′S, 64°29.70′W). We found evidence of high levels of anthropogenic burning from AD 400 to AD 1280, corroborating dated occupation layers in two nearby excavated habitation mounds. The charcoal decline pre-dates the arrival of Europeans by at least 100 yr, and challenges the notion that the mounds culture declined because of European colonization. We show that the surrounding savanna soils were sufficiently fertile to support crops, and the presence of maize throughout the record shows that the area was continuously cultivated despite land-use change at the end of the earth mounds culture. We suggest that burning was largely confined to the savannas, rather than forests, and that pre-Columbian deforestation was localized to the vicinity of individual habitation mounds, whereas the inter-mound areas remained largely forested.     

Postglacial vegetation history of Mitkof Island, Alexander Archipelago, southeastern Alaska

An AMS radiocarbon-dated pollen record from a peat deposit on Mitkof Island, southeastern Alaska provides a vegetation history spanning ∼12,900 cal yr BP to the present. Late Wisconsin glaciers covered the entire island; deglaciation occurred > 15,400 cal yr BP. The earliest known vegetation to develop on the island (∼12,900 cal yr BP) was pine woodland (Pinus contorta) with alder (Alnus), sedges (Cyperaceae) and ferns (Polypodiaceae type). By ∼12,240 cal yr BP, Sitka spruce (Picea sitchensis) began to colonize the island while pine woodland declined. By ∼11,200 cal yr BP, mountain hemlock (Tsuga mertensiana) began to spread across the island. Sitka spruce-mountain hemlock forests dominated the lowland landscapes of the island until ∼10,180 cal yr BP, when western hemlock (Tsuga heterophylla) began to colonize, and soon became the dominant tree species. Rising percentages of pine, sedge, and sphagnum after ∼7100 cal yr BP may reflect an expansion of peat bog habitats as regional climate began to shift to cooler, wetter conditions. A decline in alders at that time suggests that coastal forests had spread into the island's uplands, replacing large areas of alder thickets. Cedars (Chamaecyparis nootkatensis, Thuja plicata) appeared on Mitkof Island during the late Holocene.     

Postglacial vegetation, fire, and climate history of the Siskiyou Mountains, Oregon, USA

The forests of the Siskiyou Mountains are among the most diverse in North America, yet the long-term relationship among climate, diversity, and natural disturbance is not well known. Pollen, plant macrofossils, and high-resolution charcoal data from Bolan Lake, Oregon, were analyzed to reconstruct a 17,000-yr-long environmental history of high-elevation forests in the region. In the late-glacial period, the presence of a subalpine parkland of Artemisia, Poaceae, Pinus, and Tsuga with infrequent fires suggests cool dry conditions. After 14,500 cal yr B.P., a closed forest of Abies, Pseudotsuga, Tsuga, and Alnus rubra with more frequent fires developed which indicates more mesic conditions than before. An open woodland of Pinus, Quercus, and Cupressaceae, with higher fire activity than before, characterized the early Holocene and implies warmer and drier conditions than at present. In the late Holocene, Abies and Picea were more prevalent in the forest, suggesting a return to cool wet conditions, although fire-episode frequency remained relatively high. The modern forest of Abies and Pseudotsuga and the present-day fire regime developed ca. 2100 cal yr B.P. and indicates that conditions had become slightly drier than before. Sub-millennial-scale fluctuations in vegetation and fire activity suggest climatic variations during the Younger Dryas interval and within the early Holocene period. The timing of vegetation changes in the Bolan Lake record is similar to that of other sites in the Pacific Northwest and Klamath region, and indicates that local vegetation communities were responding to regional-scale climate changes. The record implies that climate-driven millennial- to centennial-scale vegetation and fire change should be considered when explaining the high floristic diversity observed at present in the Siskiyou Mountains.     

贵州西部晚全新世植被演替、气候变化与火灾历史

贵州西部位于北亚热带云贵高原山地湿润-半湿润气候区,西南季风是该区域水汽来源的主要气候系统。贵州六盘水娘娘山有连片分布的垫状泥炭沼泽沉积,较完整记录了过去的植被和气候历史,是研究气候-植被-火灾-人类活动变化的理想场所。本研究以六盘水娘娘山1999 m海拔的一处泥炭湿地钻孔上部52 cm岩芯为研究材料,通过AMS ¹⁴C测年获得年代框架,采用孢粉和炭屑分析,重建了该地区晚全新世气候变化和人类活动叠加影响下的植被演替及火灾活动历史。结果表明：3500~3100 cal.a B.P. 期间,当地亚热带常绿阔叶林繁盛,火灾活动为气候控制为主的森林火灾,但火灾活动并未改变阔叶林的总体面貌;3100~600 cal.a B.P. 期间,气候呈变干趋势,阔叶类木本植物显著减少,当地植被从亚热带常绿阔叶林转变为疏林草地和针叶类疏林,极可能是趋于冷干的气候环境的结果;大约600 cal.a B.P. 之后,当地植被演变为开阔林,同时,出现大颗粒炭屑(>125 μm)以及伴人花粉的明显增加,表明人类农业活动高强度的刀耕火种已经扩张到较高海拔山区。区域对比显示,西南地区在3500 cal.a B.P. 以来,主要以区域性火灾为主,而3100 cal.a B.P. 以后的火灾活动受到气候变干和人类活动的双重影响,特别是600 cal.a B.P. 以来,人类活动(刀耕火种)成为局地火灾和植被更替的主要因素。

     

A long‐term vegetation history of the Mojave–Colorado desert ecotone at Joshua Tree National Park

Thirty‐eight dated packrat middens were collected from upper desert (930–1357 m) elevations within Joshua Tree National Park near the ecotone between the Mojave Desert and Colorado Desert, providing a 30 ka record of vegetation change with remarkably even coverage for the last 15 ka. This record indicates that vegetation was relatively stable, which may reflect the lack of invasion by extralocal species during the late glacial and the early establishment and persistence of many desert scrub elements. Many of the species found in the modern vegetation assemblages were present by the early Holocene, as indicated by increasing Sørenson's Similarity Index values. C₄ grasses and summer‐flowering annuals arrived later at Joshua Tree National Park in the early Holocene, suggesting a delayed onset of warm‐season monsoonal precipitation compared to other Sonoran Desert and Chihuahuan Desert localities to the east, where summer rains and C₄ grasses persisted through the last glacial–interglacial cycle. This would suggest that contemporary flow of monsoonal moisture into eastern California is secondary to the core processes of the North American Monsoon, which remained intact throughout the late Quaternary. In the Holocene, northward displacement of the jet stream, in both summer and winter, allowed migration of the subtropical ridge as far north as southern Idaho and the advection of monsoonal moisture both westward into eastern California and northward into the southern Great Basin and Colorado Plateau. Copyright © 2009 John Wiley & Sons, Ltd.     

Modern pollen-rain characteristics of tall terra firme moist evergreen forest, southern Amazonia

The paucity of modern pollen-rain data from Amazonia constitutes a significant barrier to understanding the Late Quaternary vegetation history of this globally important tropical forest region. Here, we present the first modern pollen-rain data for tall terra firme moist evergreen Amazon forest, collected between 1999 and 2001 from artificial pollen traps within a 500 × 20 m permanent study plot (14°34′50″S, 60°49′48″W) in Noel Kempff Mercado National Park (NE Bolivia). Spearman's rank correlations were performed to assess the extent of spatial and inter-annual variability in the pollen rain, whilst statistically distinctive taxa were identified using Principal Components Analysis (PCA). Comparisons with the floristic and basal area data of the plot (stems ≥10 cm d.b.h.) enabled the degree to which taxa are over/under-represented in the pollen rain to be assessed (using R-rel values). Moraceae/Urticaceae dominates the pollen rain (64% median abundance) and is also an important constituent of the vegetation, accounting for 16% of stems ≥10 cm d.b.h. and ca. 11% of the total basal area. Other important pollen taxa are Arecaceae (cf. Euterpe), Melastomataceae/Combretaceae, Cecropia, Didymopanax, Celtis, and Alchornea. However, 75% of stems and 67% of the total basal area of the plot ≥10 cm d.b.h. belong to species which are unidentified in the pollen rain, the most important of which are Phenakospermum guianensis (a banana-like herb) and the key canopy-emergent trees, Erisma uncinatum and Qualea paraensis.     

Reconstruction of climate and vegetation changes of Lake Bayanchagan (Inner Mongolia): Holocene variability of the East Asian monsoon

A high-resolution pollen and Pediastrum record, spanning 12,500 yr, is presented for Lake Bayanchagan (115.21°E, 41.65°N, and 1355 m a.s.l.), southern Inner Mongolia. Individual pollen taxa (PT-MAT) and the PFT affinity scores (PFT-MAT) were used for quantitative climatic reconstruction from pollen and algal data. Both techniques indicate that a cold and dry climate, similar to that of today, prevailed before 10,500 cal yr B.P. The wettest climate occurred between 10,500 and 6500 cal yr B.P., at which time annual precipitation was up to 30–60% higher than today. The early Holocene increases in temperature and precipitation occurred simultaneously, but mid-Holocene cooling started at approximately 8000 cal yr B.P., 1500 yr earlier than the drying. Vegetation reconstruction was based on the objective assignment of pollen taxa to the plant functional type. The results suggest that this region was dominated by steppe vegetation throughout the Holocene, except for the period 9200 to 6700 cal yr B.P., when forest patches were relatively common. Inner Mongolia is situated at the limit of the present East Asian monsoon and patterns of vegetation and climate changes in that region during the Holocene probably reflect fluctuations in the monsoon's response to solar insolation variations. The early to middle Holocene monsoon undoubtedly extended to more northern latitudes than at present.     

Postglacial fire, vegetation, and climate history in the Clearwater Range, Northern Idaho, USA

The environmental history of the Northern Rocky Mountains was reconstructed using lake sediments from Burnt Knob Lake, Idaho, and comparing the results with those from other previously published sites in the region to understand how vegetation and fire regimes responded to large-scale climate changes during the Holocene. Vegetation reconstructions indicate parkland or alpine meadow at the end of the glacial period indicating cold-dry conditions. From 14,000 to 12,000 cal yr B.P., abundant Pinus pollen suggests warmer, moister conditions than the previous period. Most sites record the development of a forest with Pseudotsuga ca. 9500 cal yr B.P. indicating warm dry climate coincident with the summer insolation maximum. As the amplification of the seasonal cycle of insolation waned during the middle Holocene, Pseudotsuga was replaced by Pinus and Abies suggesting cool, moist conditions. The fire reconstructions show less synchroneity. In general, the sites west of the continental divide display a fire-frequency maximum around 12,000–8000 cal yr B.P., which coincides with the interval of high summer insolation and stronger-than-present subtropical high. The sites on the east side of the continental divide have the highest fire frequency ca. 6000–3500 cal yr B.P. and may be responding to a decrease in summer precipitation as monsoonal circulation weakened in the middle and late Holocene. This study demonstrated that the fire frequency of the last two decades does not exceed the historical range of variability in that periods of even higher-than-present fire frequency occurred in the past.     

The last glaciation and relative sea level history of Northwest Ellesmere Island,Canadian high arctic

Philips Inlet and Wootton Peninsula are located at 82°N and 85°W on the northwest coast of Ellesmere Island and are composed of three bedrock controlled zones: (1) 900 m undulating plateau dissected by fiords; (2) a deeply fretted cirque terrain >1200m; (3) a 300m plateau bounded by coastal cliffs. Each zone contains different glacier morphologies and these control glacigenic sediment and landform assemblages. The extent of the last glaciation is mapped using the distribution of moraines, kames, meltwater channels and glacimarine sediments. Glaciers advanced on average <10 km from their present margins and many piedmont lobes coalesced and floated in the sea. Morainal banks were deposited at the grounding lines of floating glaciers, and where debris-charged basal ice occurred, subaqueous fans were deposited upon deglaciation. Marine shells dating 20.2 ka BP (<2km from present ice margin) and 14.9ka BP (from a morainal bank) document full glacial marine fauna. Thirty-three radiocarbon dates document glacier retreat patterns and are used to reconstruct the postglacial sea level history (glacioisostatic rebound pattern). An equidistant shoreline diagram is constructed using the 8.5ka BP shoreline as a guide. Tilts from 0.73-0.85m/km are calculated for this shoreline. Using two firm control points and tilts from elsewhere on northern Ellesmere Island, the 10.1 ka BP (full glacial) marine limit descends from 117m as at the fiord heads to 63 m asl at the north coast. Deglaciation started with a pronounced calving phase throughout the field area between 10.1 and 7.8ka BP. This chronology is similar to that from northeast Ellesmere Island and attests to an early Holocene warming trend recorded in high arctic ice cores. A maximum lag of 2.1 ka exists between the field area and locations to the south of the Grant Land Mountains suggesting differences in glacioclimatic regimes on either side of the mountain range. Persistent reconstructions of all-pervasive ice sheets for the last glaciation of the area are obsolete and should be abandoned.     

23,000 yr of vegetation history of the Upper Lerma, a tropical high-altitude basin in Central Mexico

Pollen analysis on a 9.54-m sediment core from lake Chignahuapan in the upper Lerma basin, the highest intermontane basin in Central Mexico (2570 m asl), documents vegetation and limnological changes over the past ∼23,000 ¹⁴C yr. The core was drilled near the archaeological site of Santa Cruz Atizapán, a site with a long history of human occupation, abandoned at the end of the Epiclassic period (ca. 900 AD). Six radiocarbon AMS dates and two well-dated volcanic events, the Upper Toluca Pumice with an age of 11,600 ¹⁴C yr B.P. and the Tres Cruces Tephra of 8500 ¹⁴C yr B.P., provide the chronological framework for the lacustrine sequence. From ca. 23,000 ¹⁴C yr B.P. to ca. 11,600 ¹⁴C yr B.P. the plant communities were woodlands and grasslands based on the pollen data. The glacial advances MII-1 and MII-2 correlate with abundant non-arboreal pollen, mainly grasses, from ca. 21,000 to 16,000 ¹⁴C yr B.P., and at ca. 12,600 ¹⁴C yr B.P. During the late Pleistocene, lake Chignahuapan was a shallow freshwater lake with a phase of lower level between 19,000 and 16,000 ¹⁴C yr B.P. After 10,000 ¹⁴C yr B.P., tree cover in the area increased, and a more variable lake level is documented. Late Holocene (ca. 3100 ¹⁴C yr B.P.) deforestation was concurrent with human population expansion at the beginning of the Formative period (1500 B.C.). Agriculture and manipulation of the lacustrine environment by human lakeshore populations appear at 1200 ¹⁴C yr B.P. (550 A.D.) with the appearance of Zea mays pollen and abundant charcoal particles.     

Late Quaternary vegetation and climate history of the central Bering land bridge from St. Michael Island, western Alaska

Pollen analysis of a sediment core from Zagoskin Lake on St. Michael Island, northeast Bering Sea, provides a history of vegetation and climate for the central Bering land bridge and adjacent western Alaska for the past ≥30,000 ¹⁴C yr B.P. During the late middle Wisconsin interstadial (≥30,000-26,000 ¹⁴C yr B.P.) vegetation was dominated by graminoid-herb tundra with willows (Salix) and minor dwarf birch (Betula nana) and Ericales. During the late Wisconsin glacial interval (26,000-15,000 ¹⁴C yr B.P.) vegetation was graminoid-herb tundra with willows, but with fewer dwarf birch and Ericales, and more herb types associated with dry habitats and disturbed soils. Grasses (Poaceae) dominated during the peak of this glacial interval. Graminoid-herb tundra suggests that central Beringia had a cold, arid climate from ≥30,000 to 15,000 ¹⁴C yr B.P. Between 15,000 and 13,000 ¹⁴C yr B.P., birch shrub-Ericales-sedge-moss tundra began to spread rapidly across the land bridge and Alaska. This major vegetation change suggests moister, warmer summer climates and deeper winter snows. A brief invasion of Populus (poplar, aspen) occurred ca.11,000-9500 ¹⁴C yr B.P., overlapping with the Younger Dryas interval of dry, cooler(?) climate. During the latest Wisconsin to middle Holocene the Bering land bridge was flooded by rising seas. Alder shrubs (Alnus crispa) colonized the St. Michael Island area ca. 8000 ¹⁴C yr B.P. Boreal forests dominated by spruce (Picea) spread from interior Alaska into the eastern Norton Sound area in middle Holocene time, but have not spread as far west as St. Michael Island.     

Holocene beaver damming, fluvial geomorphology, and climate in Yellowstone National Park, Wyoming

We use beaver-pond deposits and geomorphic characteristics of small streams to assess long-term effects of beavers and climate change on Holocene fluvial activity in northern Yellowstone National Park. Although beaver damming has been considered a viable mechanism for major aggradation of mountain stream valleys, this has not been previously tested with stratigraphic and geochronologic data. Thirty-nine radiocarbon ages on beaver-pond deposits fall primarily within the last 4000 yr, but gaps in dated beaver occupation from ~ 2200–1800 and 950–750 cal yr BP correspond with severe droughts that likely caused low to ephemeral discharges in smaller streams, as in modern severe drought. Maximum channel gradient for reaches with Holocene beaver-pond deposits decreases with increasing basin area, implying that stream power limits beaver damming and pond sediment preservation. In northern Yellowstone, the patchy distribution and cumulative thickness of mostly < 2 m of beaver-pond deposits indicate that net aggradation forced by beaver damming is small, but beaver-enhanced aggradation in some glacial scour depressions is greater. Although 20th-century beaver loss and dam abandonment caused significant local channel incision, most downcutting along alluvial reaches of the study streams is unrelated to beaver dam abandonment or predates historic beaver extirpation.     

50,000 years of vegetation and climate history on the Colorado Plateau, Utah and Arizona, USA

Sixty packrat middens were collected in Canyonlands and Grand Canyon National Parks, and these series include sites north of areas that produced previous detailed series from the Colorado Plateau. The exceptionally long time series obtained from each of three sites (> 48,000 ¹⁴C yr BP to present) include some of the oldest middens yet discovered. Most middens contain a typical late-Wisconsinan glaciation mixture of mesic and xeric taxa, evidence that plant species responded to climate change by range adjustments of elevational distribution based on individual criteria. Differences in elevational range from today for trees and shrubs ranged from no apparent change to as much as 1200 m difference. The oldest middens from Canyonlands NP, however, differ in containing strictly xeric assemblages, including middens incorporating needles of Arizona single-leaf pinyon, far north of its current distribution. Similar-aged middens from the eastern end of Grand Canyon NP contain plants more typical of glacial climates, but also contain fossils of one-seed juniper near its current northern limit in Arizona. Holocene middens reveal the development of modern vegetation assemblages on the Colorado Plateau, recording departures of mesic taxa from low elevation sites, and the arrival of modern dominant components much later.     

An 8000-year record of vegetation, climate, and human disturbance from the Sierra de Apaneca, El Salvador

An ∼8000-cal-yr stratigraphic record of vegetation change from the Sierra de Apaneca, El Salvador, documents a mid-Holocene warm phase, followed by late Holocene cooling. Pollen evidence reveals that during the mid-Holocene (∼8000-5500 cal yr B.P.) lowland tropical plant taxa were growing at elevations ∼200-250 m higher than at present, suggesting conditions about 1.0°C warmer than those prevailing today. Cloud forest genera (Liquidambar, Juglans, Alnus, Ulmus) were also more abundant in the mid-Holocene, indicating greater cloud cover during the dry season. A gradual cooling and drying trend began by ∼5500 cal yr B.P., culminating in the modern forest composition by ∼3500 cal yr B.P. A rise in pollen from weedy plant taxa associated with agriculture occurred ∼5000 cal yr B.P., and pollen from Zea first appeared in the record at ∼4440 cal yr B.P. Human impacts on local vegetation remained high throughout the late Holocene, but decreased abruptly following the Tierra Blanca Joven (TBJ) eruption of Volcán Ilopango at ∼1520 cal yr B.P. The past 1500 years are marked by higher lake levels and periodic depositions of exogenous inorganic sediments, perhaps indicating increased climatic variability.     

Paleoenvironmental proxy records from Lake Hovsgol, Mongolia, and a synthesis of Holocene climate change in the Lake Baikal watershed

Here we discuss paleoenvironmental evolution in the Baikal region during the Holocene using new records of aquatic (diatom) and terrestrial vegetation changes from Hovsgol, Mongolia's largest and deepest lake. We reconcile previous contradictory Baikal timescales by constraining reservoir corrections of AMS dates on bulk sedimentary organic carbon. Synthesis of the Holocene records in the Baikal watershed reveals a northward progression in landscape/vegetation changes and an anti-phase behavior of diatom and biogenic silica proxies in neighboring rift lakes. In Lake Baikal, these proxies appear to be responsive to annual temperature increases after 6 ka, whereas in Lake Hovsgol they respond to higher precipitation/runoff from 11 to 7 ka. Unlike around Lake Baikal, warmer summers between 6 and 3.5 ka resulted in the decline, not expansion, of forest vegetation around Lake Hovsgol, apparently as a result of higher soil temperatures and lower moisture availability. The regional climatic proxy data are consistent with a series of 500-yr time slice Holocene GCM simulations for continental Eurasia. Our results allow reevaluation of the concepts of ‘the Holocene optimum’ and a ‘maximum of the Asian summer monsoon’, as applied to paleoclimate records from continental Asia.     

Holocene vegetation and climate dynamics of the boreal alpine ecotone of northwestern Fennoscandia

Climate change with respect to summer temperature throughout the Holocene is inferred from oscillations in the local Pinus sylvestris, Alnus incana and Betula pubescens forest‐lines, as recorded by fossil pollen and plant macrofossils in lake sediments at four altitudinal levels. Mt Skrubben (848 m a.s.l.), in Dividalen, was deglaciated down to below 280 m a.s.l. during 10 800–10 300 cal. yr BP. Betula pubescens established 10100 cal. yr BP at 280 m a.s.l. and expanded up to near the summit during the next 700 years. Birch woodland prevailed on the mountain plateau until 3300 cal. yr BP. Local Pinus sylvestris stands are recorded up to 400 m a.s.l. at 8450 cal. yr BP and >548 m a.s.l. about 8160 cal. yr BP. Alnus incana expanded from 400 to nearly 790 m a.s.l. during the period 7900–7600 cal. yr BP. The maximum forest distribution lasted until ca. 6000 cal. yr BP. Marked climatic deteriorations caused lowering of the forest‐lines around 4600 and 3000 cal. yr BP. Reconstruction of the summer temperature indicated mean July temperatures at 400 m a.s.l. of 1.5–3°C above the present during the period of maximum forest expansion, whereas >3°C above the present temperature at 548 m a.s.l. This is in accordance with other regional temperature reconstructions from northern Europe. Copyright © 2008 John Wiley & Sons, Ltd.     

Postglacial vegetation, climate, and fire history along the east side of the Andes (lat 41-42.5°S), Argentina

The history of the low-elevation forest and forest-steppe ecotone on the east side of the Andes is revealed in pollen and charcoal records obtained from mid-latitude lakes. Prior to 15,000 cal yr BP, the vegetation was characterized by steppe vegetation with isolated stands of Nothofagus. The climate was generally dry, and the sparse vegetation apparently lacked sufficient fuels to burn extensively. After 15,000 cal yr BP, a mixture of Nothofagus forest and shrubland/steppe developed. Fire activity increased between 13,250 and 11,400 cal yr BP, contemporaneous with a regionally defined cold dry period (Huelmo/Mascardi Cold Reversal). The early-Holocene period was characterized by an open Nothofagus forest/shrubland mosaic, and fire frequency was high in dry sites and low in wet sites; the data suggest a sharp decrease in moisture eastward from the Andes. A shift to a surface-fire regime occurred at 7500 cal yr BP at the wet site and at 4400 cal yr BP at the dry site, preceding the expansion of Austrocedrus by 1000-1500 yr. The spread of Austrocedrus is explained by a shift towards a cooler and wetter climate in the middle and late Holocene. The change to a surface-fire regime is consistent with increased interannual climate variability and the onset or strengthening of ENSO. The present-day mixed forest dominated by Nothofagus and Austrocedrus was established in the last few millennia.