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1.
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.  相似文献   

2.
Extensive portions of the continental shelf off the coast of British Columbia were subaerially exposed during Late Wisconsinan deglaciation due to lowering of relative sea level by as much as 150 m. Paleoecological analyses were conducted at two sites on the emergent continental shelf where terrestrial surfaces with in situ conifers are preserved. The woody plant remains confirm that, during the latest period of subaerial exposure, terrestrial vegetation was established on the continental shelf. Microscopic identification of fossil wood, and analyses of pollen and plant macrofossils from the associated paleosols and overlying shallow pond sediments indicate that productive Pinus contorta-dominated communities with abundant Alnus crispa and ferns grew on the shelf adjacent to and on the Queen Charlotte Islands around 12,200 14C yr B.P. Dwarf shrubs including Salix and Empetrum, and herbaceous plants such as Heracleum lanatum and Hippuris vulgaris, were also important components of the shelf vegetation. Near northern Vancouver Island, mixed coniferous forests dominated by Pinus contorta with Picea, Tsuga spp., Alnus spp., and ferns occupied the shelf at 10,500 14C yr B.P.  相似文献   

3.
Stratigraphic palynology and radiocarbon chronology of two bogs and a lake on the northwestern Olympic Peninsula serve to record the environmental sequence postdating the Fraser maximum of the Juan de Fuca lobe. Wastage of the lobe in the terminal area began before 14,460 ± 200 BP. Differential downwasting followed, and the last remnants of dead ice probably disappeared some time before 9,380 ± 180 BP. Ablation moraine became sufficiently thick in the course of wastage for a vegetation cover to become established. Arboreal remains of this cover, found buried in till, date between 12,020 ± 210 and 13,380 ± 250 BP.Communities of Pinus contorta first succeeded on deglaciated surfaces during the Vashon Stade. Environmental conditions were comparable to those prevailing in the modern subalpine forest, and average July temperature stood near 12°C. Later, during the Everson Interstade (11,000–13,000 BP.), Alnus and Picea multiplied as temperature increased posibly to as much as 14°. During the Sumas Stade (10,000–11,000 BP.), temperature was again ca. 12°, the cooler climate halting wastage and the spread of Alnus and enabling communities of Picea, Tsuga heterophylla, and T. mertensiana to temporarily achieve stability.Postglacial environments through the Hypsithermal (ca. 3,000 BP) were dominated principally by Alnus. Alnus, succeeded in turn by Picea, invaded the landscape, following the recession of alpine glaciers and the rise in elevation of the snowline. For a time, as suggested by a peak of Pseudotsuga, temperature may have reached close to 17° and annual precipitation less than 760 mm. Arboreal communities were relatively open while light-requiring Pteridium remained conspicuous in the record. After 3000 BP during Neoglaciation, climate became sufficiently cool and moist to favor the development of extensive, closed communities of Tsuga, Picea, Thuja, and other hygrophilous species.  相似文献   

4.
High-resolution macroscopic charcoal analysis was used to reconstruct a 14,300-year-long fire history record from the lower Columbia River Valley in southwestern Washington, which was compared to a previous vegetation reconstruction for the site. In the late-glacial period (ca. 14,300-13,100 cal yr BP), Pinus/Picea-dominated parkland supported little to no fire activity. From the late-glacial to the early Holocene (ca. 13,100-10,800 cal yr BP), Pseudotsuga/Abies-dominated forest featured more frequent fire episodes that burned mostly woody vegetation. In the early to middle Holocene (ca. 10,800-5200 cal yr BP), Quercus-dominated savanna was associated with frequent fire episodes of low-to-moderate severity, with an increased herbaceous (i.e., grass) charcoal content. From the middle to late Holocene (ca. 5200 cal yr BP to present), forest dominated by Pseudotsuga, Thuja-type, and Tsuga heterophylla supported less frequent, but mostly large or high-severity fire episodes. Fire episodes were least frequent, but were largest or most severe, after ca. 2500 cal yr BP. The fire history at Battle Ground Lake was apparently driven by climate, directly through the length and severity of the fire season, and indirectly through climate-driven vegetation shifts, which affected available fuel biomass.  相似文献   

5.
Hager Pond, a mire in northern Idaho, reveals at least five pollen zones since sediments formed after the last recession of continental ice (>9500 yr BP). Zone I (>9500-8300 yr BP) consists mainly of diploxylon pine, plus low percentages of Abies, Artemisia, and Picea. SEM examination of conifer pollen at selected levels in the zone reveals that Pinus albicaulis, P. monticola, and P. contorta are present in unknown proportions. The zone resembles modern pollen spectra from the Abies lasiocarpa-P. albicaulis association found locally today only at high elevation. Presence of whitebark pine indicates a cooler, moister climate than at present, but one which was rapidly replaced in Zone II (8300-7600 yr BP) by warmer, drier conditions as inferred by prominence of grass with diploxylon pine. Zone III (7600-3000 yr BP) was probably dominated by Pseudotsuga menziesii, plus diploxylon pine and prominent Artemisia and denotes a change in vegetation but continuation of the warmer drier conditions. Beginning at approximately 3000 yr BP Picea engelmannii, Abies lasiocarpa, and/or A. grandis and diploxylon pine were dominants and the inferred climate became cooler and moister concomitant with Neoglaciation. The modern climatic climax (Zone V), with Tsuga heterophylla as dominant, has emerged in approximately the last 1500 yr.  相似文献   

6.
Analysis of pollen, spores, macrofossils, and lithology of an AMS 14C-dated core from a subarctic fen on the Kenai Peninsula, Alaska reveals changes in vegetation and climate beginning 14,200 cal yr BP. Betula expansion and contraction of herb tundra vegetation characterize the Younger Dryas on the Kenai, suggesting increased winter snowfall concurrent with cool, sunny summers. Remarkable Polypodiaceae (fern) abundance between 11,500 and 8500 cal yr BP implies a significant change in climate. Enhanced peat preservation and the occurrence of wet meadow species suggest high moisture from 11,500 to 10,700 cal yr BP, in contrast to drier conditions in southeastern Alaska; this pattern may indicate an intensification and repositioning of the Aleutian Low (AL). Drier conditions on the Kenai Peninsula from 10,700 to 8500 cal yr BP may signify a weaker AL, but elevated fern abundance may have been sustained by high seasonality with substantial snowfall and enhanced glacial melt. Decreased insolation-induced seasonality resulted in climatic cooling after 8500 cal yr BP, with increased humidity from 8000 to 5000 cal yr BP. A dry interval punctuated by volcanic activity occurred between 5000 and 3500 cal yr BP, followed by cool, moist climate, coincident with Neoglaciation. Tsuga mertensiana expanded after ~ 1500 cal yr BP in response to the shift to cooler conditions.  相似文献   

7.
Pollen, plant macrofossil, and charcoal records from Spruce Pond (41°14′22″N, 74°12′15″W), southeastern New York, USA dated by AMS provide details about late-glacial–early Holocene vegetation development in the Hudson Highlands from >12410 to 9750 14C yr BP. Prior to 12410 yr BP, vegetation was apparently open, dominated by herbs and shrubs (Cyperaceae, Gramineae, Tubuliflorae, Salix, Alnus, Betula), possibly with scattered trees (Picea and Pinus). However, Picea macrofossils are not found until 12410 yr BP. Development of a temperature deciduous–boreal-coniferous forest featuring Quercus, Fraxinus, Ostrya/Carpinus, Pinus, Picea, and Abies occurs between 12410 and 11140 yr BP. A return of predominantly boreal forest taxa between 11140 and 10230 yr BP is interpreted as an expression of the Younger Dryas cooling event. Holocene warming at 10230 yr BP is signalled by arrival of Pinus strobus, coincident with expansion of Quercus-dominated forest. Fire activity, as inferred from charcoal influx, appears to have increased as woodland developed after 12410 yr BP. Two charcoal influx peaks occur during Younger Dryas time. Early Holocene fire activity was relatively high but decreased for approximately 100 yr prior to the establishment of Tsuga canadensis in the forest at 9750 yr BP. © 1997 by John Wiley & Sons, Ltd.  相似文献   

8.
Here, we present two high-resolution records of macroscopic charcoal from high-elevation lake sites in the Sierra Nevada, California, and evaluate the synchroneity of fire response for east- and west-side subalpine forests during the past 9200 yr. Charcoal influx was low between 11,200 and 8000 cal yr BP when vegetation consisted of sparse Pinus-dominated forest and montane chaparral shrubs. High charcoal influx after ∼ 8000 cal yr BP marks the arrival of Tsuga mertensiana and Abies magnifica, and a higher-than-present treeline that persisted into the mid-Holocene. Coeval decreases in fire episode frequency coincide with neoglacial advances and lower treeline in the Sierra Nevada after 3800 cal yr BP. Independent fire response occurs between 9200 and 5000 cal yr BP, and significant synchrony at 100- to 1000-yr timescales emerges between 5000 cal yr BP and the present, especially during the last 2500 yr. Indistinguishable fire-return interval distributions and synchronous fires show that climatic control of fire became increasingly important during the late Holocene. Fires after 1200 cal yr BP are often synchronous and corroborate with inferred droughts. Holocene fire activity in the high Sierra Nevada is driven by changes in climate linked to insolation and appears to be sensitive to the dynamics of the El Niño-Southern Oscillation.  相似文献   

9.
We present here the results of pollen analysis of two sequences of about 8.06 m and 11.90 m length, originating from two adjacent peat bogs in the southern part of Transylvania province, Romania (155 and 122 pollen spectra). The vegetation record, which is supported by 17 14C dates, begins in the Late Glacial interstadial when forest recolonisation began with the development of Pinus, without a pioneer Betula phase. Picea began to expand from regional refuges. After a well‐defined Younger Dryas, the Holocene opens with the expansion of Betula, Ulmus and Picea, followed, at about 10 400 cal. yr BP, by Fraxinus, Quercus and Tilia. The Corylus optimum is correlated with the Atlantic chronozone (after 8600 cal. yr BP). The local establishment of Carpinus occurred at about 6500 cal. yr BP, with a maximum at about 5700 cal. yr BP. Fagus pollen is regularly recorded after 8200 cal. yr BP. This taxon became dominant at about 3700 cal. yr BP. The first indications of human activities appear at around 7200 cal. yr BP. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

10.
Pollen and macrofossil analyses of a core spanning 26,000 yr from Davis Lake reveal late Pleistocene and Holocene vegetational patterns in the Puget Lowland. The core ranges lithologically from a basal inorganic clay to a detritus gyttja to an upper fibrous peat and includes eight tephra units. The late Pleistocene pollen sequence records two intervals of tundra-parkland vegetation. The earlier of these has high percentages of Picea, Gramineae, and Artemisia pollen and represents the vegetation during the Evans Creek Stade (Fraser Glaciation) (ca. 25,000–17,000 yr B.P.). The later parkland interval is dominated by Picea, Tsuga mertensiana, and Gramineae. It corresponds to the maximum ice advance in the Puget Lowland during the Vashon Stade (Fraser Glaciation) (ca. 14,000 yr B.P.). An increase in Pinus ontorta pollen between the two tundra-parkland intervals suggests a temporary rise in treeline during an unnamed interstade. After 13,500 yr B.P., a mixed woodland of subalpine and lowland conifers grew at Davis Lake during a period of rapid climatic amelioration. In the early Holocene, the prolonged expansion of Pseudotsuga and Alnus woodland suggests dry, temperate conditions similar to those of present rainshadow sites in the Puget Lowland. More-mesic forests of Tsuga eterophylla, Thuja plicata, and Pseudotsuga, similar to present lowland vegetation, appeared in the late Holocene (ca. 5500 yr B.P.).  相似文献   

11.
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.  相似文献   

12.
Pollen and macrofossil analyses of a sediment core from Beaver Pond (60° 37′ 14″ N, 154° 19′ W, 579 m a.s.l.) reveal a record of regional and local postglacial vegetation change in south‐western Alaska. The chronology is based on five AMS (accelerator mass spectrometry) 14C ages obtained from terrestrial plant macrofossils. Pollen and macrofossil records suggest that open herb and shrub tundra with e.g. Poaceae, Cyperaceae, Artemisia, Vaccinium and Salix prevailed on the landscape before ca. 14 000 cal a BP. The shift from herb‐ to shrub‐dominated tundra (Salix, subsequent Betula expansion) possibly reflects climatic warming at the beginning of the Bølling period at ca. 14 700–14 500 and around 13 500 cal a BP. Vegetation (Betula shrub tundra) remained relatively stable until the early Holocene. Macrofossil influx estimates provide evidence for greater biomass in Betula shrub tundra during the early postglacial period than today. Charcoal accumulation rates suggest tundra fire activity was probably greater from ca. 12 500 to 10 500 cal a BP, similar to results from elsewhere in Alaska. The pollen and macrofossil records of Beaver Pond suggest the prevalence of low shrub tundra (shrub Betula, Betula nana, Vaccinium, Ledum palustre, Ericaceae) and tall shrub tundra (Alnus viridis ssp. crispa, Salix) between 10 000 and 4000 cal a BP. This Holocene vegetation type is comparable with that of the modern treeless wet and moist tundra in south‐western Alaska. The expansion of Picea glauca occurred ～4000 cal a BP, much later than that of A. viridis (ssp. crispa), whereas in central and eastern Alaska Picea glauca expanded prior to or coincident with Alnus (viridis). At sites located only 200–400 km north‐east of Beaver Pond (Farewell and Wien lakes), Picea glauca and Betula forests expanded 8000–6000 cal a BP. Unfavourable climatic conditions and soil properties may have inhibited the expansion and establishment of Picea across south‐west Alaska during the mid and late Holocene. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

13.
Macrofossil, pollen, lithostratigraphy, mineral magnetic measurements (SIRM and magnetic susceptibility), loss‐on‐ignition, and AMS radiocarbon dating on sediments from two former crater lakes, situated at moderate altitudes in the Gutaiului Mountains of northwest Romania, allow reconstruction of Late Quaternary climate and environment. Shrubs and herbs with steppe and montane affinities along with stands of Betula and Pinus, colonised the surroundings of the sites prior to 14 700 cal. yr BP and the inferred climatic conditions were cold and dry. The gradual transition to open Pinus–Betula forests, slightly higher lake water temperatures, and higher lake productivity, indicate more stable environmental conditions between 14 700 and 14 100 cal. yr BP. This development was interrupted by cooler and drier climatic conditions between 14 100 and 13 800 cal. yr BP, as inferred from a reduction of open forests to patches, or stands, of Pinus, Betula, Larix, Salix and Populus. The expansion of a denser boreal forest, dominated by Picea, but including Pinus, Larix, Betula, Salix, and Ulmus started at 13 800 cal. yr BP, although the forest density seems to have been reduced between 13 400 and 13 200 cal. yr BP. Air temperature and moisture availability gradually increased, but a change towards drier conditions is seen at 13 400 cal. yr BP. A distinct decrease in temperature and humidity between 12 900 and 11 500 cal. yr BP led to a return of open vegetation, with patches of Betula, Larix, Salix, Pinus and Alnus and individuals of Picea. Macrofossils and pollen of aquatic plants indicate rising lake water temperatures and increased aquatic productivity already by ca. 11 800 cal. yr BP, 300 years earlier than documented by the terrestrial plant communities. At the onset of the Holocene, 11 500 cal. yr BP, forests dominated by Betula, Pinus and Larix expanded and were followed by dense Ulmus forests with Picea, Betula and Pinus at 11 250 cal. yr BP. Larix pollen was not found, but macrofossil evidence indicates that Larix was an important forest constituent at the onset of the Holocene. Moister conditions were followed by a dry period starting about 10 600 cal. yr BP, which was more pronounced between 8600 and 8200 cal. yr BP, as inferred from aquatic macrofossils. The maximum expansion of Tilia, Quercus, Fraxinus and Acer between 10 700 and 8600 cal. yr BP may reflect a more continental climate. A drier and/or cooler climate could have been responsible for the late expansion (10 300 cal. yr BP) and late maximum (9300 cal. yr BP) of Corylus. Increased water stress, and possibly cooler conditions around 8600 cal. yr BP, may have caused a reduction of Ulmus, Tilia, Quercus and Fraxinus. After 8200 cal. yr BP moisture increased and the forests included Picea, Tilia, Quercus and Fraxinus. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

14.
High-resolution charcoal and pollen analyses were used to reconstruct a 4600-yr-long history of fire and vegetation near Taylor Lake in the wettest forests of coastal Oregon. Today, fires in these forests are rare because the season of ignition does not coincide with months of dry fuels. From ca. 4600 to 2700 cal yr B.P. fire episodes occurred at intervals of 140±30 yr while forest vegetation was dominated by disturbance-adapted taxa such as Alnus rubra. From ca. 2700 cal yr B.P. to the present, fire episodes have become less common, occurring at intervals of 240±30 yr, and fire-sensitive forest taxa, such as Tsuga heterophylla and Picea sitchensis, have become more prominent. Fire occurrence during the mid-Holocene was similar to that of the more xeric forests in the eastern Coast Range and suggests that summer drought was widespread. After ca. 2700 cal yr B.P., a decrease in fire episode frequency suggests that cooler conditions and possibly increased summer fog allowed the establishment of present-day Picea sitchensis forests within the watershed. These results provide evidence that fire has been an important disturbance agent in the Coast Range of Oregon, and variations in fire frequency and climate have led to the establishment of present-day forests.  相似文献   

15.
Pollen analysis from a peat core 7.0 m in length, taken from a bog near Bisoca, in a mid-altitude area of the Buzăului Subcarpathian mountains, is used to reconstruct the postglacial vegetation history of the region. The vegetation record, which is supported by twelve 14C dates, starts at the end of the Late Glacial period. At the Late Glacial/Holocene transition, open vegetation was replaced by forest, suggesting a fast response to climatic warming. The Holocene began with the expansion of Betula, Pinus and Ulmus, followed, after 11,000 cal yr BP, by Fraxinus, Quercus, Tilia and Picea. The rapid expansion of these taxa may be due to their existence in the area during the Late Glacial period. At ca. 9200 cal yr BP, Corylus expanded, reaching a maximum after 7600 cal yr BP. The establishment of Carpinus occurred at ca. 7200 cal yr BP, with a maximum at ca. 5700 cal yr BP. Fagus pollen is regularly recorded after 7800 cal yr BP and became dominant at ca. 2000 cal yr BP. The first indications of human activities appear around 3800 cal yr BP.  相似文献   

16.
Pollen records in the Kootenai and Fisher River drainages in western Montana reveal a fivezone sequence of Holocene vegetation change. Deposition of Glacier Peak Ash-Layer G (ca. 10,540 ± 660 yr B.P.) in the lowermost sediments (clay intermixed with pebbles) at Tepee Lake gives a minimum date for the initiation of sedimentation. Initial vegetation on the newly deglaciated terrain was dominated by Pinus (probably white bark pine) with small amounts of Gramineae, Picea and Abies, reflecting a relatively cool, moist macroclimate. Two vegetation units appear to contribute to Pollen Zone II (ca. 11,000–7100 yr B.P.): arboreal communities with pines, along with Pseudotsuga or Larix, or both, and treeless vegetation dominated by Artemisia. Pollen Zone II represents an overall warmer macroclimate than occurred upon ice withdrawal. After ca. 7100 yr B.P. (Pollen Zone III) diploxylon pines became a major pollen contributor near both Tepee Lake and McKillop Creek Pond, indicating an expansion of xerophytic forest (P. contorta and P. ponderosa) along with an increase in the prominence of Pseudotsuga menziesii or Larix occidentalis, or both. Artemisia briefly expanded coverage near Tepee Lake concomitant with the Mazama ashfall ca. 6700 yr B.P. A short-term climatic trend with more available water began after ca. 4000 yr B.P. as Abies (probably A. grandis) along with Picea engelmannii became a more regular component of the forest surrounding both sites. Emergence of the modern macroclimate is indicated primarily with the first regular appearance of Tsuga heterophylla in the pollen record by ca. 2700 yr B.P., synchronous with the development of western hemlock forest within the same latitudes in northern Idaho and northeastern Washington.  相似文献   

17.
Chihuahueños Bog (2925 m) in the Jemez Mountains of northern New Mexico contains one of the few records of late-glacial and postglacial development of the mixed conifer forest in southwestern North America. The Chihuahueños Bog record extends to over 15,000 cal yr BP. An Artemisia steppe, then an open Picea woodland grew around a small pond until ca. 11,700 cal yr BP when Pinus ponderosa became established. C/N ratios, δ13C and δ15N values indicate both terrestrial and aquatic organic matter was incorporated into the sediment. Higher percentages of aquatic algae and elevated C/N ratios indicate higher lake levels at the opening of the Holocene, but a wetland developed subsequently as climate warmed. From ca. 8500 to 6400 cal yr BP the pond desiccated in what must have been the driest period of the Holocene there. C/N ratios declined to their lowest Holocene levels, indicating intense decomposition in the sediment. Wetter conditions returned after 6400 cal yr BP, with conversion of the site to a sedge bog as groundwater levels rose. Higher charcoal influx rates after 6400 cal yr BP probably result from greater biomass production rates. Only minor shifts in the overstory species occurred during the Holocene, suggesting that mixed conifer forest dominated throughout the record.  相似文献   

18.
Pollen, micro-charcoal and total carbon analyses on sediments from the Turbuta palaeolake, in the Transylvanian Basin of NW Romania, reveal Younger Dryas to mid-Holocene environmental changes. The chronostratigraphy relies on AMS 14C measurements on organic matter and U/Th TIMS datings of snail shells. Results indicate the presence of Pinus and Betula open woodlands with small populations of Picea, Ulmus, Alnus and Salix before 12,000 cal yr BP. A fairly abrupt replacement of Pinus and Betula by Ulmus-dominated woodlands at ca. 11,900 cal. yr BP likely represents competition effects of vegetation driven by climate warming at the onset of the Holocene. By 11,000 cal yr BP, the woodlands were increasingly diverse and dense with the expansion of Quercus, Fraxinus and Tilia, the establishment of Corylus and the decline of upland herbaceous and shrubs taxa. The marked expansion of Quercus accompanied by Tilia between 10,500 and 8000 cal yr BP could be the result of low effective moisture associated with both low elevation of the site and with regional change towards a drier climate. At 10,000 cal yr BP, Corylus spread across the region, and by 8000 cal yr BP it replaced Quercus as a dominant forest constituent, with only little representation of Picea abies. Carpinus became established around 5500 cal yr BP, but it was only a minor constituent in local woodlands until ca. 5000 cal yr BP. Results from this study also indicate that the woodlands in the lowlands of Turbuta were never closed.  相似文献   

19.
Excavations in the late-glacial Presumpscot Formation at Portland, Maine, uncovered tree remains and other terrestrial organics associated with marine invertebrate shells in a landslide deposit. Buds of Populus balsamifera (balsam poplar) occurred with twigs of Picea glauca (white spruce) in the Presumpscot clay. Tree rings in Picea logs indicate that the trees all died during winter dormancy in the same year. Ring widths show patterns of variation indicating responses to environmental changes. Fossil mosses and insects represent a variety of species and wet to dry microsites. The late-glacial environment at the site was similar to that of today's Maine coast. Radiocarbon ages of 14 tree samples are 11,907 ± 31 to 11,650 ± 50 14C yr BP. Wiggle matching of dated tree-ring segments to radiocarbon calibration data sets dates the landslide occurrence at ca. 13,520 + 95/−20 cal yr BP. Ages of shells juxtaposed with the logs are 12,850 ± 65 14C yr BP (Mytilus edulis) and 12,800 ± 55 14C yr BP (Balanus sp.), indicating a marine reservoir age of about 1000 yr. Using this value to correct previously published radiocarbon ages reduces the discrepancy between the Maine deglaciation chronology and the varve-based chronology elsewhere in New England.  相似文献   

20.
We reconstructed a 10,500-yr fire and vegetation history of a montane site in the North Cascade Range, Washington State based on lake sediment charcoal, macrofossil and pollen records. High-resolution sampling and abundant macrofossils made it possible to analyze relationships between fire and vegetation. During the early Holocene (> 10,500 to ca. 8000 cal yr BP) forests were subalpine woodlands dominated by Pinus contorta. Around 8000 cal yr BP, P. contorta sharply declined in the macrofossil record. Shade tolerant, mesic species first appeared ca. 4500 cal yr BP. Cupressus nootkatensis appeared most recently at 2000 cal yr BP. Fire frequency varies throughout the record, with significantly shorter mean fire return intervals in the early Holocene than the mid and late Holocene. Charcoal peaks are significantly correlated with an initial increase in macrofossil accumulation rates followed by a decrease, likely corresponding to tree mortality following fire. Climate appears to be a key driver in vegetation and fire regimes over millennial time scales. Fire and other disturbances altered forest vegetation at shorter time scales, and vegetation may have mediated local fire regimes. For example, dominance of P. contorta in the early Holocene forests may have been reinforced by its susceptibility to frequent, stand-replacing fire events.  相似文献   

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