The Holocene

The significance and cause of the decline in biomass burning across the Americas after ad 1500 is a topic of considerable debate. We synthesized charcoal records (a proxy for biomass burning) from the Americas and from the remainder of the globe over the past 2000 years, and compared these with paleoclimatic records and population reconstructions. A distinct post-ad 1500 decrease in biomass burning is evident, not only in the Americas, but also globally, and both are similar in duration and timing to ‘Little Ice Age’ climate change. There is temporal and spatial variability in the expression of the biomass-burning decline across the Americas but, at a regional–continental scale, ‘Little Ice Age’ climate change was likely more important than indigenous population collapse in driving this decline.

Past plant abundance may be reconstructed from pollen data if dispersal distances of pollen and pollen productivities of each taxon are known. Using surface sediment samples from small and medium sized, closed and near circular lakes from lowland Central Europe, we tested the validity of three pollen dispersal models by comparing empirical pollen data from each lake with simulated pollen data derived from applying various pollen dispersal models to vegetation data from rings situated up to 100 km from each site. Pollen assemblages simulated with a Lagrangian stochastic (LS) model best fit real pollen assemblages, simulations with the commonly used Prentice model on pollen dispersal underestimated the amount of pollen arriving from distances larger than 10 km and overestimated the differences in dispersal distances between lighter (Pinus) and heavier (Fagus, Picea) pollen grains. The LS model appeared to provide more appropriate simulations. Pollen productivity estimates (PPEs) calculated for the data set showed that the choice of the dispersal model has great impact on the results. If derived with the Prentice model, PPEs for Fagus and Picea are three times higher than with the LS model. Studies on pollen productivities thus need to consider the apparent limitations of the Prentice model. We suggest an alternative approach, which uses simulations instead of the extended R-value model, to calculate PPEs. The approach is flexible in the use of dispersal functions and produced consistent results for two independent data sets from small and medium sized lakes.

Model validation experiments are fundamental to ensure that the peat growth models correspond with the diversity in nature. We evaluated the Holocene Peatland Model (HPM) simulation against the field observations from a chronosequence of peatlands and peat core data. The ongoing primary peatland formation on the isostatically rising coast of Finland offered us an exceptional opportunity to study the peatland succession along a spatial continuum and to compare it with the past succession revealed by vertical peat sequences. The current vegetation assemblages, from the seashore to a 3000 year old bog, formed a continuum from minerotrophic to ombrotrophic plant communities. A similar sequence of plant communities was found in the palaeovegetation. The distribution of plant functional types was related to peat thickness and water-table depth (WTD) supporting the assumptions in HPM, though there were some differences between the field data and HPM. Palaeobotanical evidence from the oldest site showed a rapid fen–bog transition, indicated by a coincidental decrease in minerotrophic plant functional types and an increase in ombrotrophic plant functional types. The long-term mean rate of carbon (C) accumulation varied from 2 to 34 g C/m² per yr, being highest in the intermediate age cohorts. Mean nitrogen (N) accumulation varied from 0.1 to 3.9 g N/m² per yr being highest in the youngest sites. WTD was the deepest in the oldest sites and its variation there was temporally the least but spatially the highest. Evaluation of the HPM simulations against the field observations indicated that HPM reasonably well simulates peatland development, except for very young peatlands.

The issue of continuity in deciduous oakwood vegetation has been in the forefront of woodland ecological studies for many decades. The two basic questions that emerge from existing research are whether or not oakwoods can be characterized by long-term stability and what may be the driving forces of the observed stability or change. To answer these questions in a well-defined case study, we examined the history of a large subcontinental oakwood (Dúbrava) in the southeastern Czech Republic with interdisciplinary methods using palaeoecological and archival sources. Palaeoecology allowed us to reconstruct the vegetation composition and fire disturbances in Dúbrava in the past 2000 years, while written sources provided information about tree composition and management from the 14th century onwards. The pollen profiles show that the present oakwood was established in the mid-14th century with an abrupt change from shrubby, hazel-dominated vegetation to oak forest. This change was most probably caused by a ban on oak felling in ad 1350. From the 14th to the late 18th centuries Dúbrava had multiple uses, of which wood-pasture and hay-cutting kept the forest considerably open. The second remarkable change was dated to the late 18th century, when multiple-use management was abandoned and Dúbrava was divided into pasture-only and coppice-only parts. The last major shift occurred in the mid-19th century, when modern forestry and Scotch pine plantation became dominant. We conclude that Dúbrava Wood did not show stability in the long run and that its species composition has dramatically changed during the last two millennia. The most important driving force in the shaping and maintenance of the unique vegetation of Dúbrava was human management.

Pollen and charcoal records of a 150 cm long lake sediment core from Taibai Lake in the middle reach of the Yangtze River reveal seven major changes in regional vegetation over the last 1500 years. During the period c. ad 480–710, evergreen broadleaved forest dominated by Castanopsis/Lithocarpus and evergreen oaks occurred in the Taibai Lake catchment. From c. ad 710 to 1050, the vegetation shifted to a mixed conifer and broadleaved forest, with Pinus expanding at the expense of Castanopsis/Lithocarpus. Between c. ad 1050 and 1320, evergreen broadleaved forest reoccupied the studied area. From c. ad 1320 to 1650, the area of primary forest decreased markedly with synchronous reduction in broadleaved trees and Pinus. Between c. ad 1650 and 1740, the biomass declined rapidly, while secondary Pinus forest began to expand. During c. ad 1740–1950, forest extent increased slightly compared with the previous stage, but the landscape was still secondary forest with the minimum proportion of broadleaved trees. After c. ad 1950, the biomass in the surrounding area was very low, with vegetation types similar to that at present (secondary Pinus forest and mixed conifer and broadleaved forest). A detrended correspondence analysis (DCA) with 125 samples and 34 main pollen types is used to separate human and climatic impacts on vegetation. Then, the pollen assemblage, DCA, charcoal record, and magnetic susceptibility are combined to discuss the key factors inducing these vegetation changes. The vegetation changes were mainly controlled by the climatic changes, with the weak impacts of human activities before c. ad 1320. Since then, the intensity of human influences on vegetation increased gradually, entering a transitional period of main controlling factors of vegetation changes from nature to human activities. After c. ad 1740, the vegetation changes were chiefly controlled by human activities, and the climatic signal was weak.

High-resolution pollen, plant macrofossil and magnetic susceptibility (MS) data are presented from an alpine lake sediment core from west-central Colorado, recording changes in vegetation and sedimentation for the latest Pleistocene and Holocene (c. the last 12.5 ka; 1 ka = 1000 cal. yr BP). During the Younger Dryas chron (c. 12.9–11.5 ka), Artemisia steppe or tundra grew around the lake, but by the earliest Holocene (10.7–9.5 ka) a subalpine Picea and Abies parkland was established there. Picea remained important through the early Holocene, but also bristlecone and lodgepole pines (Pinus aristata and P. contorta) grew around the lake. Warming conditions are indicated from 9.5 ka, lasting until c. 4.5–3.5 ka, which may have been the warmest period, with greatest development of monsoonal conditions. Trees subsequently retreated downslope from Kite Lake c. 150–200 m during the last 3.5 ka, establishing their present treeline position. A decrease in total Pinus and increases in Artemisia and piñon (P. edulis) indicate a trend toward progressive climate cooling and enhanced winter precipitation. These long-term climatic trends correlate with Holocene changes in summer insolation.

We evaluated the potential of maize pollen concentrations in lake sediment profiles to serve as indicators of the extent of prehistoric agriculture in neotropical lake basins using records from a network of five sediment cores recovered from Laguna Zoncho, Costa Rica. The watershed of this small (0.75 ha) lake in the Diquís archaeological region has a c. 3000 year history of prehistoric agriculture and subsequent forest recovery, as documented through previous studies of pollen, charcoal, diatoms, and phosphorus fractions in a single core recovered from the center of the lake. In our new network of cores, we compared maize pollen concentrations with two independent proxies for the scale of agriculture in the same cores: abundance of organic matter (OM), which is an indicator of soil erosion, and bulk sediment stable carbon isotope ratios of organic matter (δ¹³C_OM), which reflect the proportion of forested and cleared land within the watershed. In none of the five cores did maize pollen concentrations correspond with either OM or δ¹³C_OM, suggesting that sedimentary maize pollen concentrations are not sensitive to the scale of maize agriculture in small neotropical watersheds. We found maize pollen in relatively high concentrations in two of the four cores taken near the lakeshore, but the others contained little or no maize pollen. The core from the center of the lake consistently recorded maize pollen, a finding that we attribute to sediment-focusing processes.

Based on radiocarbon dating and our analysis of plant and animal remains from Buziping, a Majiayao (5300–4300 BP) and Qijia (4200–3800 BP) period site located in Dingxi, Gansu Province, China, and our review of archaeobotanical studies in the Western Loess Plateau and adjacent areas, we discuss subsistence strategies during the Majiayao and Qijia periods. We also discuss the development of agriculture in the Western Loess Plateau and its influence on cultural expansion during the late Neolithic period. Humans settled at Buziping for the first time during the Majiayao period (4890–4710 cal. yr BP by ¹⁴C dating). Charred seeds from the site indicate that people engaged in millet-based agricultural production. People continued this type of agriculture during a second phase of occupation (4130–3880 cal. yr BP by ¹⁴C dating) during the Qijia period, but the proportion of foxtail millet to broomcorn millet increased from the Majiayao to Qijia period. Raising domestic animals was another aspect of subsistence during the Qijia period. The main domestic animals were likely pigs and dogs, although hunting of wild animals also took place. Subsistence at Buziping site was affected by the rapid development of intensive agriculture that diffused across eastern Gansu Province during the late Neolithic. Our work suggests that millet-based agriculture spread from east to west across the Western Loess Plateau and likely promoted the expansions of those two cultures in the area during the Majiayao period and early–mid Qijia period. Climate change might have also promoted Majiayao and Qijia expansions and probably facilitated the adoption of rain-fed agriculture in this region.

Dendroglaciological investigations near Mt. Waddington in the central British Columbia Coast Mountains provide an enhanced perspective of Holocene glacial activity. Field investigations at Confederation, Franklin, and Jambeau glaciers led to the discovery of subfossil wood mats encased in glacial deposits and glacially sheared stumps buried beneath till. Radiocarbon-dated wood collected from valley-bottom and lateral moraine sites at Confederation Glacier suggest that an early-Holocene advance occurred at c. 5665 cal. yr BP, followed by succeeding intervals of glacier expansion at c. 3700 and 3500 cal. yr BP. At Jambeau Glacier detrital wood mats buried close to the contemporary lateral moraine crests document glacier expansion at c. 3000 cal. yr BP. Detrital subfossil wood buried in lateral moraines at the confluence of Confederation and Franklin glaciers records distinct episodes of ‘Little Ice Age’ glacier expansion as early as c. 1212 cal. yr ad, and suggests the glacier surface continued to thicken until at least c. 1330–1410 cal. yr ad. An interval of downwasting and retreat followed, before late ‘Little Ice Age’ advances such as those at Jambeau Glacier were overwhelming valley-bottom forests by c. 1740 cal. yr ad. With the exception of the previously unrecognized advance of Confederation Glacier at c. 3700 cal. yr BP, our dendroglaciological findings corroborate the emerging record of Holocene glacier activity in the British Columbia Coast Mountains.

During an archaeological survey in Pajala parish, northernmost Sweden, clusters of quartz waste from knapping and burnt bone were discovered on a glaciofluvial gravel plateau close to Aareavaara village in the Muonio River valley. Sampled materials from a larger area and small-scale excavations (in total 6 m²) are interpreted as resulting from short-stay hunter-gatherer camps. Radiocarbon dating on burnt bones suggest an age of occupancy at ~10,700 cal. yr BP, which is more or less contemporary with ‘Komsa Phase’ sites on the north coast of Norway (~300–360 km northwards). The Aareavaara site should thus be the oldest known archaeological site to date in northern Sweden. A palaeoenvironmental reconstruction, based on pollen analysis of sediment cores from two nearby lakes and radiocarbon dating of macrofossils for construction of time/depth sedimentation curves, suggests a deglaciation age of the area corresponding to occupation by early man (~10,700 cal. yr BP). Aareavaara was at the time of deglaciation situated in a transitional zone between subaqueous and subaerial ice-margin retreat from the northeast towards the southwest, with higher hills and plateaux forming an archipelago in the Ancylus Lake with highest shorelines formed at ~170 m a.s.l. The hunter-gatherer camp sites at Aareavaara were thus, both in time and space, located in close proximity to the retreating ice sheet margin, but also in a waterfront location, in fact on an island in the Ancylus Lake. Our pollen data suggest a subarctic birch woodland tundra landscape characterized by open vegetation, including occasional birch trees and an abundance of willow and dwarf birch.

A reconstructed pattern of Lateglacial and Holocene hydrological changes in the area of the former lakes Dūbas, Pelesa and Matara is presented. The investigated basin is situated in southeastern Lithuania, beyond the marginal ridge of the Weichselian Glaciation, on the margin of the sandy plain that is the watershed between the Ūla and Katra rivers. Pollen analysis, radiocarbon dating, loss-on-ignition measurements and GIS-based simulation of water level fluctuations have been applied in order to obtain new information on the development of the Ūla–Katra watershed area. The results of multiproxy investigations revealed the history of the development and extinction of Dūbas, Pelesa and Matara lakes. A single basin, covering a large part of the study area, had formed after the retreat of the continental ice sheet from southeastern Lithuania. This basin was divided into three separate lakes during the Allerød Interstadial as a result of a drop in water level. Blocking of the drainage by aeolian sediments around the early Boreal caused another rise in the water-table and the successive merging of the separate lakes. Probably as a result of river capture, the single lake was drained abruptly after c. 2000 years. Another river-capture event in the 19th century caused abrupt drainage of the study area, which led to the final extinction of the three lakes.

Charred wood analyses have been performed on three Mesolithic sites located on the Causse de Gramat, a karstic plateau in southwestern France (Lot department): Les Fieux, les Escabasses and le Cuzoul de Gramat. The sites yielded occupations dating from the early to the late Mesolithic (9th to the 6th millennia cal. bc). In the absence of palynological data, charcoal analysis allowed us to characterise the woody environment exploited by Mesolithic hunter-gatherers. Our results point at the minor place of softwoods in the landscape and the local importance of Rosaceae prunoideae, marking an original type of pre-forest vegetation. In order to retrace the local vegetation dynamics, these results are synthesized and discussed thanks to a correspondence analysis, which includes previous anthracological data from Azilian and middle Mesolithic levels of two other sites of the Causse. The homogeneity of the results speaks in favour of the stability of the local environment and confirms the specific vegetation features of the area.

A multiproxy approach to a sediment sequence at Lake Saint-Point in the French Jura Mountains gives evidence of a strong coupling between changes in terrestrial and lacustrine ecosystems throughout the Holocene. The early Holocene (11,700–10,200 cal. BP) is characterised by the recovery of terrestrial and lake ecosystems favoured by climatic warming. During the middle Holocene (10,600–6200 cal. BP), the climatic optimum coincided with an extension of deciduous forests into the catchment area, while lake sedimentation is dominated by authigenic carbonates and low detrital inputs. After 6200 cal. BP, the Neoglacial favoured expansion of Abies-Fagus forests and increasing detrital inputs to the lake where ostracod fauna declined and changed in composition. After 1200 cal. BP, human impact was responsible for extensive forest clearings in the catchment area, while the lake basin shows contrasting pictures with increasing detrital input, resuming sedimentation of authigenic carbonates and changes in dominant ostracod species. Orbitally driven climatic variations were the dominant factor of environmental changes until c. 1200 cal. BP. Around 2600 cal. BP, human impact increased and became the major factor in the catchment area and the lake basin from 1200 cal. BP onwards. Finally, the Saint-Point record offers a clear illustration of how gradual changes in insolation or increasing human impact may provoke, even under temperate climatic conditions, abrupt responses in mid-European terrestrial and lake ecosystems, and how differences in the dates of tipping points revealed by proxies suggest specific threshold values depending on the sensitivity of indicators used and on their role in the different compartments of these ecosystems.