A 250 year comparison of historical, macrofossil and pollen records of aquatic plants in a shallow lake

1. Sedimentary remains of aquatic plants, both vegetative (turions, leaves, spines) and reproductive (fruits, seeds, pollen), may provide a record of temporal changes in the submerged vegetation of lakes. An independent assessment of the degree to which these remains reflect past floristic change is, however, rarely possible. 2. By exploiting an extensive series of historical plant records for a small shallow lake we compare plant macrofossil (three cores) and pollen (one core) profiles with the documented sequence of submerged vegetation change since c. 1750 AD. The data set is based on 146 site visits with 658 observations including 42 taxa classified as aquatic, spanning 250 years. 3. Approximately 40% of the historically recorded aquatic taxa were represented by macro‐remains. In general macrofossils underestimated past species diversity, with pondweeds (three of eight historically recorded Potamogeton species were found) particularly poorly represented. Nonetheless, several taxa not reported from historical surveys (e.g. Myriophyllum alterniflorum and Characeae) were present in the sediment record. 4. The pollen record revealed taxa which left no macro‐remains (e.g. Littorella uniflora), and the macrofossil record provided improved taxonomic resolution for some taxa (e.g. Potamogeton) and a more reliable record of persistence, appearance and loss of others (e.g. Myriophyllum spp. and Nymphaeaceae). 5. Detrended correspondence analysis indicated that changes in the community composition evidenced by the palaeolimnological and historical records were synchronous and of a similar magnitude. Both records pointed to a major change at around 1800, with the historical record suggesting a more abrupt change than the sedimentary data. There was good agreement on a subsequent change c. 1930. 6. The palaeolimnological data did not provide a complete inventory of historically recorded species. Nevertheless, these results suggest that combined macrofossil and pollen records provide a reliable indication of temporal change in the dominant components of the submerged and floating‐leaved aquatic vegetation of shallow lakes. As such palaeolimnology may provide a useful tool for establishing community dynamics and successions of plants over decadal to centennial timescales.     

Vegetation history and lake-level changes from the Younger Dryas to the present in Eastern Pyrenees (France): pollen, plant macrofossils and lithostratigraphy from Lake Racou (2000 m a.s.l.)

A multi-proxy palaeoecological investigation including pollen, plant macrofossil, radiocarbon and sedimentological analyses, was performed on a small mountain lake in the Eastern Pyrenees. This has allowed the reconstruction of: (1) the vegetation history of the area based on five pollen diagrams and eight AMS¹⁴C dates and (2) the past lake-level changes, based on plant macrofossil, lithological and pollen analysis of two stratigraphical transects correlated by pollen analysis. The palaeolake may have appeared before the Younger Dryas; the lake-level was low and the vegetation dominated by cold steppic grasslands. The lake-level rose to its highest level during the Holocene in the Middle Atlantic (at ca. 5060±45 b.p.). Postglacial forests (Quercetum mixtum and Abieto-Fagetum) developed progressively in the lower part of the valley, while dense Pinus uncinata forests rapidly invaded the surroundings of the mire and remained the dominant local vegetation until present. The observed lowering of the lake levels during the Late Atlantic and the Subboreal (from 5060 ± B.P. to 3590±40 b.p.) was related to the overgrowth of the mire. The first obvious indications of anthropogenic disturbances of the vegetation are recorded at the Atlantic/Subboreal boundary as a reduction in the forest component, which has accelerated during the last two millennia.     

Response of tundra ecosystem in southwestern Alaska to Younger-Dryas climatic oscillation

Climatic warming during the last glacial–interglacial transition (LGIT) was punctuated by reversals to glacial‐like conditions. Palaeorecords of ecosystem change can help document the geographical extent of these events and improve our understanding of biotic sensitivity to climatic forcing. To reconstruct ecosystem and climatic variations during the LGIT, we analyzed lake sediments from southwestern Alaska for fossil pollen assemblages, biogenic‐silica content (BSiO₂%), and organic‐carbon content (OC%). Betula shrub tundra replaced herb tundra as the dominant vegetation of the region around 13 600 cal BP (cal BP: ¹⁴C calibrated calendar years before present), as inferred from an increase of Betula pollen percentages from << 5% to >> 20% with associated decreases in Cyperaceae, Poaceae, and Artemisia. At c. 13 000 cal BP, a decrease of Betula pollen from 28 to << 5% suggests that shrub tundra reverted to herb tundra. Shrub tundra replaced herb tundra to resume as the dominant vegetation at 11 600 cal BP. Higher OC% and BSiO₂% values suggest more stable soils and higher aquatic productivity during shrub‐tundra periods than during herb‐tundra periods, although pollen changes lagged behind changes in the biogeochemical indicators before c. 13 000 cal BP. Comparison of our palaeoecological data with the ice‐core dδ¹⁸O record from Greenland reveals strikingly similar patterns from the onset through the termination of the Younger Dryas (YD). This similarity supports the hypothesis that, as in the North Atlantic region, pronounced YD climatic oscillations occurred in the North Pacific region. The rapidity and magnitude of ecological changes at the termination of the YD are consistent with greenhouse experiments and historic photographs demonstrating tundra sensitivity to climatic forcing.     

Increase in benthic trophic reliance on methane in 14 French lakes during the Anthropocene

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Can we detect a west Norwegian tree line from modern samples of plant remains and pollen? Results from the DOORMAT project

In the DOORMAT (Direct Observation of Recent Macrofossils Across Treeline) project, the modern representation of local vegetation by pollen and plant remains (plant macrofossils) across a west Norwegian tree line, composed of Betula pubescens and Pinus sylvestris, has been studied over 2 years. The aim was to discover if the modern tree line could be detected and therefore how precisely past tree-line movements could be reconstructed and related to Holocene climate changes by using one proxy or a combination of both. Traps were placed in the vegetation from 663 to 1,120 m a.s.l., spanning the pine altitudinal species limit, the birch tree-line ecotone, and the vegetation zones up to the mid-alpine zone. Three traps were also set in the small lake Trettetjørn close to the modern tree line at 800 m a.s.l. Traps were emptied twice a year to sample both summer and winter seasons. Macrofossils represent their local vegetation well. However, tree Betula remains were trapped above the tree line and Pinus and Picea remains were recorded 1.0–1.5 km away from their sources, demonstrating considerable dispersal capacity. This shows that rare macrofossil remains do not necessarily represent the local presence of these trees. Aerial tree pollen deposition in traps at the upper limit of pine woodland and in the subalpine birch woodland was unexpectedly low, whereas pollen accumulation rates (PAR) were orders of magnitude higher in the lake traps. We hypothesise that the lake receives regional pollen rain washed in from its catchment by snow meltwater and that high values in traps are due to continuous suspension of pollen in the lake water during summer. The interpretation of tree-line changes from existing Holocene pollen and plant macrofossil data from Trettetjørn was supported and refined by the DOORMAT macrofossil data, but the modern pollen data were anomalous.     

Hydrologic and anthropogenic influences on aquatic macrophyte development in a large,shallow lake in China

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A millennial‐scale chronicle of evolutionary responses to cultural eutrophication in Daphnia

For an accurate assessment of the anthropogenic impacts on evolutionary change in natural populations, we need long‐term environmental, genetic and phenotypic data that predate human disturbances. Analysis of c. 1600 years of history chronicled in the sediments of South Center Lake, Minnesota, USA, revealed major environmental changes beginning c. 120 years ago coinciding with the initiation of industrialised agriculture in the catchment area. Population genetic structure, analysed using DNA from dormant eggs of the keystone aquatic herbivore, Daphnia pulicaria, suggested no change for c. 1500 years prior to striking shifts associated with anthropogenic environmental alterations. Furthermore, phenotypic assays on the oldest resurrected metazoan genotypes (potentially as old as c. 700 years) indicate significant shifts in phosphorus utilisation rates compared to younger genotypes. Younger genotypes show steeper reaction norms with high growth under high phosphorus (P), and low growth under low P, while ‘ancient’ genotypes show flat reaction norms, yet higher growth efficiency under low P. Using this resurrection ecology approach, environmental, genetic and phenotypic data spanning pre‐ and post‐industrialised agricultural eras clearly reveal the evolutionary consequences of anthropogenic environmental change.     

Differences in the exploitation of bream in three shallow lake systems and their relation to water quality

SUMMARY 1. The development of bream populations, water transparency, chlorophyll‐a concentration, extent of submerged vegetation and densities of the zebra mussel, Dreissena polymorpha, were analysed in three shallow eutrophic lake systems subject to different fish management. 2. In Lake Veluwemeer, the bream population was reduced from c. 100 to 20 kg ha^?1 after 5 years of fishing. The mortality caused by the fishery was estimated at 38% of bream >15 cm in addition to a 13% natural mortality of bream >17 cm. The decline was followed by an expansion of the Chara beds present in the shallow parts, an increase in water transparency in the open‐water zone, an increase in the density of zebra mussels and a decrease in chlorophyll‐a concentrations. 3. The newly created Lake Volkerak showed trends opposite to those in Lake Veluwemeer. Bream colonised the lake in 1988 and reached a biomass of c. 140 kg ha^?1 in 1998. The water transparency decreased from a maximum of 3 m to c. 1 m and the chlorophyll‐a concentration increased from 5 to 45 μg L^?1. Submerged vegetation colonised up to 20% of the total lake area in the first 5 years after creation of the lake in 1987 but decreased to 10% as turbidity increased. 4. Seine fishery in the Frisian lake system did not appear to affect the bream population despite annual catches as high as 40–50 kg ha^?1. The estimated natural mortality of fish >15 cm was 15% and mortality by fishery was 26%. The high loss was apparently compensated by good recruitment and high growth rates resulting from a c. 1 °C higher water temperature during the years when bream were removed by fishing. There was only a slight decrease in chlorophyll‐a concentrations and a slight increase in water transparency. 5. The results of this study suggest that the effects of bream exploitation in eutrophic lakes can vary depending on the efficiency of the fishery, recruitment success and temperature regime. In the absence of fishery, bream dominated the fish community in the study lakes and apparently prevented D. polymorpha and submerged vegetation from establishing because of physical disturbance, enhanced internal P‐loading and resettling of resuspended sediments.     

Estimating past floristic diversity in montane regions from macrofossil assemblages

The relationship between the diversity of higher plant macrofossils in surface sediments of lakes and the surrounding vegetation is examined in two mountain regions; Grødalen in central Norway and the south-east Cairngorms in Scotland. Two lake sediment cores from each area were also analysed to examine vegetation history and to estimate changes in biodiversity through the Holocene. The diversity of present day vegetation in each region was estimated using both quadrat data and classified satellite images of the study areas. The mean surface sample macrofossil representation of species recorded in quadrats collected within 250 m of the lakes was c. 17%. This figure drops to only c. 2% when the satellite imagery of the same area is used to provide a maximal species list. The macrofossil data from the Norwegian cores show that deglaciation in this region occurred earlier on the mountain summit than in the valley and that the maximum tree line elevation was during the interval 9100–4400 ¹⁴C yr BP . In the Cairngorms the maximum tree line elevation was prior to c. 4500 ¹⁴C yr BP . The changes in higher plant diversity recorded at these sites through the Holocene show that c. 4000 ¹⁴C yr BP the reduction in the tree line resulted in decreased β-diversity at higher altitudes but an increase at the lower altitude as the forest cover opened up. Under conditions of climatic warming it is likely areas that come to lie below the tree line will experience reduced diversity and that a permanent loss of biodiversity would result from a severe reduction in the area above the tree line.     

Beringia as a glacial refugium for boreal trees and shrubs: new perspectives from mapped pollen data

Aim Beringia, far north‐eastern Siberia and north‐western North America, was largely unglaciated during the Pleistocene. Although this region has long been considered an ice‐age refugium for arctic herbs and shrubs, little is known about its role as a refugium for boreal trees and shrubs during the last glacial maximum (LGM, c. 28,000–15,000 calibrated years before present). We examine mapped patterns of pollen percentages to infer whether six boreal tree and shrub taxa (Populus, Larix, Picea, Pinus, Betula, Alnus/Duschekia) survived the harsh glacial conditions within Beringia. Methods Extensive networks of pollen records have the potential to reveal distinctive temporal–spatial patterns that discriminate between local‐ and long‐distance sources of pollen. We assembled pollen records for 149 lake, peat and alluvial sites from the Palaeoenvironmental Arctic Sciences database, plotting pollen percentages at 1000‐year time intervals from 21,000 to 6000 calibrated years before present. Pollen percentages are interpreted with an understanding of modern pollen representation and potential sources of long‐distance pollen during the glacial maximum. Inferences from pollen data are supplemented by published radiocarbon dates of identified macrofossils, where available. Results Pollen maps for individual taxa show unique temporal‐spatial patterns, but the data for each taxon argue more strongly for survival within Beringia than for immigration from outside regions. The first increase of Populus pollen percentages in the western Brooks Ranges is evidence that Populus trees survived the LGM in central Beringia. Both pollen and macrofossil evidence support Larix survival in western Beringia (WB), but data for Larix in eastern Beringia (EB) are unclear. Given the similar distances of WB and EB to glacial‐age boreal forests in temperate latitudes of Asia and North America, the widespread presence of Picea pollen in EB and Pinus pollen in WB indicates that Picea and Pinus survived within these respective regions. Betula pollen is broadly distributed but highly variable in glacial‐maximum samples, suggesting that Betula trees or shrubs survived in restricted populations throughout Beringia. Alnus/Duschekia percentages show complex patterns, but generally support a glacial refugium in WB. Main conclusions Our interpretations have several implications, including: (1) the rapid post‐glacial migration rate reported for Picea in western Canada may be over estimated, (2) the expansion of trees and shrubs within Beringia should have been nearly contemporaneous with climatic change, (3) boreal trees and shrubs are capable of surviving long periods in relatively small populations (at the lower limit of detection in pollen data) and (4) long‐distance migration may not have been the predominant mode of vegetation response to climatic change in Beringia.     

The use of aquatic vegetation in lake assessment: testing the sensitivity of macrophyte metrics to anthropogenic pressures and water quality

The relationships between aquatic vegetation, anthropogenic pressures and water quality in 83 Polish lowland lakes were analysed in order to select the best responding macrophyte metrics to be used in ecological status assessment. Several metrics describing the syntaxonomic composition, abundance and spatial structure of macrophytes were tested in three morphological lake types (deep regular-shaped, deep ribbon-shaped and shallow) separately by using the Spearman rank correlation coefficients. Among all the parameters tested, only some anthropogenic pressure parameters (the percentage share of urban areas and forests in a catchment, the pollution load from point sources) and all the water quality indicators (TP, TN, Chla, SD) were significantly correlated with most of the macrophyte metrics, although for different lake types the strength and significance of relationships varied substantially. The macrophyte metrics which best responded to anthropogenic pressures and water quality changes were: (i) in deep regular-shaped lakes: the percentage share of Chara phytocenoses and rush phytocenoses in the total phytolittoral area, the maximum depth of plant distribution and the colonisation index; (ii) in deep ribbon-shaped lakes: the hydrophytes/helophytes area ratio, the percentage share of submerged and rush phytocenoses in the total phytolittoral area; (iii) in shallow lakes: the percentage share of Chara phytocenoses and rush phytocenoses in the total phytolittoral area, and the hydrophytes/helophytes area ratio. These metrics can then be used when elaborating new or refining existing macrophyte-based methods of ecological status assessment.     

The long-term ecology of the lost forests of La Laguna, Tenerife (Canary Islands)

Aim We report the first analysis of the long‐term ecology of Tenerife, in order to establish a pre‐colonization base‐line and to assess the effect of human activity and the role of climatic variation on vegetation during the Late Holocene. Location A former lake bed in the city of La Laguna (Tenerife, Canary Islands, Spain). Methods A sedimentary sequence of over 2 m was obtained from the former lake bed. Fossil pollen and microfossil charcoal concentrations were analysed. Radiocarbon dating of the sequence indicates that it spans approximately the last 4700 years. The pollen diagram was zoned using optimal splitting within psimpoll 4.25. Results Three pollen zones were differentiated: (1) in Zone L1 (c. 4700–2900 cal. yr bp ) a mixed forest was dominated by Quercus, Carpinus, Myrica and Pinus; (2) in Zone L2 (c. 2900–2000 cal. yr bp ) the laurel forest taxa increased, while Pinus, Juniperus and Phoenix declined; and (3) Zone L3 (c. 2000–400 cal. yr bp ) was characterized by the decline of Carpinus and Quercus and the abundance of laurel forest taxa (e.g. Myrica). Neither Carpinus nor Quercus was hitherto considered to be native to the Canary Islands. Their decline started c. 2000 years ago, coinciding with microfossil charcoal evidence of increased burning and with archaeological evidence for the first human settlement on Tenerife. Main conclusions Between c. 4700 and 2000 cal. yr bp , the composition of the forest in the valley of La Laguna was very different from what it is at present. In particular, Quercus and Carpinus appear to have been significant components, alongside components of the present‐day laurel forest, and the native pine (Pinus canariensis) forest and thermophilous woodland were also more prevalent in the region (but probably not within the lake basin itself) until 3000 cal. yr bp . The subsequent decline of Quercus and Carpinus led to the establishment of the present laurel forest in the region and a shift to more open vegetation types. These changes indicate that the aboriginal inhabitants of the islands, the Guanches, had a far more profound impact on the vegetation of Tenerife than hitherto realized.     

The 'oriental' component of the Balkan flora: evidence of presence on the Thracian Plain during the Weichselian late-glacial

Aim To obtain palaeobotanical evidence enabling evaluation of the viability of the hypothesis that the ‘oriental’ element of the Balkan flora reached south‐east Europe from Turkey prior to the Holocene, probably via the Thracian Plain during a late Quaternary glacial stage but no later than the late Weichselian. Location Ezero wetland, northern Thracian Plain, Bulgaria. Methods We undertook analyses of pollen and microspores, plant macrofossils, wood fragments and molluscs recovered from sediments deposited in the Ezero wetland during the late Weichselian and Weichselian late‐glacial. Sediment chronology was determined using radiocarbon age estimates. Results Six metres of sediments were recovered from the basin, of which the lower 3 m, extending from c. 15,450 cal yr bp to the early Allerød, was analysed. A major hiatus occurred after c. 13,900 cal yr bp , the overlying sediments being of late Holocene age. Palaeobotanical evidence indicates predominantly open vegetation during the Weichselian late‐glacial, although macrofossil remains of woody taxa demonstrate the local presence of patches of wooded steppe and gallery forest. Changes in the composition of the steppe vegetation, and in the nature of the sediments deposited in the basin, indicate changes in climatic conditions, especially in the hydrological regime and in the moisture available to vegetation. After an initially relatively moister phase, the final centuries of the late Weichselian were drier, as was a short interval that may correlate with the Older Dryas. Moister conditions characterize intervals corresponding to the Bølling and Allerød sub‐units of the Weichselian late‐glacial interstadial. Although the pollen evidence is thus consistent with that from previous studies of this period in south‐east Europe and south‐west Asia, indicating predominantly open steppe vegetation, the macrofossil evidence indicates the persistent local presence of woody taxa. The woody taxa recorded include Celtis tournefortii‐type and Juniperus cf. J. excelsa, two taxa today characteristic of the wooded steppes of Anatolia and members of the ‘oriental’ element of the southern Balkan flora, as well as Rosaceae Subfams. Maloideae and Prunoideae, Alnus and Fraxinus. Main conclusions The late Weichselian vegetation of the northern Thracian Plain included patches of wooded steppe that supported members of the ‘oriental’ element of the modern Balkan flora. The presence of such taxa renders viable the hypothesis that they could have reached south‐east Europe from Turkey via the Thracian Plain during glacial times. Such hypotheses in historical biogeography can be evaluated critically using the evidence obtained from plant macrofossil analyses in combination with that from pollen analysis.     

Dynamics of North Patagonian rainforests from fine‐resolution pollen,charcoal and tree‐ring analysis,Chonos Archipelago,Southern Chile

Abstract Fine‐resolution palaeoecological and dendrochronological methods were used to investigate the impacts of climate change, and natural and anthropogenic disturbances on vegetation in the North Patagonian rainforest of southern Chile at decadal to century timescales during the late Holocene. A lake sediment mud–water interface core was collected from the northern Chonos Archipelago and analysed for pollen and charcoal. Dendrochronological analysis of tree cores collected from stands of Pilgerodendron uviferum close to the lake site was incorporated into the study. The combined analysis showed that the present mosaic of vegetation types in this region is a function of environmental changes across a range of timescales: millennial climate change, more recent natural and anthropogenic disturbances, and possibly short‐term climatic variations. Of particular interest is the spatiotemporal distribution of Pilgerodendron uviferum dieback/burning in the Chonos Archipelago region.     

The long history of Cannabis and its cultivation by the Romans in central Italy, shown by pollen records from Lago Albano and Lago di Nemi

The cores from the Albano and Nemi lakes, near Rome, were studied within the European Union funded PALICLAS project and provided high resolution records of the Late-glacial and Holocene. Pollen evidence of increasing human influence on vegetation was recorded in the Holocene parts of both diagrams, and the Cannabis (hemp) curve was one of the major signs. In this paper we present unambiguous pollen evidence from the Cannabaceae records for the cultivation of hemp in central Italy by the Romans. The oldest records of Cannabis and Humulus (hop) date from to the Late-glacial. Hop pollen values rise during the mid Holocene, while hemp pollen becomes more abundant from ca. 3000 cal B.P. onwards. The highest earliest hemp peak (21%) is dated to the 1st century A.D. This ‘Cannabis phase’, with the abrupt rise of hemp pollen soon after the rise of cultivated trees (Castanea, Juglans and Olea) is associated with the increase in cereals and ruderal plants. This unambiguous proof of cultivation by Romans around 2000 B.P. occurs as well as a long lasting pre-Roman presence of hemp in the area, which is natural and possibly also anthropogenic. Subsequent clear episodes of cultivation in the medieval period were found. Received February 4, 2002 / Accepted September 13, 2002 Correspondence to: Anna Maria Mercuri, e-mail: mercuri.annamaria@unimo.it     

Similarity between contemporary vegetation and plant remains in the surface sediment in Mediterranean lakes

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Palaeoecological studies at the Kaliakra area, northeastern Bulgarian Black Sea coast: 6000?years of natural and anthropogenic change

A 7.5 m sediment core from Lake Bolata, a small former coastal liman lake in northeastern Bulgaria, was analyzed for pollen and plant macrofossil content in attempt to trace the changes in the vegetation, human impact and the influence of the Black Sea during the last ca. 6,000 years. Lake Bolata started its existence when the rising Black Sea level reached the elevation of the bottom of the depression. By that time nearly all tree species were already present in this area with the exception of Carpinus orientalis. The comparison of the arrival time of oriental hornbeam at different sites along the Bulgarian Black Sea confirmed the reliability of the local radiocarbon chronology. For the period under study the vegetation of the region around the site can be described as a forest-steppe due to NAP values higher than 40% of the pollen sum. Forests on the slopes of the river canyon consisted of Quercus spp., Acer, Carpinus orientalis, Fraxinus ornus, Fagus and possibly Tilia. Riverine forests formed stands composed of Salix, Alnus, Ulmus, Fraxinus, Carpinus betulus and Vitis as a liana. The vegetation on the Cape Kaliakra plateau, bordering the canyon, was dominated by diverse herbs and most of these could be considered xerophytes. The oldest occupation period belongs to the Eneolithic, documented by the occurrences of Triticum-type pollen at 5570–5170 cal b.p. (3620–3220 b.c.). The next period of higher continuous Cerealia-type values corresponds to 3450–1830 cal b.p. (1500 b.c.–a.d. 120). The macrofossil record chiefly provides evidence about plant communities of aquatics and helophytes.     

Tree line identification from pollen data: beyond the limit?

Aim The boreal tree line is a prominent biogeographic feature, the position of which reflects climatic conditions. Pollen is the key sensor used to reconstruct past tree line patterns. Our aims in this study were to investigate pollen–vegetation relationships at the boreal tree line and to assess the success of a modified version of the biomization method that incorporates pollen productivity and dispersal in distinguishing the tree line. Location Northern Canada (307 sites) and Alaska (316 sites). Methods The REVEALS method for estimating regional vegetation composition from pollen data was simplified to provide correction factors to account for differential production and dispersal of pollen among taxa. The REVEALS‐based correction factors were used to adapt the biomization method and applied as a set of experiments to pollen data from lake sediments and moss polsters from the boreal tree line. Proportions of forest and tundra predicted from modern pollen samples along two longitudinal transects were compared with those derived from a vegetation map by: (1) a tally of ‘correct’ versus ‘incorrect’ assignments using vegetation in the relevant map pixels, and (2) a comparison of the shape and position of north–south forest‐cover curves generated from all transect pixels and from pollen data. Possible causes of bias in the misclassifications were assessed. Results Correcting for pollen productivity alone gave fewest misclassifications and the closest estimate of the modern mapped tree line position (Canada, + 300 km; Alaska, + 10 km). In Canada success rates were c. 40–70% and all experiments over‐predicted forest cover. Most corrections improved results over uncorrected biomization; using only lakes improved success rates to c. 80%. In Alaska success rates were 70–80% and classification errors were more evenly distributed; there was little improvement over uncorrected biomization. Main conclusions Corrected biomization should improve broad‐scale reconstructions of spatial patterns in forest/non‐forest vegetation mosaics and across climate‐sensitive ecotones. The Canadian example shows this is particularly the case in regions affected by taxa with extremely high pollen productivity (such as Pinus). Improved representation of actual vegetation distribution is most likely if pollen data from lake sediments are used because the REVEALS algorithm is based on the pollen dynamics of lake‐based systems.     

Reliability of pollen ratios for environmental reconstructions on the Tibetan Plateau

Aim Pollen ratios are widely used to gain palaeovegetation and palaeoclimatic information from fossil pollen spectra, although their applicability has seldom been tested with modern pollen data. I used a data set of 113 lake‐surface sediments from the eastern Tibetan Plateau to test the reliability of several pollen ratios. Location The lake‐surface pollen spectra cover a wide range of vegetation types (temperate desert, temperate steppe, alpine desert, alpine steppe, high‐alpine meadow, sub‐alpine shrub, coniferous and mixed forest) and climatic conditions (mean July temperature, T_July: 4.0–17.4°C; mean annual precipitation, P_ann: 104–670 mm). Methods Lake‐surface sediments were analysed palynologically, and several pollen ratios were calculated. These ratios were interpreted with respect to vegetation and climatic conditions. Results The arboreal pollen sum (AP) was highest in samples from forested areas and was significantly correlated with P_ann (r² = 0.44). In non‐forested areas, samples from large lakes and from lakes surrounded by sparse vegetation had increased AP values, suggesting that AP is a useful vegetation density indicator. Artemisia/Chenopodiaceae (A/C) ratios were lowest in desert areas and were positively correlated with P_ann (r² = 0.25). The aridity pollen index was inappropriate for inferring (palaeo‐)climatic information from samples on the eastern Tibetan Plateau as it had no significant correlation with the environmental factors. Artemisia/Cyperaceae (A/Cy) ratios had a significant correlation with T_July (r² = 0.23), but only a weak correlation with P_ann, which indicates that the A/Cy ratio is applicable as a temperature indicator. Furthermore, it is a valuable tool for the differentiation of high‐alpine meadow from steppe vegetation. Main conclusions AP sum, A/C ratio and A/Cy ratio are useful tools for qualitative and semi‐quantitative palaeoenvironmental reconstruction on the Tibetan Plateau; however, the results obtained should not be interpreted quantitatively.     

Centennial‐scale changes to the aquatic vegetation structure of a shallow eutrophic lake and implications for restoration

1. We investigate long‐term (>200 years) changes to the composition and spatial structure of macrophyte communities in a shallow, eutrophic lake (Barton Broad, eastern England) and consider the implications for lake restoration. 2. Historical macrophyte data were assembled from a variety of sources: existing plant databases, museum herbaria, journal articles, old photographs and eyewitness accounts. Additionally, two types of sediment core sample were analysed for plant macro‐remains and pollen; bulk basal samples from multiple core sites analysed to provide information on ‘pre‐disturbance’ macrophyte communities and two whole cores analysed to determine historical change. 3. Prior to the late 1800s, macrophyte communities were diverse and included a multilayered mosaic of short‐stature submerged taxa and taller submerged and floating‐leaved species. With the progression of eutrophication after around 1900, the former community was displaced by the latter. Diversity was maintained, however, since an encroaching Schoenoplectus–nymphaeid swamp generated extensive patches of low‐energy habitat affording refugia for several macrophytes otherwise unable to withstand the hydraulic forces associated with open water conditions. When this swamp vegetation disappeared in the 1950s, many of the ‘dependent’ aquatic macrophytes also declined leaving behind a sparse, species‐poor community (as today) resilient to both eutrophication and turbulent open waters. 4. The combination of historical and palaeolimnological data sources offers considerable benefits for reconstructing past changes to the aquatic vegetation of lakes and for setting restoration goals. In this respect, our study suggests that successful restoration might often be better judged by reinstatement of the characteristic structure of plant communities than the fine detail of species lists; when nutrients are low and the structure is right, the right species will follow.