Changes in the relative abundance of Polyarthra vulgaris and P. dolichoptera,following the elimination of fish

In the last decade palynological investigations of the sediments of large lakes in the USSR (Ladoga, Onega, Kubenskoye, Vozhe, Lacha, Beloye) and the Baltic Sea have been carried out. Limnic and marine sediments are notable for their high pollen and spore concentrations and pollen diagrams from these deposits have a regional significance. The diagrams from Lakes Onega and Ladoga belong to the Kola-Karelian type while the sediments of Lacha, Vozhe, Kubenskoye and Beloye belong to the North-Russian type (Neustadt 1957). Three types of Holocene pollen diagrams are registered for the Baltic Sea. Modern and fossil pollen spectra reflect the composition of the zonal vegetation. Late-glacial sediments are found in several profiles of lake and sea deposits. Holocene deposits are divided into eight stratigraphic zones: Late-glacial sediments (DR₂-Al-DR₃) and Post-glacial sediments (PB, BO, AT, SB, SA). The zonations of lake and sea sediments can be clearly correlated.     

Modern pollen representation of source vegetation in the Qaidam Basin and surrounding mountains, north-eastern Tibetan Plateau

We use a data set of 35 surface pollen samples from lake sediments, moss polsters and top soils on the north-eastern Tibetan Plateau to explore the relationship between modern pollen assemblages and contemporary vegetation patterns. The surface pollen transect spanned four vegetation zones––alpine meadow, steppe, steppe desert and desert––under different climatic/elevational conditions. Relative representation (R _rel) values and Principal Components Analysis (PCA) were used to determine the relationships between modern pollen and vegetation and regional climate gradients. The results show that the main vegetation zones along the regional and elevational transects can be distinguished by their modern pollen spectra. Relative to Poaceae, a high representation of Artemisia, Nitraria and Chenopodiaceae was found, while Cyperaceae and Gentiana showed values in the middle range, and Ranunculaceae, Asteraceae, Ephedra and Fabaceae had low relative representation values. PCA results indicate a high correlation between the biogeoclimatic zones and annual precipitation and annual temperature and July temperature. The Artemisia/Chenopodiaceae ratio and the Artemisia/Cyperaceae ratio are useful tools for qualitative and semi-quantitative palaeoenvironmental reconstruction on the north-eastern Tibetan Plateau. Surface lake sediments are found to have different palynomorph spectra from moss cushion and soil samples, reflecting the larger pollen source area in the contemporary vegetation for lakes.     

Spatial variation of modern pollen in Oregon and southern Washington, USA

Surface sediments from 95 lakes provide information on the spatial variation of modern pollen spectra in Oregon and southern Washington. Percentages for 13 pollen types were compared within and between vegetation zones to characterize regional patterns of pollen spectra. The percentage data were also compared with climate variables to determine relationships between pollen percentages and regional climate gradients. The composition of modern pollen spectra corresponds well with the distribution of the pollen producers. Most pollen assemblages were generally dominated by Pinus, but those west of the Cascade Range were dominated by Alnus. Low percentages of Pseudotsuga/Larix, Tsuga mertensiana, Abies, and Picea pollen coincided with local occurrence of the trees. The distributions of the pollen data were arranged along gradients of temperature and effective moisture. West of the Cascade Range, Alnus, Tsuga heterophylla, Pseudotsuga/Larix, and Cupressaceae pollen were abundant and correlate well with moderate temperature and high effective moisture. In the shrub-steppe and woodlands east of the Cascade Range, where effective moisture is low, Artemisia, Cupressaceae, and Pinus pollen were dominant. At high elevations, Pinus, T. mertensiana, Abies, and Picea were common pollen types in areas with short growing seasons and high effective moisture. Pollen percentages collected from lake surface sediments, moss polsters, and soils were compared within a number of vegetation types to assess their similarity. The three types of sample yielded similar results for forested areas, but lake sediment samples from upper- and lower-treeline sites captured a more regional picture of the vegetation.     

Determinants of pollen dispersal in the East Asian steppe at different spatial scales

Two sites from the East Asian steppe, the Mu Us Sandland as a regional case and the Anguli Nuur catchment as a local one, were chosen to detect roles of vegetation, climate, landform, and human disturbance on pollen dispersal. 1) Vegetation: The semi-arid steppe vegetation is characterized by Artemisia and Chenopodiaceae pollen under various vegetation conditions; however, no evident correlation between pollen percentages and corresponding plant species cover is found. 2) Climate: Samples under different Mean Annual Precipitations (MAPs) clearly distinguish themselves in the Mu Us Sandland, implying MAP-determined surface pollen spectra in regional scale. 3) Landform: Surface pollen assemblages in the azonal psammophytic vegetation and lowland meadow, show insignificant variance from the zonal steppe vegetation. The azonal halophilous vegetation, mostly distributed in the lowland or near the lake shore, leads to higher than average percentages of Chenopodiaceae pollen. Signal of exotic Pinus pollen is also strengthened in the lowland and lake sediment. 4) Human disturbance: The role of human disturbance on surface pollen assemblages is weak, as shown in the Mu Us Sandland.

This study also provides theoretical bases for quantitative reconstructions of palaeoclimate and palaeovegetation based on fossil pollen spectra from lake sediments and lowland soils in the semi-arid East Asian steppe. We suggest that calibration against locally dispersed pollen taxa is necessary to reliably reconstruct changes in vegetation pattern through time, for example, a factor of 1.75 for the widely used A/C (Artemisia/Chenopodiaceae) ratio is suggested according to the pollen assemblages in the surface layers of the lake sediment vs. slope soil in Anguli Nuur. However, uncertainties also exist for such calibration considering the dynamics of local-scale azonal vegetation.     

南京紫金山山麓湖泊表层沉积物的孢粉分析

本研究对南京紫金山山麓玄武湖和前湖共17个湖泊表层沉积物样品进行孢粉分析,旨在获得花粉组合新数据,揭示湖泊表层花粉与现生植被的关系,为开展第四纪湖泊地层孢粉学研究提供现代过程的参考.研究结果表明,花粉组合在剔除非自然分布的种植木本植物干扰花粉外,主要以木本花粉的松属(Pinus)和栎属(Quercus)占优势,其他常见...     

Upper Pleistocene and Holocene lakes in the An Nafud,Saudi Arabia

Summary Two major lake periods were discovered in the sand sea of An Nafud and the surrounding areas. In Upper Pleistocene large lakes occurred around the dune area and in the interior of the sand sea. Their deposits were formed between 34 000 and 24 000 BP. The lakes were not conformous to the present dune relief. They had an extension of several km² and a depth of ca. 10 m. Holocene lakes (8 400–5 400 BP) were of minor extension and restricted to the interdune depressions. Lake sediments consist of cemented sand, calcareous crusts and diatomites. Upper Pleistocene lakes were fresh water lakes, the Halocene lakes were mostly swamps depending on rising and falling aquifers in the dunes. Plant remains as pollen and macro rests show that the environmental changes didn't exceed the system of semidesert comparable to the modern plant cover. However, the Upper Pleistocene lake deposits contain some more soudanian elements as the Holocene sediments in the pollen spectra. Climatically the lake formations are interpreted as depending on a stronger influence of the mediterranean cyclones or an interaction of them with monsoonal air masses.     

Delineation of vegetation shaded ox-bow lakes in Ganges flood plain,India

Floating vegetation cover over the ox-bow lake withstands against its sharp delineation. A good many spectral indices are successfully used for water body delineation. But how far these are applicable in vegetation-shaded ox-bow lakes is a research question. The study also aimed that if the existing indices are not satisfactory and how a new index could be endorsed for resolving the problem. The study additionally monitored the ox-bow lake and vegetation cover area from 1991 to 2021 based on Landsat satellite images.Normalized differences water index (NDWI), Modified NDWI (MNDWI), Re-modified NDWI (RmNDWI), and Normalized Difference Vegetation Index (NDVI) spectral indices were used for delineating ox-bow lakes and multiple accuracy test measures revealed that these are not highly satisfactory. Vegetation inclusive aggregated water index (ViAWI) was built by coupling mentioned spectral indices with the vegetation index and the ensemble map was found more accurate.Monitoring the ox-bow lake area clearly showed that these declined in the last 30 years irrespective of the historical drainage modification legacy of the major rivers to which the ox-bow lakes belonged. Aquatic vegetation cover within ox-bow lakes changed dynamically.The endorsed ViAWI would be a good approach for resolving wetland delineation shaded with floating vegetation and it could be used in other regional units worldwide. Quantitative information regarding ox-bow lake and vegetation cover within ox-bow lakes would be valuable data support for adopting ox-bow lake conservation and restoration planning.     

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.     

A comparison of sedimentary DNA and pollen from lake sediments in recording vegetation composition at the Siberian treeline

Reliable information on past and present vegetation is important to project future changes, especially for rapidly transitioning areas such as the boreal treeline. To study past vegetation, pollen analysis is common, while current vegetation is usually assessed by field surveys. Application of detailed sedimentary DNA (sedDNA) records has the potential to enhance our understanding of vegetation changes, but studies systematically investigating the power of this proxy are rare to date. This study compares sedDNA metabarcoding and pollen records from surface sediments of 31 lakes along a north–south gradient of increasing forest cover in northern Siberia (Taymyr peninsula) with data from field surveys in the surroundings of the lakes. sedDNA metabarcoding recorded 114 plant taxa, about half of them to species level, while pollen analyses identified 43 taxa, both exceeding the 31 taxa found by vegetation field surveys. Increasing Larix percentages from north to south were consistently recorded by all three methods and principal component analyses based on percentage data of vegetation surveys and DNA sequences separated tundra from forested sites. Comparisons of the ordinations using procrustes and protest analyses show a significant fit among all compared pairs of records. Despite similarities of sedDNA and pollen records, certain idiosyncrasies, such as high percentages of Alnus and Betula in all pollen and high percentages of Salix in all sedDNA spectra, are observable. Our results from the tundra to single‐tree tundra transition zone show that sedDNA analyses perform better than pollen in recording site‐specific richness (i.e., presence/absence of taxa in the vicinity of the lake) and perform as well as pollen in tracing vegetation composition.     

Quantifying Regional Vegetation Cover Variability in North China during the Holocene: Implications for Climate Feedback

Validating model simulations of vegetation-climate feedback needs information not only on changes in past vegetation types as reconstructed by palynologists, but also on other proxies such as vegetation cover. We present here a quantitative regional vegetation cover reconstruction for North China during the Holocene. The reconstruction was based on 15 high-quality lake sediment profiles selected from 55 published sites in North China, along with their modern remote sensing vegetation index. We used the surface soil pollen percentage to build three pollen-vegetation cover transfer models, and used lake surface sediment pollen data to validate their accuracy. Our results showed that vegetation cover in North China increased slightly before its maximum at 6.5 cal ka BP and has since declined significantly. The vegetation decline since 6.5 cal ka BP has likely induced a regional albedo change and aerosol increase. Further comparison with paleoclimate and paleovegetation dynamics in South China reproduced the regional cooling effect of vegetation cover decline in North China modelled in previous work. Our discussion demonstrates that, instead of reconstructing vegetation type from a single site, reconstructing quantitative regional vegetation cover could offer a broader understanding of regional vegetation-climate feedback.     

立地尺度植被重建模型MARCO POLO的应用分析——以柴达木盆地为例

立地尺度植被定量重建是对利用孢粉进行古气候和古环境研究的一个重要补充。一些新的定量重建模型相继提出,但其适用性仍需进一步验证。以柴达木盆地24个样方表土孢粉及4个湖泊表层沉积孢粉为研究对象,利用MARCO POLO模型重建立地尺度植被盖度并分析该模型的适用性。研究结果表明,由于地形开阔且风力较大,干旱区的大型湖泊沉积物中往往容易富集超代表性孢粉,使这些孢粉的区域信号增强,从而降低孢粉类型筛选的正确率。若湖泊与样方间的距离较近,湖泊表层孢粉能够更好地反映样方所在区域的孢粉特征,则有利于提高孢粉类型筛选的正确率。MARCO POLO模型中的孢粉代表性R值是将孢粉含量转化为植被盖度的关键参数。由于受到孢粉自身生产、传播、保存以及外部环境等多方面因素的影响,不同样方的R值可能差异很大。因此在运行该模型前需保证R值能够准确反映立地尺度中各孢粉类型与植被盖度的关系。仅在以柴达木盆地为代表的干旱区对该模型进行检验,今后仍需要在不同植被或气候区进行深入研究以全面了解该模型的适用条件。     

The vegetation of Nyumba ya Mungu reservoir, Tanzania

A study was made of the aquatic macrophyte ecology of Nyumba ya Mungu; a nine year old man-made lake in northern Tanzania. The vegetation is described with the aid of percentage cover histograms and standing crop estimates. Concurrent physical, chemical and biological studies helped in ecological interpretations. Distinct vegetation communities were found in the deltaic swamps of the two major inflow rivers and it is concluded that this was due to water chemical differences. The water of the more alkaline River Kikuletwa appeared to influence the greater part of the perimeter swamps as the species composition largely coincided with that reported for higher pH, higher conductivity African lakes. Typha domingensis swamp was predominant and extensive, whilst Cyperus alopecuroides and Paspalidium geminatum were also common emergent species. Floating and submerged plants were relatively unimportant in the lake with respect to cover. The major zonations of species appeared to be a function of water depth, while chemistry seemed the main factor controlling vegetation pattern. In conclusion the significance of interrelationships between the macrophytes and other aquatic organisms is discussed.     

A taphonomic study of modern pollen assemblages from dung and surface sediments in arid environments of Spain

Coastal plant communities in arid southeastern Spain are characterized by insect-pollinated scrub species, which fail to occur in Quaternary pollen sequences from valleys, marshlands and marine cores. We investigate pollen–vegetation relationships in samples from soil surfaces, animal dung, and sediments in depressions or basins that, in theory, should have pollen spectra that are comparable to those from sedimentary basins elsewhere. Pollen spectra from basins or depressions are very susceptible to long-distance wind and water transport. This can mask representation of pollen from the surrounding insect-pollinated vegetation, as can over-representation of basin-margin halophilous and hydrophilous pollen. Pollen spectra from biogenic materials of animal origin are the best analogues of local and regional vegetation, and show the best analytical potential in terms of pollen concentration and taxon diversity. Pollination properties of the species studied indicate they will rarely be found in most conventional pollen records. It cannot be stressed too strongly that insight into Quaternary vegetation of arid regions demands complementary pollen analysis of coprolites, urine-cemented deposits, and cave sediments with preserved biotic remains, in addition to water-lain sediments.     

Modelling pollen sedimentation in Danish lakes at c.ad 1800: an attempt to validate the POLLSCAPE model

Aims To validate the POLLSCAPE simulation model of pollen dispersal and deposition, and evaluate the effect of factors such as pollen productivity, wind speed and regional plant abundance, using a data set of ad 1800 pollen assemblages and historical land cover data. Location Denmark. Methods ad 1800 land cover from historical maps is digitized for 2000 m radii around 30 Danish lakes (3.5–33 ha). The simulation model POLLSCAPE is used to predict sedimentary pollen assemblages in the lakes from the plant abundance data inferred from these maps, with different model parameter settings for wind speed, pollen productivity, regional pollen loading, etc. The model predictions are compared with observed ad 1800 pollen assemblages from the lake sediment records. Furthermore, pollen productivity is estimated from the ad 1800 pollen and vegetation data using the Extended R‐value model. Results Generally the model reproduces the patterns in the observed pollen assemblages, and for most pollen types there are significant correlations between observed and predicted pollen proportions. The pollen proportions predicted by the POLLSCAPE model are sensitive to the pollen productivity estimates used, the regional background pollen loading and average wind speed. There is a difference in background pollen loading between eastern and western Denmark, especially of Calluna pollen. The fit between predicted and observed pollen assemblages is best at wind speeds around 2.5 m s^?1, and decreases rapidly at lower wind speeds. The pollen productivity estimates from the ad 1800 data set are comparable with estimates from moss polsters in modern analogues of traditional cultural landscapes in Sweden and Norway. Main conclusions The POLLSCAPE model reproduces the patterns in the observed pollen assemblages from the lakes well, considering the uncertainty in the historical plant abundance data. This study indicates that the simulation model can be a useful tool for investigating relationships between vegetation and pollen composition, but also that the simulated pollen proportions are sensitive to the pollen productivity estimates, the regional background and to wind speed.     

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.     

Taxon-related pollen source areas for lake basins in the southern Alps: an empirical approach

The pollen/vegetation relationship in broadleaved forests dominated by Castanea sativa was analysed using an empirical approach. The pollen content of surface sediments of three lake basins of different sizes (6.3, 22.2, and 101.2 ha) in Ticino (southern Switzerland) was used for a comparison with the surrounding vegetation. We surveyed the vegetation around the two small lakes, Lago di Origlio and Lago di Muzzano, and estimated the relative crown coverage of tree species. The regional vegetation outside the lake catchment (ca. >1 km) was determined with the data from the first Swiss National Forest Inventory. For the third large lake, basin of Ponte Tresa, we used only this latter approach for comparison with pollen data. We compare uncorrected and corrected pollen percentages with vegetational data that were processed with distance-weighting functions. To assess the degree of correspondence between pollen and vegetation data we define a ratio pollen/vegetation, which allows a comparison at the taxon level. The best fit between total pollen load and vegetation is reached for a distance from the lake shore of ca. 300 m for Lago di Origlio (150×350 m in size) and of ca. 600 m for Lago di Muzzano (300×750 m in size). Beside these general patterns, our analysis reveals taxon-specific pollen dispersal patterns that are in agreement with results from previous studies in northern Europe. Ratios of species with local (proximal) and long-distance (distal) pollen dispersal provide evidence that pollen dispersal mechanisms can influence the size of the taxon-related pollen source area, from small (100–400 m) to large (>5 km) for the same lake. The proportion of distal species increases with increasing lake size, highlighting the predominance of atmospheric pollen transport. We conclude that the large species-related differences in pollen source areas have to be taken into account when the provenance at a site is estimated and discussed.     

Chemical composition of modern and pre-acidification sediments in the Tatra Mountain lakes

Concentrations of major nutrients (C, N, P) and acid soluble metals (Ca, Mg, K, Al, Fe, Mn, Pb, and Zn) were determined in modern (0–1 cm) and pre-acidification (5–10 cm) sediment layers collected from 37 alpine and 3 forest lakes in the Tatra Mountains (Slovakia, Poland) in 1996–1998. Sediment composition reflected catchment characteristics and productivity of lakes. In the sediments of alpine lakes, C and N concentrations decreased and Mg increased with a decreasing proportion of vegetation and soil in the catchment. Decreasing Ca:Mg ratios in sediments along the vegetation gradient was inverse to that in water, and could be associated with different ratios of cations in water leachate from catchments and in solids which enter the lake due to soil erosion. Phosphorus concentrations increased with the proportion of moraine areas, with till soils rich in P. Concentrations of C, N, P, and Ca in sediments positively correlated to their concentrations in water. Sediment concentrations of Al and Al:Ca ratios increased with decreasing sediment and water pH. A negative correlation between water pH and concentrations of organic C in water and sediments indicated the important impact of organic acids on the acid status of the lakes exposed to higher terrestrial export of organic matter. Compared to the pre-acidification period, the modern sediments had significantly higher Fe, Mn, Zn, Pb, and K, but lower Mg concentrations. The Zn and Pb enrichment was more evident in oligotrophic alpine lakes than in more productive forest lakes and was independent of lake water or sediment pH. Fe and Mn concentrations in the modern sediments were higher than in ambient soils and bedrock, while those in pre-acidification sediments were similar to contemporary soils and to the rock layer. The enrichment of the modern sediments with Fe and Mn thus probably resulted from both their redox recycling and ecosystem acidification.     

Representation of tundra vegetation by pollen in lake sediments of northern Alaska

Aim To understand better the representation of arctic tundra vegetation by pollen data, we analysed pollen assemblages and pollen accumulation rates (PARs) in the surface sediments of lakes. Location Modern sediment samples were collected from seventy‐eight lakes located in the Arctic Foothills and Arctic Coastal Plain regions of northern Alaska. Methods For seventy of the lakes, we analysed pollen and spores in the upper 2 cm of the sediment and calculated the relative abundance of each taxon (pollen percentages). For eleven of the lakes, we used ²¹⁰Pb analysis to determine sediment accumulation rates, and analysed pollen in the upper 10–15 cm of the sediment to estimate modern PARs. Using a detailed land‐cover map of northern Alaska, we assigned each study site to one of five tundra types: moist dwarf‐shrub tussock‐graminoid tundra (DST), moist graminoid prostrate‐shrub tundra (PST) (coastal and inland types), low‐shrub tundra (LST) and wet graminoid tundra (WGT). Results Mapped pollen percentages and multivariate comparison of the pollen data using discriminant analysis show that pollen assemblages vary along the main north–south vegetational and climatic gradients. On the Arctic Coastal Plain where climate is cold and dry, graminoid‐dominated PST and WGT sites were characterized by high percentages of Cyperaceae and Poaceae pollen. In the Arctic Foothills where climate is warmer and wetter, shrub‐dominated DST, PST and LST were characterized by high percentages of Alnus and Betula pollen. Small‐scale variations in tundra vegetation related to edaphic variability are also represented by the pollen data. Discriminant analysis demonstrated that DST sites could be distinguished from foothills PST sites based on their higher percentages of Ericales and Rubus chamaemorus pollen, and coastal PST sites could be distinguished from WGT sites based on their higher percentages of Artemisia. PARs appear to reflect variations in overall vegetation cover, although the small number of samples limits our understanding of these patterns. For coastal sites, PARs were higher for PST than WGT, whereas in the Arctic Foothills, PARs were highest in LST, intermediate in DST, and lowest in PST. Main conclusion Modern pollen data from northern Alaska reflect patterns of tundra vegetation related to both regional‐scale climatic gradients and landscape‐scale edaphic heterogeneity.     

Pollen deposition in a small closed drainage basin lake

A moss cushion and fifteen lake bottom samples from a small volcanic crater lake in southeastern South Australia were examined for pollen. The distribution of pollen types is analysed in relation to the distance from source areas of Eucalyptus woodland and Pinus radiata plantations, and in relation to water depth in the lake.Most taxa were only detectable in small quantities within a few metres of the source plants. The decline in these local pollen types led to a relative increase in regional pollen types in the centre of the lake. This is a problem which makes interpretation of the extent of the redeposition factor difficult for some taxa. The redeposition factor was small for extralocal taxa including Pinus radiata and Eucalyptus spp. as their relative frequencies declined noticeably toward the centre of the lake. Regional types included Casuarina stricta, Chenopodiaceae, Polygonaceae and Poaceae pollen. Aquatic pollen was derived within the lake and does not contribute to the regional pollen rain. The majority of these taxa have highest values near the source plants but were also deposited elsewhere in the lake. The exceptions to this were Triglochin and Typha angustifolin pollen; the former showed a distribution pattern which had little relation to the occurrence of the plant while the latter had highest relative frequencies away from the source plants. The implications in relation to water depth interpretation from fossil pollen profiles are discussed.     

On the interpretation of fossil Poaceae pollen in the lowland humid neotropics

Poaceae pollen is abundant in fossil records and is often used as a paleoclimatic indicator. A common interpretation is to link increases in Poaceae pollen abundance to increased regional aridity. However, the representation of Poaceae pollen is influenced by a number of factors, such as the proportion of other plants in the flora that are anemophilous, the size of local marshes, and the influence of humans on the landscape. Abundant anemophilous trees are likely to mask the contribution of Poaceae pollen, whereas floras that are primarily entomophilous are likely to produce a pollen spectrum containing an over-representation of Poaceae. As most fossil pollen data are drawn from flooded settings, it is critically important that palynologists recognize the Poaceae pollen contribution derived from floating grasses and marshes that surround their coring site. Interpretations that ignore changes in effective lake size and assume that Poaceae percentage is a simple indicator of regional vegetation change are likely to overstate ‘dry’ episodes and transitions from wet forest to scrub environments. Human occupation of a site is sometimes manifested in increased Poaceae pollen abundance in lake sediments. The duration, intensity and land-use associated with occupation are all variables that influence Poaceae pollen representation. Very high (50–90%) abundances of Poaceae pollen provide a strong indicator of savanna habitats, but trying to determine transitional vegetation types between savanna and wet forest is best determined using other taxa. Indeed, reliance on Poaceae abundance as an indicator of paleoprecipitation is potentially very misleading when it is in the fossil record at moderate abundances.