Search results for: J. B. Birks

Items from 1 to 19 out of 19 results

article

Kohonen Artificial Neural Networks and the IndVal Index as Supplementary Tools for the Quantitative Analysis of Palaeoecological Data

M. Płóciennik, A. Kruk, D. J. Michczyńska, J. B. Birks

Geochronometria > 2015 > Vol. 42, no.1 > 189--201

We applied two widely-used methods for data partitioning — constrained incremental sum-of-squares (CONISS) and Optimal Partitioning (OP) along with two supplementary methods, a Kohonen artificial neural network (self-organising map, SOM) and the indicator value (IndVal) index, for the quantitative analysis of subfossil chironomid assemblages from a palaeolake in Central Poland. The samples, taken...

article

Pollen‐based quantitative reconstructions of Holocene regional vegetation cover (plant‐functional types and land‐cover types) in Europe suitable for climate modelling

A.‐K. Trondman, M.‐J. Gaillard, F. Mazier, S. Sugita, more

Global Change Biology > 21 > 2 > 676 - 697

We present quantitative reconstructions of regional vegetation cover in north‐western Europe, western Europe north of the Alps, and eastern Europe for five time windows in the Holocene [around 6k, 3k, 0.5k, 0.2k, and 0.05k calendar years before present (bp)] at a 1° × 1° spatial scale with the objective of producing vegetation descriptions suitable for climate modelling. The REVEALS model was applied...

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Quantification of UV‐B flux through time using UV‐B‐absorbing compounds contained in fossil Pinus sporopollenin

K. J. Willis, A. Feurdean, H. J. B. Birks, A. E. Bjune, more

New Phytologist > 192 > 2 > 553 - 560

•UV‐B radiation currently represents c. 1.5% of incoming solar radiation. However, significant changes are known to have occurred in the amount of incoming radiation both on recent and on geological timescales. Until now it has not been possible to reconstruct a detailed measure of UV‐B radiation beyond c. 150 yr ago. •Here, we studied the suitability of fossil Pinus spp. pollen to record variations...

article

Orchid species richness along Himalayan elevational gradients

Kamal P. Acharya, Ole R. Vetaas, H. J. B. Birks

Journal of Biogeography > 38 > 9 > 1821 - 1833

Aim This study explores the relationship between orchid species richness along elevational gradients in the central and eastern Himalaya. Specifically, we aimed to test predictions of maximum richness at different elevational levels for orchid species geographically restricted to central and eastern Himalaya (endemics), and total orchid richness, including different life forms. We then compared and...

article

Effect of uneven sampling along an environmental gradient on transfer-function performance

R. J. Telford, H. J. B. Birks

Journal of Paleolimnology > 2011 > 46 > 1 > 99-106

We investigate the effect that uneven sampling of the environmental gradient has on transfer-function performance using simulated community data. We find that cross-validated estimates of the root mean squared error of prediction can be strongly biased if the observations are very unevenly distributed along the environmental gradient. This biased occurs because species optima are more precisely known...

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A modern pollen–climate calibration set based on lake sediments from the Tibetan Plateau and its application to a Late Quaternary pollen record from the Qilian Mountains

Ulrike Herzschuh, H. J. B. Birks, Steffen Mischke, Chengjun Zhang, more

Journal of Biogeography > 37 > 4 > 752 - 766

Aim Fossil pollen spectra from lake sediments on the Tibetan Plateau have been used for qualitative climate reconstruction, but no modern pollen–climate calibration set based on lake sediments is available to infer past climate quantitatively. This study aims to develop such a dataset and apply it to fossil data. Location The Tibetan Plateau, between 30 and 40° N and 87 and 103° E. Methods We collected...

article

Quantitative summer-temperature reconstructions for the last 2000 years based on pollen-stratigraphical data from northern Fennoscandia

A. E. Bjune, H. Seppä, H. J. B. Birks

Journal of Paleolimnology > 2009 > 41 > 1 > 43-56

Quantitative reconstructions of mean July temperatures (T _jul) based on new and previously published pollen-stratigraphical data covering the last 2000 years from 11 lakes in northern Fennoscandia and the Kola Peninsula are presented. T _jul values are based on a previously published pollen-climate transfer function for the region with a root-mean-square error of prediction (RMSEP) of 0.99°C. The most...

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Estimating the amount of compositional change in late-Quaternary pollen-stratigraphical data

H. J. B. Birks

Vegetation History and Archaeobotany > 2007 > 16 > 2-3 > 197-202

Detrended canonical correspondence analysis is used to estimate the amount of palynological change or compositional turnover in ten Holocene pollen-stratigraphical sequences from Setesdal, southern Norway. The results, when the analyses are standardised for the same time interval, show that the highest amounts of change occurred at sites in the south of Setesdal where there is a richer tree flora...

article

Are cladoceran fossils in lake sediment samples a biased reflection of the communities from which they are derived?

G. R. Kattel, R. W. Battarbee, A. Mackay, H. J. B. Birks

Journal of Paleolimnology > 2007 > 38 > 2 > 157-181

A study on the taphonomy of Cladocera was carried out in a small (9 ha), oligotrophic mountain loch, Loch Coire Fionnaraich (LCFR) in northwest Scotland. Four approaches were used. First, the fossil assemblage of Cladocera in the core-top sample taken from the deepest basin (14 m) of the loch were compared with the fossil assemblages of Cladocera in surface sediments along eight depth transects with...

article

Winifred Tutin (1915–2007)

H. J. B. Birks, Hilary H. Birks

Journal of Paleolimnology > 2007 > 38 > 4 > 601-605

article

Aquatic Biota and the Detection of Climate Change: Are there Consistent Aquatic Ecotones?

E. Heegaard, A. F. Lotter, H. J. B. Birks

Journal of Paleolimnology > 2006 > 35 > 3 > 507-518

In this study, we analyse the cumulative rate of compositional change along an altitudinal gradient in the Swiss Alps for three different groups of aquatic organisms – Cladocera, chironomids, and diatoms. In particular, we are interested in the magnitude of unusually large changes in species composition that allows the detection of critical ecotones for each of these three organism groups. The estimated...

article

Palaeolimnological evidence for recent climatic change in lakes from the northern Urals, arctic Russia

Nadia Solovieva, Vivienne J. Jones, Larisa Nazarova, Stephen J. Brooks, more

Journal of Paleolimnology > 2005 > 33 > 4 > 463-482

The recent sediments from two deep arctic lakes, Mitrofanovskoe and Vanuk-ty, situated in the permafrost belt within the Bol’shezemel’skaya Tundra in the northern Ural region, were studied for diatoms, chironomids, spheroidal carbonaceous particles and stable lead isotopes. The magnitudes and rates-of-change in diatom and chironomid assemblages were numerically estimated. Instrumental climate records...

article

The identification of wingless Betula fruits in Weichselian sediments in the Gross Todtshorn borehole (Lower Saxony, Germany) the occurrence of Betula humilis Schrank.

Holger Freund, Hilary H. Birks, H. J. B. Birks

Vegetation History and Archaeobotany > 2001 > 10 > 2 > 107-115

Abstract.Betula fruits lacking their wings were recovered from the Gross Todtshorn borehole, Germany, covering the late Eemian interglacial, the Early Weichselian, and the Weichselian pleniglacial. In order to identify them, a data-set of seven size and shape variables was made from modern fruits of B. pubescens, B. pendula, B. nana, and B. humilis. These measurements were summarised by principal...

article

Terrestrial pollen record of recent land-use changes around nine North African lakes in the CASSARINA Project

Sylvia M. Peglar, Hilary H. Birks, H. J. B. Birks, A.A. Fathi, more

Aquatic Ecology > 2001 > 35 > 3-4 > 431-448

Pollen analyses and related plant macrofossil records are presented from short cores from nine North African lakes in the EU-funded CASSARINA project. Terrestrial pollen reflects human impact on the vegetation and landscape over the last 150–100 years. Pollen changes, aided by radiometric dating, could be correlated with historical developments. The chronology of the landscape changes date other biostratigraphical...

article

Surface-sediment and epilithic diatom pH calibration sets for remote European mountain lakes (AL:PE Project) and their comparison with the Surface Waters Acidification Programme (SWAP) calibration set

N. G. Cameron, H. J. B. Birks, V. J. Jones, F. Berges, more

Journal of Paleolimnology > 1999 > 22 > 3 > 291-317

A modern diatom-pH calibration data-set consisting of surface-sediment diatom assemblages from 118 lakes and 530 taxa is presented. The AL:PE data-set is from high-altitude or high-latitude lakes in the Alps, Norway, Svalbard, Kola Peninsula, UK, Slovenia, Slovakia, Poland, Portugal, and Spain (pH range = 4.5-8.0; DOC range = 0.2-3.2 mg l^-1). In addition, 92 epilithon samples from 22 high-altitude...

article

Predicting Heavy Metal Concentrations in the Surface Sediments of Norwegian Headwater Lakes from Atmospheric Deposition: An Application of a Simple Sediment-Water Partitioning Model

J. F. Boyle, H. J. B. Birks

Water, Air, & Soil Pollution > 1999 > 114 > 1-2 > 27-51

Data from 96 headwater lakes from Norway are used to model heavy metal concentrations in surficial lake sediments in relation to atmospheric deposition. The study evaluates the application of sediment-water partitioning models at the field scale and finds optimum values for the partition coefficients. The impact of environment (sediment type, lake water pH, etc.) on K_Dvalues is explored directly by...

article

The Influence of Excimer Formation on Solvent-Solute Energy Transfer in Organic Liquid Scintillators

J. B. Birks, J. M. de C. Conte, G. Walker

IEEE Transactions on Nuclear Science > 1966 > 13 > 3 > 148 - 152

Excimer (excited dimer) formation occurs in all the aromatic hydrocarbon solvents used in liquid scintillators. The influence of this process on solvent-solute energy transfer has been studied for a model system: 9, 10-diphenyl anthracene (DPA) in 1, 6-dimethyl naphthalene (DMN), diluted by n-heptane (Hp). Data on the fluorescence lifetimes, spectra and quantum yields and the viscosity of DMN in Hp...

article

The Scintillation Process in Alkali Halide Crystals

J. B. Birks

IEEE Transactions on Nuclear Science > 1964 > 11 > 3 > 4 - 11

The scintillation process in impurity-activated alkali halide crystals is considered by analogy to that in binary solid organic solutions. A general relation is proposed for the differential scintillation efficiency dL/dE as the product of the efficiencies of the primary and secondary scintillation processes. The primary efficiency is influenced by electron-hole recombination and ionization quenching...

article

The Scintillation Process in Organic Systems

J. B. Birks

IRE Transactions on Nuclear Science > 1960 > 7 > 2 > 2 - 11

The scintillation process in organi: crystals and solutions is described, and a mechanism is proposed to account for the origin of the fast and slow scintillation components. The intermolecular energy migration and transfer processes in pure and mixed crystals and in plastic and liquid solutions are discussed quantitatively, and the influence of temperature and thickness on the scintillation and fluorescence...

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