Anneli Poska

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

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 on 636 pollen records from lakes and bogs to reconstruct the past cover of 25 plant taxa grouped into 10 plant-functional types and three land-cover types [evergreen trees, summer-green (deciduous) trees, and open land]. The model corrects for some of the biases in pollen percentages by using pollen productivity estimates and fall speeds of pollen, and by applying simple but robust models of pollen dispersal and deposition. The emerging patterns of tree migration and deforestation between 6k bp and modern time in the REVEALS estimates agree with our general understanding of the vegetation history of Europe based on pollen percentages. However, the degree of anthropogenic deforestation (i.e. cover of cultivated and grazing land) at 3k, 0.5k, and 0.2k bp is significantly higher than deduced from pollen percentages. This is also the case at 6k in some parts of Europe, in particular Britain and Ireland. Furthermore, the relationship between summer-green and evergreen trees, and between individual tree taxa, differs significantly when expressed as pollen percentages or as REVEALS estimates of tree cover. For instance, when Pinus is dominant over Picea as pollen percentages, Picea is dominant over Pinus as REVEALS estimates. These differences play a major role in the reconstruction of European landscapes and for the study of land cover-climate interactions, biodiversity and human resources.

Plant macrofossil evidence for an early onset of the Holocene summer thermal maximum in northernmost Europe

Holocene summer temperature reconstructions from northern Europe based on sedimentary pollen records suggest an onset of peak summer warmth around 9,000 years ago. However, pollen-based temperature reconstructions are largely driven by changes in the proportions of tree taxa, and thus the early-Holocene warming signal may be delayed due to the geographical disequilibrium between climate and tree populations. Here we show that quantitative summer-temperature estimates in northern Europe based on macrofossils of aquatic plants are in many cases ca. 2 °C warmer in the early Holocene (11,700–7,500 years ago) than reconstructions based on pollen data. When the lag in potential tree establishment becomes imperceptible in the mid-Holocene (7,500 years ago), the reconstructed temperatures converge at all study sites. We demonstrate that aquatic plant macrofossil records can provide additional and informative insights into early-Holocene temperature evolution in northernmost Europe and suggest further validation of early post-glacial climate development based on multi-proxy data syntheses.

Holocene vegetation and land-use history in the environs of Lake Kahala, northern Estonia

Pollen profiles, based cores taken in Lake Kahala and from the adjoining mire, were used to establish general vegetation history and to reconstruct the extent and types of land-use over most of the Holocene. Modern pollen deposition was studied using moss polsters and the results were used in the interpretation of the fossil pollen data in terms of former land-use practices. The modern-day samples are from settlements, hay meadow and pasture, and overgrown pasture. Indications of human activity can be traced back to the Stone Age. At ca. 6400 cal. B.C., the first indications of possible woodland utilisation by humans are recorded. This may have involved grazing within the forests. From 4200 cal. B.C. onwards, animal husbandry with changing intensity was practised. Arable farming, involving cereals, was introduced to the area at ca. 1800 cal. B.C., but it was only at ca. 500 cal. B.C. that it assumed an important role in the farming economy.Secale cereale (rye) was introduced during the Roman Iron Age, intensive rye cultivation started at the end of the Iron Age, at ca. cal. A.D. 800. Ever increasing farming pressures triggered the formation of openalvars. Open landscape similar to that of today has persisted, with minor forest regeneration phases, since at least 500 cal. B.C.

Pollen size in Carex: The effect of different chemical treatments and mounting media

Pollen size and pollen aperture size for ten species of the genus Carex L., native to Estonia, have been measured using light microscopy. The species selected represent different sections of the genus, a range of habitats and different chromosome numbers. The effects of two basic chemical treatments, two mounting media and the effect of chemically induced dehydration with tertiary butyl alcohol (TBA) on the size of pollen grains were then recorded. In general pollen size and pollen aperture size of the species examined is highly variable at both intraspecific and interspecific levels. Carex hirta has notably larger pollen grains than any of the other species investigated and, although correlations between size and chromosome number in the species examined are limited, it also has the highest chromosome number. Statistically significant size differences resulted from variations in chemical treatment, mounting media and tertiary butyl alcohol (TBA) induced dehydration. Acetolysed pollen grains are larger than potassium hydroxide (KOH) treated pollen grains. Pollen grains dehydrated after chemical treatment with TBA are larger than pollen grains not dehydrated. Pollen grains mounted in silicon oil are smaller than grains mounted in glycerine. But considering the great size variation of Carex pollen grains, the size changes caused by preparation procedures fall within the size variation range of the species examined. All the samples contained a high number of deformed pollen grains and pollen grains with hardly distinguishable or no lateral apertures.

Late-Holocene expansion of a south Swedish peatland and its impact on marginal ecosystems: Evidence from dendrochronology, peat stratigraphy and palaeobotanical data

In this study, a reconstruction of the long-term development and lateral expansion of a south Swedish peat bog was performed using a multi-proxy approach, including dendrochronology, peat stratigraphy and macrofossil and pollen analyses. By combining mapping of cross-dated subfossil trees with radiocarbon-dated peat sequences, an improved approach to reconstruction of lateral peat expansion was applied. Apart from providing approximate ages of tree burial episodes, the ring-width records offer information on hydrological variations prior to the bog expansion. New bog oak, pine and alder chronologies are presented and their potential as a dating tool for peatland expansion as well as for local to regional environmental interpretations is examined. Our tree-replication records show that increased amounts of bog trees in the central parts can be linked to drier bog-surface conditions, whereas an increase in wood remains in the marginal zone is related to enhanced preservation due to lateral bog expansion. Our reconstructions of the development of the peat deposit and associated changes in the distribution of vegetation communities provide new insight into peatland responses to climate change at the end of the ‘Holocene Thermal Maximum’ (5000–4000 cal. yr BP).

The Physical and Social Effects of the Kaali Meteorite Impact — a Review

There is a concern that the world we know today will end in a global ecological disaster and mass extinction of species caused by a meteorite impact (Chapman and Morrison 1994; Chapman 2004). We are aware that rare large impacts have changed the face of our planet as reflected by extinctions at the Permian/Triassic (∼251 Ma; Becker et al. 2001), Triassic/Jurassic (∼200 Ma; Olsen et al. 2002) and Cretaceous/Tertiary (∼65 Ma; Alvarez et al. 1980) boundaries. Today astronomers can detect and predict the orbits of the asteroids/comets that can cause similar impacts. Yet, Tunguska, Meteor Crater-size and smaller meteorites that could cause local disasters are unforeseeable. However, while planning to avoid the next bombardment by cosmic bodies we can look at past interactions of human societies, environment and meteorite impacts to understand to what extent human cultures were influenced by meteorite impacts. The question is whether the past examples are relevant in the modern situation, but they are certainly useful. The Kaali crater field in Estonia, in that respect, is an excellent case study area for past human-meteorite interactions. Moreover, Kaali is not the only Holocene crater field in this region: in fact, during the last 10 000 years Estonia has been targeted at least by four crater forming impacts and there are five registered meteorite falls (Fig. 15.1). The two large craters, Neugrund and Kardla, originate from 535 and 455 Ma, respectively (Suuroja and Suuroja 2000). cr]

Surface Samples and Trapping

Abstract: Reference pollen data for use in interpreting fossil pollen assemblages may be either collected as surface samples or monitored by means of pollen traps. Surface samples can be obtained from moss polsters or lake-surface sediment, or exceptionally soil, leaf litter, or snow. The advantage of such samples is that a large number can be collected relatively quickly. Within the resulting pollen assemblage, however, the presence of each taxon has to be expressed in percentage terms. Reference material obtained from pollen traps offers more possibilities because pollen accumulation rates (PARs, grains cmâ 2 yearâ 1) can be calculated, and the record of each taxon can be considered independently. This allows comparisons over distance and between vegetation regions. The collection of such data using traps requires several years because the annual variation in pollen production, which is partly determined by climate, is great, and it is only the long-term average PAR that reflects vegetation composition. The number and location of samples and the amount of accompanying vegetation data should be appropriate for the research question to which they will be applied because there is no single standard that is suitable for the whole range of possible uses.

Modelling Spatial Compositional Data : Reconstructions of past land cover and uncertainties

In this paper, we construct a hierarchical model for spatial compositional data, which is used to reconstruct past land-cover compositions (in terms of coniferous forest, broadleaved forest, and unforested/open land) for five time periods during the past

Drivers of dissolved organic carbon export in a subarctic catchment : Importance of microbial decomposition, sorption-desorption, peatland and lateral flow

6\,000

Causes of Regional Change—Land Cover

years over Europe. The model consists of a Gaussian Markov Random Field (GMRF) with Dirichlet observations. A block updated Markov chain Monte Carlo (MCMC), including an adaptive Metropolis adjusted Langevin step, is used to estimate model parameters. The sparse precision matrix in the GMRF provides computational advantages leading to a fast MCMC algorithm. Reconstructions are obtained by combining pollen-based estimates of vegetation cover at a limited number of locations with scenarios of past deforestation and output from a dynamic vegetation model. To evaluate uncertainties in the predictions a novel way of constructing joint confidence regions for the entire composition at each prediction location is proposed. The hierarchical models ability to reconstruct past land cover is evaluated through cross validation for all time periods, and by comparing reconstructions for the recent past to a present day European forest map. The evaluation results are promising and the model is able to capture known structures in past land-cover compositions.