Marie-José Gaillard

Landscape openness and pollen records: a simulation approach

Quantitative reconstruction of the area cleared of forest in the past is essential to assess the possible indirect anthropogenic impacts on the past environment of Europe, including past climate. We apply a simul ation model of pollen dispersal and deposition (1) to re-examine the relationship between pollen and landscape openness, often uncritically inferred from non-arboreal pollen (NAP) percentages alone, and (2) to predict the relevant source area of pollen, the smallest spatial scale of vegetation that can be reconstructed from pollen records. The simulations use landscapes simplified from the modern open agricultural and semi-open forested regions in southern Sweden where traditional cultural landscapes still remain. The model is appropriate, because the simulated pollen assemblages resemble the pollen assemblages observed in each of the two landscape types, and because the simulated relationships between NAP percentages and percentage cover of open land within 1000 m agree with the empirical relationships. The simulated relevant source area of pollen is the area within 800–1000 m from both small hollows and 3-ha ponds. NAP percentages give only a rough first approximation of the percentage cover of open land. More comprehensive methods will be required to obtain quantitative estimates of open land from fossil pollen.

Holocene biomass burning and global dynamics of the carbon cycle

Fire regimes have changed during the Holocene due to changes in climate, vegetation, and in human practices. Here, we hypothesise that changes in fire regime may have affected the global CO2 concentration in the atmosphere through the Holocene. Our data are based on quantitative reconstructions of biomass burning deduced from stratified charcoal records from Europe, and South-, Central- and North America, and Oceania to test the fire-carbon release hypothesis. In Europe the significant increase of fire activity is dated approximately 6000 cal. yr ago. In north-eastern North America burning activity was greatest before 7500 years ago, very low between 7500-3000 years, and has been increasing since 3000 years ago. In tropical America, the pattern is more complex and apparently latitudinally zonal. Maximum burning occurred in the southern Amazon basin and in Central America during the middle Holocene, and during the last 2000 years in the northern Amazon basin. In Oceania, biomass burning has decreased since a maximum 5000 years ago. Biomass burning has broadly increased in the Northern and Southern hemispheres throughout the second half of the Holocene associated with changes in climate and human practices. Global fire indices parallel the increase of atmospheric CO2 concentration recorded in Antarctic ice cores. Future issues on carbon dynamics relatively to biomass burning are discussed to improve the quantitative reconstructions.

Pollen productivity estimates for the reconstruction of past vegetation cover in the cultural landscape of southern Sweden

Pollen productivity estimates (PPE) are one of the critical parameters for a quantitative recon struction of past vegetation from fossil pollen records. Modern pollen and vegetation data were collected in traditional landscapes of southern Sweden to derive PPE for the most characteristic plant taxa. The 42 selected sites are assumed to be good analogues of historical to prehistorical grasslands. A sampling method of vegetation, designed to produce plant abundance data in different distance classes, allowed the use of distance-weighted plant abundance around the surface pollen sites. PPE for 11 herb taxa and Juniperus communis were estimated by extended R-value (ERV) models, using the distance-weighted plant abundance and surface pollen data. Results using three ERV submodels, data sets from open and semi-open landscapes, and two distance-weighting methods are generally consistent. The herb taxa analysed have higher PPE than Poaceae except Cyperaceae and Compositae (sub-family) Cichorioideae. Calluna vulgaris, Rumex acetosa type, Juniperus communis and Plantago lanceolata have the highest PPE. Most of the common tree taxa in the region produce 6–8 times as much pollen per unit area as Poaceae. The present set of PPE covers most of the common herb taxa of northwest Europe and will be useful for simu lating pollen dispersal and deposition in heterogeneous landscapes of open and forested vegetation, which will help in future research project design and in the interpretation of fossil data.

The plant-macrofossil record of past lake-level changes

Plant-macrofossil analysis is one of the most useful biostratigraphical methods for the reconstruction of former lake-level changes. The distribution of submerged, floating-leaved and emergent lake-shore vegetation is mainly dependant on water depth, but water chemistry and nutrient status must also be taken into account when interpreting water-level changes. Lake-level studies should be based on the investigation of several littoral cores along a transect perpendicular to the lake-shore. Multiple cores are essential for separating genuine lake-level changes from other processes influencing the plant-macrofossil record. Physical analyses of sediment stratigraphy provide important additional information to the plant-fossil record, because natural infilling processes and erosion from the catchment must be distinguished from climatic events causing a change in the water level. Here we review several important concepts, including suitability of lakes for lake-level study, the degree of detail required in the analysis, and macrofossil records of lake-level changes, and illustrate those concepts by examples from southern Sweden and Minnesota. We discuss how to reconcile alternative hypotheses for the stratigraphic changes seen in the macrofossil assemblages.

Modern pollen/land-use relationships as an aid in the reconstruction of past land-uses and cultural landscapes: an example from south Sweden

An on-going, integrated project concerning modern pollen/vegetation/land-use relationships in south Sweden is outlined. Some preliminary results and their potential uses in palaeoecological reconstructions are discussed. Moss polsters were collected from 92 examples of vegetation/land-use areas of south Sweden (e.g. nonfertilized grazed areas, burned and grazed heaths, traditionally managed fodder-producing meadows, etc.). A total of 23 land-use (e.g. management type) and environment variables (e.g. soil chemistry) is available for 84 sites. The 84 sample data-set was analysed by canonical correspondence analysis and associated statistical testing using Monte Carlo permutation tests to explore and test patterns of modern pollen variation in relation to the environment, and by weighted averaging regression and calibration to derive local-scale environmental and land-use reconstructions from fossil pollen assemblages preserved in a soil profile.

Modern pollen/vegetation relationships from small lakes in ancient cultural landscapes of south Sweden- a first step towards quantification of landscape openness in the past

This study aims to analyse how vegetation, and in particular the degree of openness of the landscape, is reflected in pollen assemblages from surface sediment in lakes. Modern analogues of ancient cultural landscapes in southern Sweden were selected. Surface sediments from 22 small lakes (0.5–20 ha) located mainly in the forest region of southern Sweden were collected and analysed for pollen in order to enlarge and complement an earlier data set of 13 lakes collected in the open, agricultural region of southernmost Sweden. The composition of the landscape surrounding the lakes was mapped within 1000-m and 500-m radii around the lakes using Colour InfraRed (CIR) aerial photographs. The pollen and landscape data were analysed using numerical ordination techniques. The results show that, despite the large variation of landscape openness, the variation in non-arboreal pollen (NAP) is low between the sites which was not the case for the 13 lakes of the previous study. It is hypothesised that this may be due to differences in the major characteristics of the two regions in which the sites were selected, i.e. mainly treeless and intensively farmed in the previous study and mainly forested in the present investigation. The difference in background pollen appears to play a decisive role on the relative representation of NAP. This implies that the background pollen should be estimated before NAP percentages can be used for quantitative reconstruction of past landscape openness. In the 22 lakes studied, Gramineae, Cerealia (excludingSecale),Filipendula andSalix are positively correlated to cultivated land within both radii, and with open land (tree cover not exceeding 20%) within the 1000-m radius.Quercus andFagus have some positive correlation with deciduous orest within 1000-m radius. We conclude that the landscape units cultivated land, open land and deciduous forest within 1000-m radius are reasonably well reflected in the pollen assemblages and could be predicted within this area.

Estimating the spatial scale of pollen dispersal in the cultural landscape of southern Sweden

The primary aim of the study was to estimate the spatial scale of pollen dispersal and deposition for pollen assemblages from moss polsters in the cultivated landscape of southern Sweden, as a mean to improve future studies of the pollen/vegetation relationship in the region, and interpretation of fossil pollen data in terms of past cultural landscapes. This can be done by estimating the ‘relevant source area of pollen’ (RSAP) defined as the area around the pollen sampling point beyond which the pollen-vegetation relationship does not improve. Forty-two sites from nonfertilized grasslands in the traditional open agricultural (Open Region) and semi-open forested (Semi-Open Region) regions of southern Sweden were selected. The vegetation survey was performed within a 1500 m radius area around the moss polsters sampling area. The extended R-value (ERV) model was used to evaluate the pollen-plant abundance relationship. The RSAP for moss polsters in the Open Region was estimated to c. 400 m from empirical data. In the Semi-Open Region, however, the likelihood function score, an indicator of the goodness-of-fit of the data to the ERV model, showed an unexpected pattern of change, making it difficult to evaluate the RSAP. Simulations using hypothetical landscapes suggest that systematic selection of sampling sites could cause this pattern. Simulations also demonstrate that the size of vegetation patches affect the RSAP, i.e., the larger the vegetation patches are, the larger the RSAP becomes. Similar RSAP for the Open and Semi-Open Regions is obtained in simulations using the same patch size, and random selection. In the actual vegetation, patch size is comparable in the two regions, which would suggest that the RSAP for moss polsters in the Semi-Open Region is c. 400 m as well.

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.

A late Holocene record of land-use history, soil erosion, lake trophy and lake-level fluctuations at Bjäresjösjön (south Sweden)

Land-use history, soil erosion, lake trophy and lake-level fluctuations during the last 3000 years were reconstructed through a multidisciplinary palaeolimnological study (pollen, plant macrofossils, diatoms, physical and chemical analysis, magnetic measurements and radiometric methods) of a small eutrophic lake in southern Sweden (Bjäresjösjön, Scania). There are striking responses in diatom, chemical, sediment yield and magnetic records to land-use changes documented by pollen analysis or historical sources, and to lake-level changes identified from sedimentary changes. Our multidisciplinary approach assists interpretation of the processes controlling long-term changes and separation of the effects of different factors (land-use changes, lake-level fluctuations) on individual biostratigraphical records. Climate has controlled processes in the lake indirectly, through lake-level fluctuations, from the Late Bronze Age to the Viking Age (700 BC-AD 800). Since the Viking Age, land-use controlled most of the changes observed in the lakes development and soil erosion processes. Major changes in lake development occurred during the last 200 years, due to a drastic increase in soil erosion and water eutrophication during a period of agricultural modernization.

Environment and Climate in Southwestern Switzerland during the Last Termination, 15-10 ka BP

Abstract Various records from sites ranging in altitude between 372 and 2290 m along a NW-SE transect through SW Switzerland were compiled and correlated within well defined chrono- and biostratigraphic units. Deglaciation of lowland areas is estimated at ca. 15,000-14,000 BP (minimum age), while the lower part of the Rhone valley and the Jura Mountains may have become ice free later. A significant climatic warming is clearly indicated by several palaeoecological records at ca. 12,600 BP. The compiled data give no clear evidence of a cooler phase between 12,200-12,000 BP. Oxygen isotope data point to a slight and progressive decrease in the δ 18 O values between ca. 12,500-11,000 BP, which is followed by a marked drop at 11,000 BP. A very distinct change in lithology, rock glacier development and in mollusc and vegetation records characterises many low and high altitude sites ca. 300 years later, at 10,700 BP. Between 11,000 and 10,000 BP oxygen isotope records seem to respond more rapidly to the climatic shifts than the biostratigraphical records. Possible explanations for these time-lags are discussed. Our compilation shows that further multidisciplinary research in key sites would provide more precise palaeoclimatological information, both qualitatively and quantitatively. Higher time resolution and quantitative estimates of climatic parameters are needed to evaluate in detail the mechanisms and consequences of these rapid climatic changes in SW Switzerland.