Thomas Giesecke

Holocene dynamics of the southern boreal forest in Sweden

Holocene vegetation dynamics in the ecotone between the boreo-nemoral and boreal forest were reconstructed from sediment cores of two small lakes using pollen and macrofossil analyses. Competition, migration and changing climate parameters determine species dynamics in the ecotone. The spread of Picea abies was the most important event in the vegetation history of the region, probably shaping the present distribution limits of Corylus avellana, Tilia cordata and Quercus robur through competitive exclusion. The expansion of Picea abies is well documented by abundant macrofossil finds at one of the lakes, mirroring the rise in pollen percentage and confirming the presence of the tree before Picea abies pollen frequencies reached 1%. No Picea abies macrofossils were encountered before pollen was regularly found. Changing vegetation composition through migration affects both sites at different times, while a shift in atmospheric circulation pattern may be responsible for a concordant change in vegetation composition at both sites around 5700 cal. BP.

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.

Integrated varve and pollen-based temperature reconstruction from Finland: evidence for Holocene seasonal temperature patterns at high latitudes

A detailed understanding of decadal to millennial-scale climate changes requires seasonal-scale (summer-winter) reconstructions of past precipitation and temperature fluctuations. Comparing seasonally resolved varve records with pollen-based sum of growing degree-days (GDD) reconstructions from Lake Nautajärvi, we examined the intra-annual nature of climate variability in central southern Finland during the Holocene. The organic varve record and the GDD reconstruction show roughly comparable trends supporting the interpretation that both proxies predominantly reflect summer temperatures in the study area. The records suggest low but rising early-Holocene (9500 to 8500 cal. yr BP) summer temperatures. The Holocene Thermal Maximum (HTM) in the GDD record dates to about 7500 to 4500 cal. yr BP, but the organic varve record along with reconstructed changes in vegetation composition, notably a peak of Tilia pollen percentages, indicate that during the HTM there was a trend towards a more continental climate with maximum mid-summer temperatures reached at 6500 to 4500 cal. yr BP. Both records reflect the start of the post-HTM cooling at about 4500 cal. yr BP, simultaneously with an increase of the amount of catchment erosion and mineral matter influx into the lake, suggesting gradually colder and/or longer winters with high net accumulation of snow. The organic varve record and the GDD record start to diverge at 2000 cal. yr BP, possibly owing to the human influence on catchment processes. The reconstructed mid-Holocene summer temperature peak deviates from the regional climate model outputs, which suggest highest summer temperatures during the early Holocene.

The European Modern Pollen Database (EMPD) project

Modern pollen samples provide an invaluable research tool for helping to interpret the quaternary fossil pollen record, allowing investigation of the relationship between pollen as the proxy and the environmental parameters such as vegetation, land-use, and climate that the pollen proxy represents. The European Modern Pollen Database (EMPD) is a new initiative within the European Pollen Database (EPD) to establish a publicly accessible repository of modern (surface sample) pollen data. This new database will complement the EPD, which at present holds only fossil sedimentary pollen data. The EMPD is freely available online to the scientific community and currently has information on almost 5,000 pollen samples from throughout the Euro-Siberian and Mediterranean regions, contributed by over 40 individuals and research groups. Here we describe how the EMPD was constructed, the various tables and their fields, problems and errors, quality controls, and continuing efforts to improve the available data.

Calibrated pollen accumulation rates as a basis for quantitative tree biomass reconstructions

Recent investigations show that the pollen accumulation rate (PAR) of the common tree taxa is directly related to the biomass and, by inference, to the population size of the taxa around the study site. Fossil PAR records preserved in lakes provide therefore a potential proxy for quantitative biomass and population reconstructions. We use the high-resolution PAR records obtained from two accurately dated lake sediment cores in Finland to generate quantitative Holocene biomass records for Pinus, Picea and Betula, the most common tree taxa of the European Boreal forest. PAR values were calibrated to biomass values by comparing the modern PAR values with the modern biomass values and assuming a linear relationship between the past PAR and biomass values. The obtained PAR and biomass values and trends are remarkably coherent between the two records. Pinus has a stable Holocene biomass size and its modern biomass, about 20 t/ha, corresponds with the natural Pinus biomass in the study regions. In contrast, Picea immigrated from the East during the mid Holocene, had a maximum biomass, 50—60 t/ha, at 3500—1000 cal. yr BP, and declined strongly during the last 1500—1000 years as a result of increased human activity and related rise of fire frequency. Thus, the modern Picea biomass in the study regions, about 22 t/ha, is only 35—40% of the natural Picea biomass. The results of this pilot study demonstrate the potential of the calibrated PAR data in quantitative biomass and population reconstructions. Such reconstructions can provide fresh insights into the structure of past plant communities and, when combined with records reflecting palaeoclimates, natural disturbances, and human activity, can help to disentangle the long-term importance of different enviromental drivers to changes in plants populations and ecosystems.

Comment on “Glacial Survival of Boreal Trees in Northern Scandinavia”

Parducci et al. (Reports, 2 March 2012, p. 1083) fail to present convincing evidence for glacial survival of Pinus and Picea in northern Scandinavia. Their methodology does not exclude contamination. Additionally, they should consider the lack of suitable habitats, the apparent extinction of both taxa after deglacial warming, and alternative hypotheses for the distribution of the Picea genetic marker haplotype A.

Exploring holocene changes in palynological richness in northern Europe--did postglacial immigration matter?

In mid to high latitudes glacial and interglacial cycles have repeatedly changed the area available for plant growth. The speed at which plants are able to colonize areas at the onset of an interglacial is hypothesized to limit their distribution ranges even today (migrational lag). If the spread of plants would have been generally slow then plant diversity in previously glaciated areas would be expected to increase over time. We explore this hypothesis using results from six palynological investigations from two previously glaciated regions: central Sweden and north-eastern Germany. Rarefaction, slope of rank order abundance, and taxa accumulation plots were used to evaluate richness and evenness in pollen data in an attempt to separate richness from evenness. These analyses show little change in palynological richness for the northern sites throughout the Holocene. In contrast, the southern sites show an increase in richness and evenness during the early Holocene; this may be explained by the different initial conditions at the onset of the Holocene. A strong rise in palynological richness around 6000 and 1000 years ago at the southern sites can be attributed to the regional initiation of agriculture and major opening of the forest, respectively. For the northern sites there is no evidence for increased taxonomic diversity through time that could be due to delayed immigration of species.

Dendroecological investigations at Venner Moor (northwest Germany) document climate-driven woodland dynamics and mire development in the period 2450—2050 BC:

Excellently preserved subfossil pine and oak tree remains from the bottom layer of raised bog peat were dendroecologically investigated at Venner Moor (northwest Germany). Tree-ring analyses were combined with observations of stem and root morphology, preservation state, mineral soil relief, peat stratigraphy and pollen analysis to reconstruct in great detail environmental changes leading to the start of the raised bog formation. Hydrology was identified as the main determinant influencing tree growth and population dynamics at Venner Moor, as documented by different growth patterns and dying-off dates in relation to the mineral soil elevation. The woodland phase has been dendrochronological dated to the period from 2421—2077 BC (4371—4027 cal BP). In this period, a general change from more or less open landscape with dominating heath to wet pine forest and eventually to open raised bog occurred at the site. Comparisons with pine population dynamics at the nearby Voerdener Moor and with the independent Lower Saxony Bog Oak Chronology (LSBOC) indicate that the reconstructed ecological changes at Venner Moor are mainly triggered by climate variations, in particular wet shifts on the decadal timescale. This example shows the value of subfossil pine layers from northwest German bogs as a high resolution proxy archive of Holocene humidity fluctuations.

Pollen diversity captures landscape structure and diversity

Summary 1. Past changes in plant and landscape diversity can be evaluated through pollen analysis, however, pollen-based diversity indexes are potentially biased by differential pollen production and deposition. Studies examining the relationship between pollen and landscape diversity are therefore needed. The aim of this study was to evaluate how different pollen-based indexes capture aspects of landscape diversity. 2. Pollen counts were obtained from surface samples of 50 small- to medium-sized lakes in Brandenburg (north-east Germany) and compiled into two sets, with one containing all pollen counts from terrestrial plants and the second restricted to wind-pollinated taxa. Both sets were adjusted for the pollen production/dispersal bias using the REVEALS model. A high-resolution biotope map was used to extract the density of total biotopes and different biotopes per area as parameters describing landscape diversity. In addition, tree species diversity was obtained from forest inventory data. 3. The Shannon index and the number of taxa in a sample of 10 pollen grains are highly correlated and provide a useful measure of pollen type diversity which corresponds best to landscape diversity within one km of the lake and the proportion of non-forested area within seven km. Adjustments of the pollen production/dispersal bias only slightly improve the relationships between pollen diversity and landscape diversity for the restricted data set as well as for the forest inventory data and corresponding pollen types. 4. Using rarefaction analysis, we propose the following convention: pollen type diversity is represented by the number of types in a small sample (low count e.g. 10), pollen type richness is the number of types in a large sample (high count e.g. 500), and pollen sample evenness is characterized by the ratio of the two. 5. Synthesis. Pollen type diversity is a robust index that captures vegetation structure and landscape diversity. It is ideally suited for between site comparisons as it does not require high pollen counts. In concert with pollen type richness and evenness, it helps evaluating the effect of climate change and human land use on vegetation structure on long timescales.

Late-glacial and Holocene European pollen data

ABSTRACT The European Pollen Database (EPD) is a community effort to archive and make available pollen sequences from across the European continent. Pollen sequences provide records that may be used to infer past vegetation and vegetation change. We present here maps based on 828 sites from the EPD giving an overview of changes in postglacial pollen assemblages in Europe over the past 15,000 years. The maps show the distribution and abundance of 54 different pollen taxa at 500 year intervals, supported by new age-depth models and associated chronological uncertainty analysis. Results show the individualistic patterns of spread of different pollen taxa, and provide a standardized dataset for further analysis, defining a spatial context for the study of past plant and vegetation changes and other aspects of environmental history in Europe.