Early hope that the estimation of pollen accumulation-rates (PAR) from lake sediments would permit the quantitative reconstruction of past vegetational composition abated when results from several experiments uncovered the problems of the method. A review of monitoring experiments, mainly conducted between 1960 and 1980, shows that basin size, transport of pollen with flowing water and sediment redeposition and focusing are the most important factors influencing pollen deposition in lakes. With careful site selection most of these factors can be rendered unimportant or constant. It is shown that PAR estimates from three small lakes in a rather homogeneous forest region are very similar for the last 9000 years. Results from monitoring of pollen deposition in pollen traps provides important information in the interpretation of PAR. Monitoring experiments were conducted to explore the deposition of pollen on and in small lakes. Pollen deposition in pollen traps floating on small lakes was not different from pollen deposition observed on land. PAR estimates from lake sediments were comparable with the results of pollen monitoring obtained with Tauber-type traps. A proportion of airborne pollen deposited on a lake surface is lost during the transport to the lake sediment. The amount of pollen that is lost differs between pollen types.
Understanding the mechanisms of climate that produce novel ecosystems is of joint interest to conservation biologists and palaeoecologists. Here, we define and differentiate transient from accumulated novelty and evaluate four climatic mechanisms proposed to cause species to reshuffle into novel assemblages: high climatic novelty, high spatial rates of change (displacement), high variance among displacement rates for individual climate variables, and divergence among displacement vector bearings. We use climate simulations to quantify climate novelty, displacement and divergence across Europe and eastern North America from the last glacial maximum to the present, and fossil pollen records to quantify vegetation novelty. Transient climate novelty is consistently the strongest predictor of transient vegetation novelty, while displacement rates (mean and variance) are equally important in Europe. However, transient vegetation novelty is lower in Europe and its relationship to climatic predictors is the opposite of expectation. For both continents, accumulated novelty is greater than transient novelty, and climate novelty is the strongest predictor of accumulated ecological novelty. These results suggest that controls on novel ecosystems vary with timescale and among continents, and that the twenty-first century emergence of novelty will be driven by both rapid rates of climate change and the emergence of novel climate states. This article is part of a discussion meeting issue ‘The past is a foreign country: how much can the fossil record actually inform conservation?’
Natural disturbance processes such as volcanic eruptions, fire and human activities are important vegetation drivers in north Patagonia. Here, we tested the impact of volcanic ash fall and fire on vegetation composition analysing two sediment records, Lake Avutarda and Lake Bruja, located in the forest-steppe transition at 40°S. In addition, our analysis provides the first account on the history of Nothofagus alpina at its eastern distribution limits. Our results comprise the last 3000 years, indicating the persistence of the vegetation despite evident volcanic activity documented by numerous tephra layers in both records. Eleven fire episodes were identified, while redundancy analysis indicates a non-significant influence of fire activity on the vegetation. The population increase of Nothofagus alpina represents the most important change in vegetation composition in the last three millennia. We speculate that the presumed change in climate, which led to the expansion of Austrocedrus chilensis south of the study area, also caused the increase of Nothofagus alpina populations in the region.
Abstract Aim Understanding the driving forces and mechanisms of changes in past plant distribution and abundance will help assess the biological consequences of future climate change scenarios. The aim of this paper is to investigate whether modelled patterns of climate parameters 6000 years ago can account for the European distribution of Fagus sylvatica at that time. Consideration is also given to the role of non‐climatic parameters as driving forces of the Holocene spread and population expansion of F. sylvatica . Location Europe. Methods European distributions were simulated using a physiologically‐based bioclimatic model (STASH) driven by three different atmospheric general circulation model (AGCM) outputs for 6000 years ago. Results The three simulations generally showed F. sylvatica to have potentially been as widespread 6000 years ago as it is today, which gives a profound mismatch with pollen‐based reconstructions of the F. sylvatica distribution at that time. The results indicate that drier conditions during the growing season 6000 years ago could have caused a restriction of the range in the south. Poorer growth conditions with consequently reduced competitive ability were modelled for large parts of France. Main conclusions Consideration of the entire European range of F. sylvatica showed that no single driving force could account for the observed distributional limits 6000 years ago, or the pattern of spread during the Holocene. Climatic factors, particularly drought during the growing season, are the likely major determinants of the potential range. Climatic factors are regionally moderated by competition, disturbance effects and the intrinsically slow rate of population increase of F. sylvatica . Dynamic vegetation modelling is needed to account for potentially important competitive interactions and their relationship with changing climate. We identify uncertainties in the climate and pollen data, as well as the bioclimatic model, which suggest that the current study does not identify whether or not climate determined the distribution of F. sylvatica 6000 years ago. Pollen data are better suited for comparison with relative abundance gradients rather than absolute distributional limits. These uncertainties from a study of the past, where we have information about plant distribution and abundance, argue for extreme caution in making forecasts for the future using equilibrium models.
A stated aim of Dynamic European Climate-Vegetation Impacts and Interactions (DECVEG), an ESF EuroCLIMATE project, is to use and improve both dynamic vegetation models and palaeovegetation data in order to study climate-vegetation interactions in Europe. To this end, the generalised vegetation model LPJ-GUESS is used to simulate the historical tree species composition at the location of three Scandinavian pollen sites along a north-south transect, two in Sweden and one in Finland. The sites are among the few in Europe that appear to have experienced minimal human impact during the Holocene. The model is run from 10,000 years ago to present and forced with climate output from an atmospheric general circulation model. Output is compared to high-resolution paleoecological data gathered at the three sites.
Pollen productivity estimates (PPEs) are indispensable prerequisites for quantitative vegetation reconstructions. Estimates from different European regions show a large variability and it is uncertain whether this reflects regional differences in climate and soil or is brought about by different assessments of vegetation abundance. Forests represent a particular problem as they consist of several layers of vegetation and many tree species only start producing pollen after they have attained ages of several decades. Here we used detailed forest inventory data from north-eastern Germany to investigate the effect of flowering age and understory trees on PPEs. Pollen counts were obtained from 49 small to medium sized lakes chosen to represent the different forest types in the region. Surface samples from lakes within a closed forest of Fagus yielded disproportionate amounts of Fagus pollen, increasing its PPE and the variability of all other estimates. These samples were removed from further analysis but indicate a high trunk-space component that is not considered in the Prentice–Sugita pollen dispersal and deposition model. Results of the restricted dataset show important differences in PPEs based on the consideration of flowering age and understory position. The effect is largest for slow growing and/or late flowering trees like Fagus and Carpinus while it is minimal for species that flower early in their development like Betula and Alnus. The large relevant source area of pollen (RSAP) of 7 km obtained in this study is consistent with the landscape structure of the region.
Uber die Subsistenzwirtschaft der Bevolkerung des 3. und 2. Jahrtausends v. Chr. (dem Ende der Jungsteinzeit und dem Beginn der Bronzezeit) liegen bislang nur wenige bioarchaologische Daten vor. Die augenscheinliche Dominanz von Grabern und Grabfunden und das fast vollstandige Fehlen von Siedlungsnachweisen fur diese Zeit fuhrt seit mehr als 70 Jahren zu Diskussionen und verschiedenen Erklarungsmodellen. Eine dieser Hypothesen geht davon aus, dass die Menschen der Schnurkeramik- und Glockenbecherzeit Viehhirten waren und als ein reisendes Volk von Bogenschutzen angesehen werden mussen. Dem stellen wir hier Resultate aus gut datierten palaookologischen Untersuchungen entgegen, die klar beweisen, dass in Mitteleuropa (zwischen der Schweiz und Danemark, sowie zwischen Rhein und Oder) im 3. und 2. Jahrtausend v. Chr. ein voll funktionierendes Landwirtschaftssystem existiert hat. Dies bedeutet auch, dass die Siedlungen permanent bewohnt waren und die Bewohner auf den lokalen Anbau von Kulturpflanzen (z.B. Getreide und Hulsenfruchte) und auf Viehhaltung spezialisiert waren. Mit Hilfe der Pollenanalysen aus dem Grosen Treppelsee in Brandenburg (Deutschland) kann das Vorhandensein von mehreren Landwirtschaftsphasen und einer ersten Form von Grunlandwirtschaft zwischen 3200 und 800 v. Chr. gezeigt werden. Diese palaookologischen Resultate ermoglichen ein besseres Verstandnis dieser Zeitepoche, die zudem durch Neuerungen in der Tischkultur (u.a. das Auftreten von neuen Geschirrformen) und durch die Dungung von Landwirtschaftsflachen charakterisiert wird. Somit ist das augenscheinliche oder effektive Fehlen von archaologisch nachgewiesenenSiedlungen entweder durch schlechte Erhaltungsbedingungen und Forschungsstrategien begrundet, oder aber auf neue Vorlieben in der Auswahl von Siedlungslagen, Bauweise und/oder Abfallentsorgung zuruckzufuhren.
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