Comparisons of modern pollen deposition with vegetation abundance and landscape pattern in NE-Germany

Abstract

Since the introduction of pollen analysis for vegetation reconstruction, scientists have been concerned about the influence of different pollen productivities of individual taxa. A second long standing question focuses on the special extent represented by the pollen content of a sample. In addressing these two questions, surface samples were collected from lake sediments in Brandenburg (NE-Germany) and compared with vegetation data from a forest inventory database and a biotope mapping. In addition to pollen percentages, pollen accumulation rates (PAR) obtained from 210Pb dated short sediment cores were used in the comparison. Important results of this thesis are relative and absolute pollen productivity estimates (PPE and aPPE). PPEs describe the pollen production of the different taxa relative to a reference taxon, and aPPEs estimate the production per unit of vegetation, in this case per standing volume. The results demonstrate for Pinus and Fagus similar PPE-values and show that the obtained PPEs for Brandenburg are in general similar to values that were calculated in earlier studies for different regions. However, the obtained aPPE values are much higher compared to a study from northern Finland, where the value for Pinus is ten times lower than the one from Brandenburg. These differences might be explained by a higher net primary production of the same biomass in Central Europe. In addition, the effects of flowering age and forest structure on PPE calculations were evaluated. Results show the strongest influence for slow growing and/or late flowering trees like Fagus and Carpinus and a minimal effect for species that start flowering early in their development as Betula and Alnus do. In order to estimate the relative and absolute PPEs, it is necessary to determine a particular area on the site for which the vegetation can be compared to the pollen. For the relative PPE this is termed the “relevant source area of pollen” (RSAP). The RSAP is defined as the area where pollen percentage data and vegetation data fit to each other best and the fit does not improve when considering the vegetation over a larger area. The obtained RSAP of seven kilometres is larger than in similar studies. This can be explained by the landscape structure, which is in Brandenburg characterised by large vegetation patches that are determined by the different soil substrates left by the last glaciation. The results of the thesis demonstrate for the first time that PAR of the major tree taxa are linearly related to the biomass surrounding the lake, as is generally assumed. In contrary to the RSAP the source area that explains most of the variation of PAR ranges between a few hundred metres up to 2.5 kilometres from the lake basin. This area varies between the taxa, due to differences in pollen dispersal and individual abundance patterns. The finding that a large amount of pollen in small lakes originates from the nearby vegetation confirms earlier considerations which defined this as the “trunk space” component. Currently used pollen dispersal models largely ignore this component and by empirically demonstrating its importance, this study contributes to refining these models. The collected datasets were further used to compare different measures of pollen diversity to aspects of landscape diversity. While nearby vegetation determines mainly the pollen diversity, “palynological richness” is influenced by a larger area of at least seven kilometres. The evenness of pollen grains might be described by the ratio between pollen diversity and “palynological richness”, where a higher number would suggest a higher evenness of distribution of the taxa and vice versa. The results of this thesis demonstrate that the relevant area for a pollen sample depends on the question that is asked of the data. The wider area around the site is desrcibed by the pollen percentage composition and pollen type richness, while PARs and pollen-type diversity reflect the nearby vegetation abundance and diversity.