Relevant Source Area of Pollen in patchy cultural landscapes and signals of anthropogenic landscape disturbance in the pollen record: A simulation approach

Using the HUMPOL simulation computer model we explored the effects of various factors characteristic of mosaic cultural landscapes on the Relevant Source Area of Pollen (RSAP sensu Sugita) of small lakes (50 m radius), and the representation of NAP and human-impact pollen indicators in small takes. The following aspects were tested: 1) effect of variations in the spatial distribution of taxa/patches and species diversity on RSAP, 2) effect of changes in the proportion of landscape openness and size of openings on the RSAP and the pollen signal, and 3) the value of low-abundance anthropogenic indicator species to detect openness in the landscape. Moreover, we examined a methodological aspect of the MOSAIC computer program, i.e. the variability in RSAP due to inherent variations between replicates of random landscape simulation scripts using the program MOSAIC. We found differences in the RSAP estimates between runs using replicates of the same landscape created in the program MOSAIC. Such differences need to be taken into account when interpreting results involving the use of MOSAIC replicates. The simulations suggest that, if the model assumptions are valid, the RSAP is sensitive to vegetation structure in terms of patch size, and taxa/patch diversity and spatial distribution, whatever the properties of the taxa modelled. The longer the distance from each sampling site to get a sufficient cover of all taxa within the landscape, the larger the RSAP. If the spatial distribution of patches is very uneven and/or some taxa/patches are characterised by low frequencies in the randomized landscape, this distance will become longer. Further analysis of the results and new modelling experiments are necessary to provide better means for testing the presented hypothesis on the effect of spatial vegetation/landscape evenness on RSAP. The simulations indicate that, in hypothetical mosaic tree/herb landscapes including common NW European taxa of traditional cultural landscapes, RSAP estimates are relatively robust, i.e. will seldom be smaller than 1000 m and larger than 2500 m in radius in the case of 50 m radius lakes. NAP percentage is not a reliable measure of landscape openness. A NAP percentage value may represent a large range of openness percentage cover depending on the size and spatial distribution of the openness patches. Common pollen indicators of human impact and landscape openness will be represented in a pollen assemblage from small lakes when the landscape is relatively open (>= 30%) and a minimum of 300-1000 grains is counted. Findings of pollen indicators of human impact may not be as reliable indicators of land use within the RSAP as has sometimes been assumed, especially when human activity is small scale. Moreover, it is important to realize that a small number of grains may indicate relatively large overall openness cover in the landscape, especially if the open areas consist of few large clearings rather than many small clearings. (c) 2008 Elsevier B.V. All rights reserved.