From early pollen trapping experiments to the Pollen Monitoring Programme

Thomas Giesecke 0 1 2 3 4 Sonia L. Fontana 0 1 2 3 4 Willem O. van der Knaap 0 1 2 3 4 Heather S. Pardoe 0 1 2 3 4 Irena A. Pidek 0 1 2 3 4 0 W. O. van der Knaap Institute of Plant Sciences and Oeschger Centre for Climate Change Research, University of Bern , Altenbergrain 21, 3013 Bern, Switzerland 1 T. Giesecke (&) S. L. Fontana Albrecht-von-Haller-Institute for Plant Sciences, Department of Palynology and Climate Dynamics, University of Gottingen , Untere Karspule 2, 37073 Gottingen, Germany 2 Communicated by F. Bittmann 3 I. A. Pidek Institute of Earth Sciences, University of Maria Curie-Sklodowska , al . Krasnicka 2 c/d, 20-718 Lublin, Poland 4 H. S. Pardoe Department of Biodiversity and Systematic Biology, National Museum Wales , Cathays Park, Cardiff CF10 3NP, UK Pollen monitoring has become a standard investigation method for researchers in several disciplines; among them are Quaternary palynologists, who conduct experiments in order to gain insights that will help to interpret the content of pollen in sediments. A review of the literature shows how these experiments diversified during the 1920s and 1930s with an array of different research questions, ranging from pollination biology to hay fever studies. Quaternary palynologists gained renewed interest with the possibility of radiocarbon dating late Quaternary sediments and obtaining accumulation rates. Also, the comprehensive model of pollen deposition and the pollen budget studies by H. Tauber encouraged researchers to conduct similar experiments using the same type of pollen trap, which became the main trapping device for Quaternary palynologists. The high precipitation in the tropics inspired the development of alternative designs. The equipment used to assess the pollen content in the air has evolved from simple gravity devices to different types of apparatus using a vacuum pump or revolving rods that collect the pollen on impact. Silicone impregnated filters exposed perpendicularly to the wind can also yield a volumetric assessment and have proven useful in areas with a low content of pollen in the air. The literature review is followed by a brief account of the developments which established the basis for the formation of a group of scientists monitoring the pollen deposition at a network of sites using standard pollen traps, the Pollen Monitoring Programme (PMP). Over the last 15 years the network has collected a large dataset, which is now available to answer a number of research questions. A summary of selected regions and environments, for which pollen monitoring results are available, is provided to serve as a complement to the investigations mentioned above and to provide an overview that may stimulate new research. - Investigations that aim to assist in the interpretation of fossil pollen diagrams are of major importance, as they provide the tools to reconstruct past environments. In this respect, much information has been gained from the collection of surface samples of lake sediments, mosses and soil. Studies monitoring the pollen content of air and deposition of airborne pollen have also given enormous insights. Early results of diverse trapping experiments and surface sample studies are summarized by Erdtman (1943). Since the 1950s, the related research objectives of determining the quantity of pollen in the air, the amount being deposited on the ground, and the proportions of pollen types on the ground have developed independently, but have often inspired each other. The prospect of using modern pollen deposition rates as analogues for past situations (Welten 1944; Davis et al. 1973; Hicks 1994) spurred the formation of the Pollen Monitoring Programme (PMP), which is a network of researchers who have agreed to monitor the pollen deposition using standardised traps. The aim of this contribution is to provide an account of the history of pollen trapping experiments, highlighting a selection of devices that have been and are being used to trap pollen. This review is by no means comprehensive, but intends to provide an overview of past developments. A further intention is to introduce a selection of natural environments where pollen monitoring experiments are conducted as part of the PMP and which are included in several of the contributions in this volume. The beginnings of pollen trapping It is difficult to set a starting date for investigations on the quantity of pollen released by a plant, present in a volume of air, or deposited at a distance from the parent plant. Early aerobiologists like Miquel (1883), who reported on the amount of pollen in the air among other things, probably provided the first information. However, if the lecture by von Post at the 16th convention of Scandinavian naturalists in 1916 is taken as the birth hour of the quantitative study of pollen in late Quaternary deposits, then we must look to Hesselman as the pioneer of pollen trapping experiments. Following von Posts (1918) presentation, Hesselman raised the question of how to separate locally produced pollen from long distance transported pollen (Hesselman 1916; Davis 2000). While von Post pointed out the good match between the occurrence of a pollen type in surface samples and the regional presence of the parent tree, Hesselman (1919a) conducted a pollen trapping experiment designed to quantify long distance pollen deposition. Petri dishes were exposed for consecutive 24-h periods from mid May until the end of June 1918, on two light ships. The Petri dishes contained filter paper soaked in glycerine and were sheltered from the rain. The two light ships were situated in the Baltic Sea 30 and 55 km from the nearest shore and Hesselman reported a total of 16.2 and 8.82 pollen grains per mm2 over the period from mid May to the end of June. He argued that it is important to take account of this long distance component when reconstructing the spread of species. Moreover, he suggested that the expression of palynological results as absolute counts per sample, as was commonly done before von Post described the advantage of percentages, may be better suited to adjust for the long distance component. The idea was to devise threshold values for pollen accumulation rates (PAR) of different taxa that would indicate their local presence. Such threshold values were finally devised by Hicks (2001) for the distribution limits of Betula, Pinus and Picea in northern Finland and were applied to Holocene pollen diagrams by Seppa and Hicks (2006). In 1919, under Hesselmans supervision, Malmstrom (1923) conducted similar pollen trapping experiments in a large mire complex in northern Sweden. Malmstrom placed his pollen trapping Petri dishes in different vegetation types and observed that most Pinus pollen was trapped in the pine woodland, although, due to the large amount of Betula pollen, this trap had the lowest percentage of Pinus pollen. On the other hand, absolute catches of Picea pollen were similar for all traps, while (...truncated)