Fagus trees with a predominantly allospecific neighborhood (Fagus<70 category) did not only reach a higher mean annual increment but were also less sensitive in growth to environmental fluctuation, in particular to the severe drought in 2003, as compared to beech trees in pure beech neighborhood.
Beeches with a few allospecific neighbors (Fagus70-99 category) reduced growth to a smaller extent in the negative pointer year 1976 than beeches in pure stands and they also recovered more rapidly in terms of stem increment after the 1976 drought. Since 1976 frequency of drought periods increases and especially harms beech on climatically and pedologically marginal sites (Eckstein et al. 1984;
Frech 2006). Thus, the presence of allospecific neighbors seems to buffer Fagus trees from environmental hazards which result in growth reductions.
One possible mechanism by which allospecific neighbors may reduce the drought stress sensitivity of a target tree could be a relatively low water consumption of the neighboring trees, as it is the case in F. excelsior (Gebauer et al. 2008; Köcher et al. 2009). Target trees with a higher mean increment as observed in the Fagus<70 group are likely to possess an enhanced ability to cope with stressful periods and to recover more rapidly after a stress event (Bonn 1998; Pedersen 1998). Thus, a more open canopy or a better nutrient supply in allospecific neighborhoods are additional putative causes of the lower growth sensitivity in this neighborhood category. While the beech neighborhood seems to influence the growth response of Fagus to certain stress events (e.g., the drought summers 1976 and 2003), it had no significant effect on other growth responses (e.g., to the negative pointer year 1992). Moreover the neighborhood seems to have an only weak or negligible effect on the 26-yr ring chronologies, i.e. the long-term response types identified by cluster analysis.
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In the period from 1926 to 1975, beech surrounded by mainly conspecifics (Fagus100 and Fagus70-99) reached higher values of δ13C than beech neighboring influential allospecific competitors (Fagus<70). On the other hand, values of the more actual period from 1976 to 2005 are indifferent between the neighborhood classes. This leads to the conclusion that target trees today surrounded by allospecific neighbors probably benefited from thinning in order to foster more valuable broad-leaved tree species. Thinning resulted in increased soil water availability and in increased growth (Sucoff &
Hong 1974; McDowell et al. 2003). The effect vanished when the canopy closed again. Hence, the observed pattern is probably rather a secondary effect of forest management than a direct effect of neighborhood diversity on water supply of Fagus.
Generally, I assume that the neighborhood represents an environmental factor which is only secondary to climate variability, soil conditions and light availability as a factor controlling annual ring width. Thus, the neighborhood may influence the growth response under certain conditions, but this factor is too weak to determine principal types in the growth-environment relationship of Fagus.
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Acknowledgments
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Acknowledgments
First of all, I would like to thank Prof. Dr. Christoph Leuschner for providing me an interesting and sometimes challenging research topic - many thanks for the constant support and the great interest in my work. Special thanks go to Dr. Hanns Hubert Leuschner for his help and support throughout the last years.
I am grateful to Prof. Dr. Stefan Zerbe for reviewing this thesis and arousing my enthusiasm for plant ecology during one of my first ecology courses.
To all PhD students of the ‘Graduiertenkolleg 1086’: Thanks a lot for wonderful days of joint field work in Hainich National Park, discussions helping to look “beyond the plate edge” and for becoming good friends. But not only had our own research project made studying the forests of the Hainich so agreeable. I appreciate the cooperation with the management of the Hainich National Park, who gave us the opportunity to work in this wonderful forest. In this context, I would like to thank Eike Kinne and Hans Georgi, who gave valuable information on the management history of our research stands. With regard to local history also Wulf Walther of the Mühlhäuser Museen and Rosemarie Barthel of the Staatsarchiv Gotha made things easier.
Furthermore, I would like to thank the DFG for funding.
To all my colleagues of the Department of Plant Ecology: thank you for the very nice working atmosphere and entertaining lunch breaks. Field work would have been much harder without Heiko Eichner, Heiko Rubbert, Dr. Heinz Coners, Nicolai Brock, Marcel Tillmann, Erika Müller, Gabriele Krisinger and Alena Vacátková, Arantzazu Blanco Bernadeau, Christoph Zabel - thank you very much.
Further, I would like to thank Philippe Marchand and Kieran McDonald for their very useful work in Göttingen.
Special thanks to Dr. Mascha Jacob and Dr. Tobias Gebauer for sharing also late office hours with fruitful discussions and a lot of fun.
Without the big support of many people, finishing this PhD thesis would have been impossible. Many thanks to:, Dr. Nadine Weland, Dr. Catharina Meinen, PD Dr. Hermann F. Jungkunst, Dr. Stefan Fleck, Prof. Dr. Wolfgang Schmidt, PD Dr. Dirk Gansert, Peter Herreid, Laura Rose, Prof. Dr. Hermann Behling and Prof. Dr. Hartmut Laatsch.
Ein besonderer Dank geht an meine Eltern Ingrid and Erich Schmidt für ihre Unterstützung, diverse Doktorandenversorgungspakete und ihren Willen mein Interesse für Jahrringe und Bodensamen-banken zu teilen.
Finally, special thanks to my husband Dr. Andreas Mölder for the endless and invaluable support, for extending my interest in ecological history back to the Cretaceous, his enthusiasm in my historical PhD topic and his love.
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Scientist at the Northwest German Forest Research Station, Research project
“Further Development and Implementation of an EU-level Forest Monitoring System” (FutMon)
Teaching assistant for biology: “Biologie für Studenten der Medizin”, Georg-August University Göttingen, Albrecht-von-Haller-Institute for Plant Sciences PhD project „Diversity and tree neighborhood effects on the growth dynamics of European beech and the stand seed bank in temperate broad-leaved forests of variable tree diversity”, Georg-August University Göttingen,
Albrecht-von-Haller-Institute for Plant Sciences, Department of Plant Ecology and Ecosystems Research. Research training group 1086 “The role of biodiversity for biogeochemical cycles and biotic interactions in temperate deciduous forest“.
Teaching assistant for statistics: “Statistik für Biologiestudierende“,
„Einführung in die Biostatistik mit R“, Georg-August University Göttingen, Institute for Microbiology and Genetics, Department of Bioinformatics.
Student assistant: “Biologische Vielfalt und deren Bewertung“ (BioTeam), Technical University of Berlin, Institute for Landscape and Environmental Planning, Chair in Environmental and Land Economics
Student assistant: “Stromlandschaft Elbe“, Technical University of Berlin, Institute for Landscape and Environmental Planning, Chair in Environmental and Land Economics
Education
04/2005 to 03/2009 PhD program “Biological Diversity and Ecology“
08.12.2004 Diploma “Diplom-Ingenieur der Landschaftsplanung“
Diploma thesis: “Biodiversity indicators in forest ecosystems – the Solling mountains (Germany) as an example”
10/1998 to 12/2004 Study program in landscape planning, Technical University of Berlin
08/2003 to 12/2003 Stay abroad at the University of Wisconsin – Madison /USA (Urban Ecology:
Educating for the Management of Cultural Landscapes Exchange Program) 08/1991 to 07/1998 High school examination (Abitur), Gymnasium “Otto von Guericke”,
Magdeburg