This work was carried out in the Department of Biology at the University of Oulu, in the Department of Ecological and Environmental Sciences at the University of Helsinki, in Kempele Institute of Horticulture and in Häme Polytechnic Lepaa. It was supported financially by the Academy of Finland, the University of Oulu, Maiju and Yrjö Rikala Foundation, Häme Vocational Higher Education Foundation and the Horticultural Society of Oulu. All this support is gratefully acknowledged.
I am most grateful to my supervisors professor Rauni Strömmer and professor Juha Tuomi. I have worked throughout the study with Rauni. Her support and guidance have been most valuable to me. Juha"s encouragement and valuable comments during the last period of my studies are warmly appreciated. I also appreciate all the help and constructing comments professor Satu Huttunen gave me. I would like to thank Dr. Marko Hyvärinen for his support and opening my eyes so that I was able to see things from a different perspective. Professor Pekka Lähdesmäki and director Kari Laine are thanked for their encouraging attitude. Many thanks to Riikka Ojala, for being a very good assistant during my greenhouse experiment.
I owe my sincere thanks to professor Paavo Havas for believing in my idea about cover cropping research and giving me the possibility to start this work. The encouraging attitude of production manager Erkki Sipilä from Harviala Nurseries was also a decisive factor to start the research. I found the research idea from a book, which I obtained from my friend Elisa Tikanmäki, and as a result I would like to thank her, too.
I am very grateful to Tuulikki Pakonen and the assistants for their good work. I would like to give my special thanks to Tuulikki for the nice company during my study years. I acknowledge the work of the staff at the Institute of Electron Optics, director Seppo Sivonen and his research assistent Olli Taikina-Aho. Furthermore, I would also like to thank the staff at the Botanical Gardens for all their help during the greenhouse experiment. My sincere thanks go to my students in Kempele, Sirkka Tervamäki and all the others, for their valuable help in the field work. In addition, I would like to thank director Veikko Hätälä in Kempele for his positive attitude towards my experiments in the nursery. Thanks to Esa Härmä and Helena Salomaa for drawing help, Irma Lampinen and the boys for field work, and my cousin Sinikka Vuorinen and my colleague Soile Seppälä for helping me with the English language. Teresa Dias-Heikkilä, MA, Licentiate Dip., revised the language, for which I would like to thank her.
Finally, I am deeply grateful to my sister Mirja and my friends Carolin Nuortila and Oili Tarvainen for their friendship and discussions during all these years. These discussions have been very important to me.
Oulu, May, 2002 Kaarina Hänninen