The present book gives a modernized and comprehensive overview of the degradation of major litter components as well as the sum of processes when forest plant litter decomposes, releases nutrients and turns into humus. Covering mainly boreal and temperate forests, the book gives a synthesis of the different sub-processes on the basis that decomposition is microbially mediated and develops a system in the decomposition processes leading to chemical changes in litter. Further, it introduces a conceptual model for litter transformations from litter fall until the decomposing litter accumulates as humus. The effects of substrate quality, climate, and their interactions on decomposition are presented as well as examples of litter types that break the basic pattern. A system is suggested for humus accumulation rates among forest systems, a system that may be a starting point for estimates of carbon sequestration in forest systems.

When starting our work on this book we intended to summarize and synthesize the new information that had developed in the last 20-30 years in the field of plant lit­ ter decomposition. It turned out, however, that the main part of more recent work was directed towards boreal and temperate forest systems and therefore, with a fo­ cus on these ecosystems we finally concluded a synthesis that has a similarity to a case study. Still, we hope that a deeper insight into the behavior of a limited num­ ber of litter species will be of value for a generalization and also for the identifica­ tion of process systems that deviate from those presented here. We have written the book focusing on the transfer from newly shed litter to re­ calcitrant humus, describing and explaining the system of chemical changes taking place in the process both on a mechanistic basis and on a more general and re­ gionallevel, considering different climates and species. As a synthesis, this book gives some new aspects on decomposition that to some of us may be controversial. Thus, the fact that we emphasize the dominant role of microorganisms in the process may be disturbing to many readers, as well as the strong emphasis we give to the fact that humus layers actually do grow over millennia and that at a considerable rate, and thus really sequester e.g. C and N. This book is based primarily on data and conclusions made from field studies.

1 Introduction.- 2 Decomposition as a process.- 3 Decomposer organisms.- 4 Initial litter chemical composition.- 5 Changes in substrate composition during decomposition.- 6 Influence of chemical variation in litter on decomposition.- 7 Climatic environment.- 8 Influence of site factors other than climate.- 9 Decomposition of fine root and woody litter.- 10 Models that describe litter decomposition.- 11 Decomposition and ecosystem function.- 12 Human activities that influence decomposition.- Appendix I. Glossary.- References.- Appendix II. Scientific names of vascular plants.- II.1 Gymnosperms.- Firs.- Hemlocks.- Pines.- Spruces.- II.2 Angiosperms.- Alders.- Aspen.- Beeches.- Birches.- Grasses.- Maples.- Oaks.- Other woody plants.- Appendix III. Site descriptions.- Stråsan, Sweden.- Jädraås, Sweden.- Black Hawk Island, Wisconsin, USA.- Harvard Forest, Massachusetts, USA.- References.