Distribution and Function of Litter Basidiomycetes in Coniferous Forests

Björn Lindahl and Johanna Böberg

Contents

1.

Introduction

184

2.

Fungal Succession and Interactions in Forest Litter: A Molecular

Approach

184

2.1 Early Colonisers

185

2.2 The Cellulose Decomposition Phase

187

2.3 Mycorrhizal Fungi Take over at Late Decomposition Stages

188

3.

Nitrogen and Carbon Cycling by Litter Saprotrophs

189

3.1 Nitrogen Import During Early Decomposition Stages

190

3.2 Nitrogen Export During Late Saprotrophic Decomposition

Stages

191

3.3 Lignin Decomposition by Saprotrophs

192

3.4 Very Late Decomposition Stages: Humus and Mycorrhizal Fungi

192

Acknowledgements

193

References

194

Abstract The spatio-temporal distribution of basidiomycetes colonising coniferous litter, and their impact on carbon and nitrogen cycling is reviewed. After a brief phase of colonisation by phyllosphere ascomycetes and ephemeral basidiomycetes, litter is colonised by fungi with higher capacity for litter decomposition, such as Mycena species. This change in fungal community composition involves a shift in substrate utilisation from relatively readily available sugars to cellulose. The fungi overcome the nitrogen deficiency experienced during litter colonisation by translocating nitrogen from older parts of the mycelium. To retrieve nitrogen from well-decomposed litter, the fungi presumably use carbohydrates translocated from fresh litter as a co-substrate to attack polyphenol-nitrogen complexes. After a few years of decomposition, the saprotrophic fungal community is out-competed by mycorrhizal fungi. Their direct access to photo-assimilates is likely to be a major competitive advantage in highly recalcitrant and cellulose-depleted resources. Decomposition is continued by the mycorrhizal fungi, although at a very slow rate. It is not until this phase that net losses of nitrogen from the litter occur, as the mycorrhizal fungi forage for nitrogen to support

British Mycological Society Symposia Series © 2008 The British Mycologica! Society

Published by Elsevier Ltd. All rights reserved.

themselves and their host plants. In low-productivity coniferous forests, saprotrophic fungi, thus, re-cycle litter carbon to the atmosphere, whereas re-cycling of litter nitrogen to growing plants is performed largely by my-corrhizal fungi.

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