In Europe an abundance of humus taxonomies exists starting with early approaches in the late 19th century. Frequently used in an international context, they do not cover all site conditions in the European area. Although having basic concepts and general lines, the European (and North American, Canadian) classification systems differ in important parameters used for the description and classification of humus forms. These discrepancies result in incongruities, so they require adjustments when exchanging partially compatible soil data, even between nearby countries. In 2003, 26 European specialists in humus forms met in Trento (Italy) and decided to formulate rules of classification based on morphogenetic descriptions and diagnostic horizons, adapted to European ecological conditions. Taking into account old and new European and North American systems of humus forms classification, six main references (Anmoor, Mull, Moder, Mor, Amphi and Tangel) were defined, each of them further divided into more detailed categories. This inventory assigned a strong discriminatory power to the action of soil animals. Both semiterrestrial (anoxic) and terrestrial (aerated) topsoils were classified. Descriptors of diagnostic horizons were conceived in accordance with recent international soil classifications. Assigning an "ecological value" to each main humus form along a gradient from biologically active forms, degrading and incorporating all organic remains, to those characterized by the accumulation of poorly transformed organic matter, this European system of classification avoids a strong hierarchical structure and allows a flexible approach open to additional ecological contributions and renditions.2
The morpho-functional classification of humus forms proposed in a previous issue by Zanella and collaborators for Europe has been extended and modified, without any change in diagnostic horizons, in order to embrace a wide array of humus forms at worldwide level and to complete and make more effective the World Reference Base for Soil Resources. For that purpose 31 Humus Form Reference Groups (HFRGs) and a set of prefix and suffix qualifiers are proposed, following the rules erected for the WRB. An exhaustive classification key, respecting the principles of WRB, is suggested and examples of classification are given for some already well known humus forms
Question: Do tree species, with different litter qualities, affect the within‐forest distribution of forest understorey species on intermediate to base‐rich soils? Since habitat loss and fragmentation have caused ancient forest species to decline, those species are the main focus of this study.
Location: Three ancient forests, along a soil gradient from acidification‐sensitive to base‐rich, were studied: Limbrichterbosch and Savelsbos in The Netherlands and Holtkrat in Denmark.
Methods: Canopy and soil surveys along transects generated data for Redundancy Analysis on tree – humus relationships. We analysed the distribution of forest plant species with Canonical Correspondence Analysis. The explanatory factors were soil characteristics (pH, organic matter, loam content and thickness of the humus layers), external crown projection, ground water and canopy data. We further analysed the relationship between forest species and humus characteristics with Spearman correlations.
Results: Tree species have a significant impact on humus characteristics through the nature of their litter. Humus characteristics significantly explain the distribution of forest understorey species. The pH of the first 25 cm mineral soil and the thickness of the F‐ (fermentation) layer are the primary factors affecting the distribution of ancient forest species.
Conclusion: This study indicates that the species composition of the forest canopy affects the distribution of forest understorey species. Ancient forest species are more abundant and frequent underneath trees with base‐rich litter. On acidification‐sensitive soils these relationships were stronger than on more base‐rich, loamy soils.
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