Sustainability Issue #2 July 2009

Swedish forest ecosystems are dominated by pine, spruce and birch. Above ground, the occurrence and distribution of these species are quite well known. On the other hand, below ground there is a high diversity of fungi, and these are to a large extent unresearched.

The role of these species

With more than a thousand known species in Sweden, ectomycorrhizal fungi, which live in symbiosis with tree roots, make up a significant proportion of the biodiversity in the forest. Many common forest mushrooms are mycorrhizal fungi. Examples that may be mentioned are fly agaric, sickener, King Alfred's cake, slippery jack and Jersey cow bolete. The number of fungal species below ground is considerably greater than the number of sporophores seen above ground. The composition of the species depends on factors such as the kind of tree, the age of the forest, soil type, the ability of the spores to spread and become established, and the ability of the fungi to compete or coexist in the soil. However, the way these and other factors contribute to the formation and maintenance of this high diversity is almost unknown.

Climate changes and nitrogen deposition affect species composition and often result in a reduction in the number of species, when specialist species are wiped out to the benefit of generalists. Many studies have demonstrated such changes, but in order to understand the significance of this we must understand more about the role of different species in the forest ecosystem.

In forest soil, the majority of tree roots are colonised by ectomycorrhizal fungi. The mycelia of the fungi grow outwards from the roots, and since the hyphae are very thin (2-5 mm) these fungal mycelia can effectively colonise the soil and take up nutrients which are then transferred to the trees. Owing to this symbiotic relationship with the roots of the tree, the fungi have direct access to energy in the form of carbohydrates from the tree's photosynthesis. About 25 per cent of the energy that the tree binds in photosynthesis is allocated to the growth and soil activity of the ectomycorrizal fungi.

Decomposers

In the forest ecosystem nutrients circulate through decomposition of organic matter. It is the decomposer fungi which are the specialists in this. But we also know that ectomycorrhizal fungi can decompose organic matter in order to benefit from nitrogen, phosphorus and basic cations which are positive ions, for instance potassium, calcium and sodium.

Felling of a forest results in the removal of organically bound nutrients from the ecosystem. Losses of phosphorus and base cations can be compensated for through weathering of minerals in the soil. Roots, fungi and bacteria accelerate weathering through nutrient uptake, local acidification and the production of organic acids. In this way they contribute to the formation of a nutrient rich soil, the maintenance of a productive ecosystem and the rehabilitation of contaminated soil.

Stratified mineral soils

The objective of my research is to understand the great species abundance of the ectomycorrhizal fungi by analysing the functional differences in different species in relation to where in the soil profile they occur. I am chiefly interested in stratified mineral soils, podsol soils, which are common in Swedish forests.

My previous research showed that the composition of fungal species varies in such soils. Only one half of the fungal species occur in the upper organic stratum, and in the mineral soil there are many previously unknown species. I focus on the ability of fungi to take up phosphorus from organic and inorganic structures. In the soil the concentration of accessible phosphorus is always low, since it is rapidly taken up by plants and microorganisms or is bound to iron and aluminium in the mineral soil. Almost 90 per cent of soil phosphorus occurs in organic structures. The remainder is found bound to minerals and to phosphate minerals such as apatite.

In the fungal species Piloderma we have been able to see that a species which preferably grows in mineral soil produces high concentrations of siderophores which are organic molecules that form strong complexes with iron. Siderophores might contribute to the liberation of iron-bound phosphorus. The Piloderma species which occurs in the entire soil profile instead produces organic acids which may increase the weathering of minerals and may also destabilise organic matter. By defining functional differences between species in relation to their distribution in the soil, we increase our understanding of how the species abundance of fungi contributes to the long term productivity of the forest ecosystem.

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