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Mushrooms: How do their species communities vary?

How do fungal species communities vary?

Benoit Richard (Rouen University) takes the floor



Fungi are major contributors to forest biodiversity. The group includes a large number of species and represents a considerable part of the living biomass. Their activities contribute to the healthy functioning of the whole ecosystem: they help decompose organic matter and recycle the nutrients it contains, they structure the soil, they facilitate mineral uptake for trees through a vast network of mycelium filaments (mycorrhizea), etc.

Yet, compared to other organisms (like flora), little is known about the taxonomic, phenotypic and ecological diversity of fungi or about how their communities function in ecological terms (community structure, organization and determining factors).

In 1996, the ONF initiated a partnership with the French Mycology Observatory and the French Mycological Society to complete the monitoring effort on RENECOFOR plots with an inventory of fungal communities.

A first trial was conducted on 12 plots from 1996 to 1998. Then the experiment was repeated on a larger number of plots from 2003 to 2007. In all, about 60 plots were inventoried based on the observation of fruiting bodies visible above ground (sporophores). Since these fruiting bodies are often ephemeral, the inventories were repeated 12 times (over 3 consecutive years with a minimum of 4 repetitions per year) to reflect the diversity of species present on each plot as exhaustively as possible. Then followed a long period of extensive data analysis; in particular to standardise species names according to a single reference list.

This study was designed to investigate the variations in fungal community composition on the inventoried plots with various analytical tools. The study focused on the Basidiomycota, the most recorded species by far (84% of the 1,604 identified species) and the ones the participating mycologists knew the best. Plots that had not undergone at least two consecutive years of inventorying with three repeated campaigns per year were excluded from the analyses. The remaining 51 plots still encompassed a large range of environmental conditions with differing soils, climates, altitudes and atmospheric deposits and with different tree species (pedunculate oak, sessile oak, beech, Douglas fir, spruce, maritime pine, Scots pine and silver fir).

In practice, recording the number of species present (species richness) seems insufficient to assess changes in community diversity since exhaustiveness was unattainable, despite repeated campaigns on each plot. Even on the plots where inventories were carried out most often (up to more than 30 times in three years), each new inventory revealed at least one new species that had not been previously recorded.

On the other hand, species communities did vary from stand to stand in a non-random way following a specific organisational pattern. Above all, the fungal community was influenced by the dominant tree species in the stand; this is in remarkable contrast with observed patterns for other groups of organisms such as flora, where bio-geographical factors (climate) play a major role. For fungi, bio-geographic factors (altitude, latitude, precipitation) do have an influence, but the role they play is of secondary importance, on a par with "more local" soil factors (pH, carbon/nitrogen ratio, percentage base saturation).

Furthermore, the study linked the patterns of variation in fungal community composition to the more or less generalist, or specialist, nature of the species related to their habitat and ecological niche. A species was considered to be more generalist if it appeared along with numerous other species of varying nature.

Conversely, a species was deemed more specialist if it was associated to few other species, which tend to be the same from one inventory to the next. The RENECOFOR campaign revealed that most ectomycorrhizal fungi (in symbiosis with tree roots) are more generalist than saprophytic fungi (organic matter decomposers), which tend to be more specialised.

Furthermore, the fungal communities found in resinous stands were mainly composed of generalist species, while those found under broadleaves included a larger proportion of specialists. In this variation pattern from resinous to broadleaf plots, Douglas fir stands were noticeably different; the fungal communities therein hosted an intermediate range of species with the lowest number of species (probably due to the exotic nature of Douglas fir in Europe).

To conclude, though certain methodological hurdles were encountered, this pioneering initiative to inventory the fungal community produced robust, original results and has made it possible to document variation patterns in these communities over a wide range of ecological conditions. In addition, the inventory results have raised new questions related to silvicultural management and soil biodiversity preservation and have brought to light the preponderant role local factors (tree species choice, physio-chemical soil properties) play in fungal communities, which are so important in forest ecosystem functioning.

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