Forests function better when diversity of soil-borne organisms is high

One gram of Central European forest soil contains as many different organisms as an entire square kilometre above ground: approximately 7,000 species of bacteria, 3,000 species of fungus, and numerous types of fauna such as springtails, mites and ticks, nematodes and earthworms. They are all hugely important for decomposition of plant material, unlocking the nutrients contained inside. Until now, little has been known about how this enormous diversity is related to the services forests provide for society.

These services are not limited to the production of wood, food and energy or benefits such as air and water purification, protection against natural hazards or the long-term binding of carbon in wood and soil. People also enjoy the forest for its aesthetic value and as a recreational space and regard it as part of their homeland. All these advantages are linked to biodiversity, i.e. the diversity of organisms and habitats, particularly those in the soil. But how?

Researchers from Europe wanted to fill this knowledge gap. Within the framework of a European COST Action* they have conducted a search for existing scientific studies on soil biodiversity and forest services using a keyword search. Out of more than 10,000 results, just under 600 scientific publications contained indications of a relationship between diversity and forest services, e.g. that a higher diversity of plants and algae lowers the nitrogen content of soil moisture and thus improves the "water filtration" function of the ecosystem.

Soil diversity has a positive effect

Their report in the Lausanne-based journal "Frontiers in Forests and Global Change" indicates that the vast majority of the studies showed a positive effect of diversity of soil-borne organisms on the performance of European forests. High fungal diversity, for example, not only improves the nutrient supply and thus tree growth, but delights mushroom pickers and recreation seekers. Many species of earthworms and other soil-borne fauna improve nutrient turnover and significantly reduce soil erosion. Greater species diversity at the level of tree roots increases the biomass in the forest for energy or paper production.

"Subterranean diversity acts as an insurance policy or backup for certain services," states co-author Ivano Brunner of the WSL. "This has never been proven so comprehensively before." There have also been some examples of diversity having negative effects, for example an increase in pathogenic fungi that cause tree disease leads to a decline in biomass production. In principle, however, the authors assume that soil-borne organisms could actually be more important than previous studies have shown. This is because their investigation revealed that only a very minor part of all possible relationships between organisms and forest services have been studied scientifically at all.

The researchers strongly recommend conducting targeted studies on the contribution of soil biodiversity to these services and on the threats soils face in order to provide scientifically based data to policy-makers and other decision-makers. "At European level, soil biodiversity is especially threatened in areas of high population density as well as areas with intensive agricultural activity."