Tiny Forests create habitats, improve the microclimate, and reintroduce biodiversity into the city in the smallest of spaces. But what actually happens to the soil, to the fungal communities that are crucial for nutrient cycling and symbiosis with the young trees?

We are addressing these questions in an ongoing project in collaboration with MIYA forest e.V. The study is designed to run for three years and investigates the fungal and bacterial communities in ten Tiny Forests and ten adjacent urban green spaces in Berlin, Brandenburg, and the Rhine-Main region. In this article, we present the initial findings on soil fungi from the first year of data collection.

Was sind Tiny Forests und warum wird dort geforscht?

Tiny Forests are small woodland areas of around 100 m², densely planted with native tree species. You are following the method of Japanese botanist Akira Miyawaki, which involves planting around three plants per square metre. Without regular maintenance, these areas develop into multi-layered, self-regulating ecosystems within a few years. The association MIYA forest e.V., based in Eberswalde, has so far created over 40 such mini-forests in Germany.

Development of a Tiny Forest over three years. Photo: MIYA forest e.V.

As part of a socio-ecological research project, MIYA is systematically collecting data on ecological and social parameters at these sites: soil chemistry, microclimate, tree growth, CO₂ sequestration, insect diversity and community effects. MyPilz is responsible for analysing microbial diversity in the soil. Initial results from the overall project already show that soil properties have improved significantly at all sites since planting: The humus content increased, nutrient availability rose, and the soil temperature in the Tiny Forests was on average 2 to 2.4 °C lower than in adjacent lawns. These changes create favourable conditions for soil biology and further plant growth.

From a mycological perspective, the crucial question is: are these changes also reflected in the composition of fungal communities?

How to visualise soil fungal communities

Fungi in the soil are not visible to the naked eye. To systematically capture them, we use DNA metabarcoding. In this process, all DNA is extracted from soil samples, and a specific gene region of all fungi present in them is amplified and sequenced. This region, the so-called ITS region, functions like a genetic fingerprint: it is individual in many fungal species, thus enabling the assignment of species using global reference databases. We use the same principle in parallel for bacteria, with a different genetic marker. For now, we are focusing on the fungi.

At each location, three soil samples were taken from a Tiny Forest and three from an adjacent urban green space, making a total of 60 samples. Across all locations, we identified around 2,400 distinct fungal sequences and over 8,300 bacterial sequence variants. Each of these sequence variants corresponds to a distinct strain or species and allows for the comparison of microbial diversity at high resolution.

More species of fungi in the open areas, but functionality is key

The results reveal a surprising pattern at first glance. To estimate the total number of fungal species, we used the so-called Chao1 index. This statistical method also takes into account rare, as yet unrecorded species, thereby providing a more realistic picture of the actual diversity. The result: the estimated species number was significantly higher overall in the open control areas than in the Tiny Forests. Other diversity measures (Shannon index and Simpson index), which evaluate not only the number of species but also the evenness of their distribution, showed no significant differences. Thus, the fungal communities in Tiny Forests and in the control areas are similarly balanced in their structure, but differ significantly in their composition.

And it is precisely this composition that is crucial. A higher number of species alone says little about the ecological function of a soil. On the contrary, more species can also mean that more potentially plant-damaging fungi are represented. What is much more relevant here is which functional groups are present and which processes they drive in the soil.

Fewer pathogens, more symbionts and decomposers

Using the FungalTraits database, which assigns an ecological function to the mushroom species, We classified the identified fungi into functional groups (so-called guilds). The result is clear: Mycorrhizal fungi, specifically ectomycorrhizal fungi, are found in greater proportions in the Tiny Forests compared to the control areas. A difference is also observed in saprotrophic fungi: fungi that decompose wood or organic matter in the soil are more prevalent in the Tiny Forests. Both groups are essential for the development of a functional forest soil. In contrast, litter decomposers, which primarily utilise leaf litter and plant remains on the surface, dominate the control areas.

At the genus level, the Tiny Forests included, among others Tuber (Truffle) and Geopora significantly more frequently represented, both typical ectomycorrhizal partners of deciduous trees. Also increased were Coprinopsis and Parasol (Tintlingsartige), classic decomposers that rapidly break down fresh organic material. In the Frankfurt Tiny Forests, it was also noted that Mortierella off, a saprotrophic soil fungus with an important role in nutrient cycling. On the control plots, on the other hand, genera such as Fusarium, Cladosporium Black mould, Diaporthe and Dothiorella. Several of these genera include plant-pathogenic species. Fusarium was particularly prevalent in the Rhine-Main control areas and is known in agriculture as a frequent cause of wilting and rot diseases.

Links: Relative abundance of fungal guilds in Tiny Forests and control plots. Tiny Forests show a higher proportion of ectomycorrhizal and saprotrophic fungi. Right: Significantly common fungal genera in Tiny Forests (green) and control areas (grey). The font size reflects the frequency. Source: MyPilz

So, planting native tree species doesn't just change what's visible above ground. It also shifts the soil fungal community: more symbionts, an altered composition of decomposers, and fewer plant-pathogenic fungi.

The location counts just as much as the planting.

An equally important finding concerns the role of local site conditions. To this end, we investigated which factor has the greatest influence on the composition of the fungal community. The individual site accounts for the largest proportion of the differences. Whether an area was planted as a Tiny Forest or not accounts for around 9% of the differences. This may not sound like much at first, but it is within the typical range for ecological microbiome studies, in which many environmental factors act on the community simultaneously. Regional differences between Berlin/Brandenburg and the Rhine-Main area also contribute to the differences between the sites.

In practical terms, this means: soil properties, land history and the type of soil cultivation shape the fungal community at least as much as the planting itself. A sound site assessment before planting, the targeted selection of native tree species and appropriate soil preparation can help beneficial fungal communities establish themselves more quickly and sustainably.

What comes next

The results presented here are an initial snapshot. Later in the project, the same locations will be sampled again in 2027 in order to map the temporal development of the fungal and bacterial communities. Linking the microbiome data with the other parameters that MIYA is simultaneously collecting will be particularly exciting: soil chemistry, soil moisture, temperature, insect diversity, and tree growth. This will make it possible in the future to show which specific measures taken when planting a Tiny Forest have the greatest impact on the microbial community.

The bacterial communities are already showing interesting patterns. In the Tiny Forests, the community composition is more balanced, and for the bacterium 16%, the habitat type accounts for an even greater proportion of the variation than it does for fungi. We will present these results in a future post.

Our role at MyPilz

This project embodies precisely what drives us at MyPilz: combining cutting-edge microbiome research with a specific environmental issue to highlight the real-world impacts on our environment. We support the project from sampling and sequencing through to bioinformatic evaluation and interpretation, thereby shaping the future of urban green space design together with MIYA.

Our microbiome analysis is suitable for anyone who wants to know what is living and working in their soil. The method provides robust data on which microbial communities are present and how they change through interventions. This is relevant for renaturation projects as well as for agriculture, for the assessment of protected areas, or for the monitoring of greening measures. In any case, the results form a solid basis for well-informed decisions.