Hidden in the Hollow: First Insights into Fungal Communities of Tree Hollow wood mold

Deep within the forests of Germany, a quiet transformation is taking place inside the hollows of old trees. These cavities, often formed by storms or disease, are not merely empty spaces; they are living, breathing micro-worlds filled with a soft, crumbly substance known as wood mold. This material is a complex mixture of decaying wood, fallen leaves, animal remains, and microscopic life, creating a stable environment that has long been recognized as a sanctuary for insects, birds, and mammals. However, the invisible architects of this transformation—the fungi that break down the wood and build the soil-like mold—have remained largely a mystery. While scientists understand how fungi operate on fallen logs or standing dead trees, the unique conditions inside a tree hollow, where the substrate slowly shifts from wood to something resembling earth, offered a new frontier for discovery. Understanding these hidden communities is vital because fungi are the primary recyclers of the forest, turning dead matter into nutrients that sustain the entire ecosystem.

To uncover the secrets of these hidden fungal communities, researchers from the University of Bayreuth ventured into the northern Steigerwald region in July 2025. They focused on thirty-six European beech trees, each containing a hollow with at least two centimeters of accumulated wood mold. The team carefully collected samples from a depth of two to seven centimeters below the surface of the mold, ensuring they captured the heart of the habitat rather than just the surface layer. They classified these samples into three distinct stages of decay based on their appearance: early stages where the material was light in color with visible wood chunks, intermediate stages that were darker and more broken down, and advanced stages that appeared dark brown to black, resembling soil with no visible wood parts remaining.

Once the samples were collected, the researchers froze them immediately to preserve their biological integrity. Back in the laboratory, they used a technique called DNA metabarcoding to identify the fungal species present. This process involved extracting genetic material from the mold and reading specific DNA sequences that act as unique fingerprints for different fungi. By analyzing these genetic codes, the team could identify hundreds of different fungal types, ranging from common molds to specialized decomposers, without needing to grow them in a petri dish. This high-tech approach allowed them to see the full diversity of life hidden within the wood mold, revealing a community far more complex than previously imagined.

The results painted a picture of gradual change rather than sudden shifts. As the wood mold progressed from its early, wood-like state to its advanced, soil-like condition, the number of different fungal species tended to increase. The most decayed hollows, which looked most like soil, contained the highest variety of fungal life, with some samples holding nearly three hundred distinct types. However, the researchers found that this increase in variety was not a sharp jump but a slow, steady climb. The overall abundance and dominance of the most common fungal species remained surprisingly stable across all stages. It was the rarer, less common species that drove the changes, appearing and disappearing as the environment evolved. This suggests that while the main players in the fungal community remain consistent, the supporting cast of rare species shifts subtly to match the changing conditions of the decaying wood.

One of the most striking findings was the changing identity of the fungi themselves as decomposition advanced. In the early stages, the mold was dominated by fast-growing fungi that are excellent at breaking down fresh wood. As the material aged and became richer in nutrients, the community began to include more specialized species, including fungi that typically live in soil or form partnerships with tree roots. By the time the wood mold reached its most advanced stage, it supported a diverse mix of decomposers and soil-associated fungi, effectively bridging the gap between dead wood and living forest soil. This transition indicates that the hollow is not just a static container but a dynamic environment that undergoes a functional transformation, eventually becoming a habitat that supports life forms usually found on the forest floor.

Interestingly, the location of the trees did not seem to matter as much as the condition of the wood itself. The distance between the trees, whether they were close together or far apart, had no detectable effect on the types of fungi found inside. This suggests that the physical and chemical state of the wood mold is the primary force shaping these communities, acting as a filter that selects for specific types of fungi based on the stage of decay. The fungi are not limited by their ability to travel from one tree to another; instead, they are limited by whether the environment inside the hollow is suitable for them to survive and thrive.

The study concludes that tree hollows are dynamic, heterogeneous habitats that support a wide range of fungal life throughout their entire lifespan. Rather than being distinct, separate communities at each stage of decay, the fungal assemblages change gradually, with subtle shifts in composition driven by rare species and specific environmental conditions. This finding highlights the importance of preserving trees at all stages of decay. To maintain the full diversity of fungal life and the ecological processes they support, forests need a continuous supply of hollows, from those just beginning to form to those that have been decaying for decades. By protecting these structures, we ensure that the intricate, invisible web of life that recycles the forest continues to function, supporting everything from the smallest microbe to the largest forest-dwelling animal.