The relative humidity in the tree cavity and the humidity of the surrounding wooden walls, the stored honey and the honeycomb building interact directly, they aim a balance . The moisture of the respective material corresponds with the level of the relative humidity.
During the formation of a tree cavity, the cavity walls dry out and reach a moisture content far below the fiber saturation (fiber saturation is the wood balancing moisture, which adjusts at 100% relative humidity). The ventilation, the age of the cavity, the type of wood, the geometry of the tree, and the weather are all crucial influences to this.
This volume of dried wood is now available as a buffer for fluctuations in moisture. During the summer months, the humid air in the tree cavity is permanently lowered by the ventilation of the bees. The walls of the cavities then give off the moisture absorbed in the winter.
This interaction is however clearly slowed down by the propolis coating of the bees. In the past I overestimated the amount of water the wood stores during the winter. The honey stores are a larger store of moisture.