The geometry of tree cavities
Thanks to the study “The nest of the honeybee” by T. Seeley & R. Morse from 1976, we have gained a good understanding of the structure and shape of beehives in tree cavities. In the study the authors determined an average volume of 45L and a ratio of height to width of 7.
The space that has been available to the bees in tree caves since ancient times is thus significantly taller and slimmer than the space offered to them in modern beekeeping.
Thermal insulation of tree cavities
Tree caves are equipped with a considerable thermal insulation. The side walls should have had an average thickness of 30cm-70cm. Assuming an average wall thickness of 40cm, a thermal insulation of 2.670m²K/W is obtained (beech wood).
This value significantly exceeds the thermal insulation of houses for humans. The insulation is better still from the top and the bottom of the tree cave. Obviously the bees want a similarly good insulation for their habitations as we humans do for ours.
In the tree cavities the bees have more time before they have to take up the winter formation due to the good insulation. It is easy for them to maintain temperatures at which they can still move and work freely. However, the mass of the tree in the early spring also acts as a cold store, the bees can only leave the winter cluster later. The advantages and disadvantages are gladly speculated about.
Good thermal insulation makes the climate in honeycombs (not everywhere in the tree hollow) drier, the dew point is in the wood of the tree or on the cavity wall. In the case of thin-walled hives, the dew point (i.e. the point of the theoretical condensation water accumulation) is often already in the honeycomb structure.