Buffering moisture in the wood of the hollow / hive

Attention: Basically the following consideration of the buffering of moisture is correct. But the order of magnitude is wrong. The transport processes of water in wood are considerably slower than assumed here. In a complex experiment run (Tom´s Tree) I am currently determining the correct values and will then revise the calculations here. I refrain from a new assessment in advance, I don’t want to be wrong about the same topic twice in a row…

Buffering effect of the tree

The relative humidity in the tree cave and the moisture of the wood surrounding it are directly related, they aim a balance . The moisture of the wood corresponds with the level of humidity. Conversely, the same is true: In the tree cave the relative humidity also adapts to changes in the moisture of the wood.

During the formation of a tree cave, the cavern 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 cave, 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. The buffer is so large that it can easily absorb all the moisture generated by a bee colony during the winter months. During the summer months, the humid air in the tree cave is permanently lowered by the ventilation of the bees. The walls of the caverns then give off the moisture absorbed in the winter.

Due to the buffering effect of the wood, it is not necessary to transport the moisture up to the flight hole in the cold months by means of air movements. The moisture is absorbed by sorption from the wood. The bees support this process by simply fanning the warm, humid air into cooler areas of the tree cavity. Here, the relative humidity increases by cooling the air so far that the moisture is absorbed by the wood. With this system the heat of the bees is hardly lost, it is largely preserved within the tree cavity.

Calculation of the buffer size

The following is a model calculation to determine the buffer size. Various assumptions are made for this. Currently there is no data available from bee-tree species to check these assumptions.

The buffer surrounding the tree cavity is thinner on the side walls than on the ceiling and floor. This is because of the fiber direction of the wood and the different transport speeds in and across the fiber direction. Only the ripe and the heartwood are a part of the buffer, the water-bearing wood layers cannot absorb any extra moisture.

Let’s take a look at the moisture of the ripe and heartwood between the transition to the water-bearing layer and the side wall of the tree cave:
On the side of the tree cave, a balance between relative humidity (60%) and wood moisture (11.12%) is established at the end of summer. This data corresponds with a current series of measurements of a treehive.
Depending on the type of wood, the water-bearing layers have a moisture content of up to 150%.
The percentage of sapwood on the tree decreases with its age. The content of ripe and heartwood grows steadily over the years. In these wood layers, the maximum of  moisture of the wood is 75%. Consider a 200-year-old beech with a stem diameter of 1m in the area of the tree cavity (layer thickness sapwood and bark 10cm).
At the transition from ripe wood to sapwood, we assume the maximum wood moisture of ripe wood (75%). Furthermore we assume a linear course of the wood moisture between the moisture contents at the edges of the buffer. The wood volume directed towards the tree cavity is available as a buffer, but only to the point where the wood moisture is equal to the fiber saturation moisture (the wood moisture, which is adjusted at 100% relative humidity). This fiber saturation moisture content is considerably higher for beech wood (34%) than for coniferous wood (28%) for example. Therefore, there is a larger buffer available in beech wood (1.5x larger).

buffer
buffer for moisture of a hollow tree

The average moisture content of the buffer is 22.5% (beech wood). The lateral buffer volume is 0.15 m³. On the ceiling and at the bottom of the tree cavity, the buffer is provided with a larger layer. The water transport processes are about 10 times faster in the direction of the fiber than perpendicular to it. Theoretically the top of the buffer would be 4m above the tree cavity. The volume of this buffer is calculated at a total of 0.094 m³. Downward we only take into account a cylinder with a height of 1.50m, since bee-inhabited tree caves usually have their flight hole below 2m above the ground. Therefore a total of 0.391m³ of buffer with an average wood moisture content of 22.5% is available.

This volume of beech tree has a dry weight of 0.391m³ x 680kg / m³ = 266kg. The water content of the buffer is 60 kg. In a well isolated tree cave a beehive colony consumes a maximum of 5kg of honey from October to March. This contains 3.4 kg of water, which is metabolized by the bees and released into the air. The total water content of the buffer thus changes to 63.4 kg and the average wood moisture content to 23.8%. At the edge of the cavern, a moisture content of 13.6% is obtained. With this wood moisture, a relative humidity of 70% is established in the cave.

On this scale, a mold risk is nearly impossible in the tree cave. An additional dehumidification by means of natural air changes and by steam diffusion has not yet been taken into account.