Why dendrometry reveals water stress: the structure of a tree
A dendrometer measures nothing more than a distance of a few micrometers. That such a tiny movement still tells you whether a tree is thirsty comes down to the way a trunk is built. Once you know the layers of a trunk, you understand why the measurement raises the alarm so early.
In What is a dendrometer? you can read which values we calculate from the measurement. Here we take a step back: what happens in the wood and bark that makes a trunk shrink and swell?
A trunk in cross-section: five layers, each with a job
Cut through a trunk and you see these layers, from the outside in. For a trunk measurement, each layer behaves differently.
| Layer | What it does | Role in the measurement signal |
|---|---|---|
| Outer bark (cork) | Dead, water-repellent armor against drying out, browsing and sun. | Hardly moves with the tree's water balance; at most slightly with rain or dew. |
| Inner bark (phloem) | Living cells that carry sugars from the leaves to roots, fruit and growing parts. | The largest water buffer in the trunk. This is where most of the daily shrinking and swelling happens. |
| Cambium | A layer just a few cells thick that divides and makes the trunk thicker: wood to the inside, bark to the outside. | Source of permanent growth. Only works when the cells have enough water. |
| Sapwood (xylem) | The young, active wood: a bundle of millions of fine conduits that carry water from the roots to the crown. | Rigid. On warm days it draws water from the inner bark, but it shrinks much less itself. |
| Heartwood | Old wood that no longer conducts water and mainly gives the tree strength. Not clearly visible in every species. | Does not contribute to the daily movement. |
A tree drinks by pulling
A tree has no pump. Water rises because the leaves evaporate it at the top. Every water molecule that leaves the leaf pulls the next one up with it, like an unbroken thread from the roots to the crown. The brighter the sun and the drier the air, the stronger that pull.
As long as the roots supply enough water, the tree barely notices. But on a warm morning, evaporation in the crown runs ahead of what the roots can deliver. Then the tree taps its reserve: the living cells in the inner bark release water to the conduits next to them, like a sponge being squeezed. The bark gets thinner and the trunk shrinks, by tens and sometimes hundreds of micrometers.
In the evening, evaporation drops away. The pull eases, the roots catch up and the bark cells soak themselves full again. By morning the trunk is back to its old thickness, or slightly thicker if it also grew that day.
Growth only happens under pressure
A new cell in the cambium grows by taking up water and inflating against its own cell wall. That internal pressure is called turgor. If turgor drops because there is too little water, cell expansion stops, even with plenty of light and nutrients. The tree stops growing, long before a single leaf starts to droop.
That is why water stress shows up in a trunk measurement in a fixed order:
- Growth levels off: the morning maxima stop rising.
- Daily shrinkage gets deeper, because the tree draws more from its reserve.
- The trunk no longer fully recovers at night: the buffer no longer refills.
- Only then do visible signs follow, such as limp leaves, early leaf drop or slower fruit growth.
Between step 1 and step 4 there are often days to weeks. That is the time a dendrometer gives you to adjust.
Why measuring the trunk tells you more than measuring around the tree
Soil moisture tells you how much water is around the roots, the weather forecast how much the air will demand. Only the trunk shows whether supply inside the tree itself keeps up with demand. A tree with damaged roots can be thirsty in wet soil. A tree with a deep root system bridges a dry topsoil with ease. In both cases, the bark gives the right answer.
What the trunk's structure means for installation
- Measure on a sound, straight part of the trunk, away from wounds, burls and side branches. That is where the bark is most representative.
- Preferably choose the north side, or shade the sensor, so direct sun does not heat up the measurement.
- Loose flakes of outer bark make the signal noisy and can be carefully removed. The living inner bark always stays intact.
- Compare trees of the same species and age. A young fruit tree with thin bark moves differently from an old avenue tree; the trend over time says more than the absolute value.
From trunk structure to action
In the PlantData.Live portal, the trunk measurement comes in continuously, together with weather data for the measuring location. The algorithm behind the Water barometer recognizes the moment a tree draws on its reserve faster than it can refill it, and then sends a notification. That way you step in at step 2 or 3, not only at step 4.
See how that works in practice with our dendrometer with continuous measurement, or read how the air itself pulls on the tree in What is vapor pressure deficit (VPD)?.
Sources
Steppe, K., Sterck, F. & Deslauriers, A. (2015). Diel growth dynamics in tree stems: linking anatomy and ecophysiology. Trends in Plant Science, 20(6), 335–343. https://doi.org/10.1016/j.tplants.2015.03.015
Zweifel, R., Item, H. & Häsler, R. (2001). Link between diurnal stem radius changes and tree water relations. Tree Physiology, 21(12–13), 869–877. https://doi.org/10.1093/treephys/21.12-13.869
More from our knowledge base
What is a dendrometer?
Deriving growth, daily shrinkage and water deficit from trunk diameter.
What is vapor pressure deficit (VPD)?
How hard the air pulls water out of the tree.
Regulated deficit irrigation: smarter watering with less water
Why you measure water stress to save water safely.