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On seasonal trunk thermal buffering and its electrical signature in mature trees

This study demonstrates that mature trees exhibit significant seasonal thermal buffering in their trunks, driven by hydraulically linked heat transfer from deep soil rather than simple heat storage, a process that is non-invasively detectable through coherent delays in spontaneous electrical potentials.

Original authors: Jean-Baptiste Boulé, Vincent Courtillot, Dominique Gibert, Fernando Lopes, Alexis Maineult, Pierpaolo Zuddas, Jean de Bremond d'Ars

Published 2026-07-15

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CC BY 4.0

Original authors: Jean-Baptiste Boulé, Vincent Courtillot, Dominique Gibert, Fernando Lopes, Alexis Maineult, Pierpaolo Zuddas, Jean de Bremond d'Ars

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a tree trunk not just as a static pillar of wood, but as a living, breathing thermal battery that plays a very slow, very clever game of "keep your cool" against the chaotic weather outside. That's the big story behind a new study by Boulé and colleagues, who spent years watching six mature trees (three oaks and three hornbeams) in a garden in Paris to figure out how they manage their internal temperatures.

The Mystery of the Smooth Trunk
You know how on a hot summer day, the air can feel scorching, but if you stick your hand in a deep cave or a cool basement, it feels steady and mild? Well, the researchers found that tree trunks do something similar. They measured the temperature of the air, the soil deep underground (1 meter down), and the sapwood inside the tree trunks.

The air temperature was a rollercoaster, swinging wildly between freezing nights and baking days. But the inside of the tree trunk? It was surprisingly chill. It didn't swing nearly as hard. Instead of following the air's crazy mood swings, the trunk's temperature stayed much closer to the steady, deep-soil temperature. It's like the trunk has a built-in thermostat that ignores the weather report and just listens to the earth.

The "Ghost" Signal
To understand how the trunk does this, the scientists didn't just look at thermometers; they listened to the tree's electricity. Trees generate tiny electrical signals called "spontaneous electrical potentials" (SP) as water moves through their veins. Think of this like the tree's own internal Wi-Fi signal, buzzing with information about how water is flowing.

Here is the twist: The tree's electrical signal didn't match the temperature changes instantly. In fact, it was playing catch-up. The researchers found a massive delay. For five out of the six trees, the electrical signal lagged behind the temperature difference between the trunk and the soil by about 100 days. That's roughly a quarter of a year!

If the trunk were just a passive lump of wood reacting to the air, the temperature and the electricity would change at the same time. They didn't. Instead, they formed a loop, like a figure-eight drawn in the air, showing that the tree's internal systems are moving in a slow, delayed rhythm.

Ruling Out the "Heavy Blanket" Theory
You might think, "Maybe the trunk is just heavy and thick, so it takes a long time to heat up and cool down, like a big rock." The scientists tested this idea using a simple energy model. They calculated how much heat the trunk could store just by being a big, water-filled object.

The result? The trunk's ability to store heat on its own was way too weak to explain the smooth temperature they saw. The math showed that the trunk's "thermal inertia" (its natural resistance to changing temperature) was only a tiny fraction of what was needed to buffer the weather. In other words, the trunk isn't just a slow-to-warm-up rock; it's actively doing something else to stay stable.

The Hydraulic Elevator
So, what's the secret? The paper suggests that the tree is using its plumbing system—its xylem and water flow—to move heat around. Just as water carries nutrients from the roots to the leaves, it might also be carrying thermal energy. Because the soil deep underground stays at a steady temperature year-round, the water moving up from the roots acts like a thermal elevator, bringing stable "cool" or "warm" energy up to the trunk to buffer it against the wild air above.

The electrical signals (SP) act as a non-invasive way to "eavesdrop" on this slow-moving hydraulic process. The fact that the electrical signal lags by 100 days suggests that the tree's internal water transport is deeply linked to its thermal regulation, creating a slow, rhythmic dance between the roots and the trunk.

What This Means
This doesn't mean we have a perfect map of every heat molecule moving inside a tree. The authors are careful to say this is a strong suggestion based on field measurements, not a final, solved equation. They haven't directly measured the heat moving with the water yet; they've inferred it from the temperature patterns and the electrical "ghost" signals.

However, the evidence is compelling: the trunk isn't just a passive victim of the weather. It seems to be actively using its connection to the deep soil and its internal water flow to create a stable, buffered home for its living tissues. This could be a crucial survival trick, protecting the delicate cells that make new wood and leaves from the stress of sudden temperature spikes or drops.

In short, trees might be smarter than we thought. They aren't just standing there waiting for the sun; they are running a slow, silent, hydraulic heating and cooling system that keeps their insides comfortable, even when the world outside is going crazy. And if you want to know what they're doing, you don't need to cut them open; you just need to listen to their electricity.

Technical Summary: Seasonal Trunk Thermal Buffering and Its Electrical Signature in Mature Trees

Problem Statement
While the thermal buffering role of tree canopies is well-documented, the thermal dynamics of tree trunks remain poorly understood, particularly regarding the extent to which hydraulic transport contributes to buffering trunk temperature against atmospheric variability. Trunks contain living tissues (vascular cambium, phloem) whose physiological functions are temperature-dependent. Although it is conceptually plausible that xylem sap flow connects the trunk to stable deep-soil thermal reservoirs, acting as a carrier of thermal energy, direct field evidence for sustained thermo-hydraulic coupling in mature trees is limited. Separating passive thermal inertia from hydraulically mediated heat transport is challenging, and there is a need for non-invasive indicators of these slow hydraulic and thermal processes.

Methodology
The study was conducted over multiple years (2022–2025) in a temperate urban forest garden in Paris, France, monitoring six mature trees (three oaks and three hornbeams). The experimental design involved:

Continuous Monitoring: Simultaneous recording of sapwood temperature (at 50 cm and 100 cm heights), local air temperature, soil temperature at 1 m depth, and spontaneous electrical potential (SP) generated by electrokinetic processes associated with sap movement.
Data Processing: Raw time series were down-sampled to daily means. Singular Spectrum Analysis (SSA) was employed to isolate dominant seasonal components, filtering out short-term meteorological fluctuations.
Analytical Approaches:
- Phase-Space Analysis: Lissajous representations (hysteresis plots) were used to visualize the relationship between SP and the trunk-soil temperature difference ( $\Delta T$ ).
- Phase Estimation: The Hilbert transform was applied to quantify instantaneous phase lags between the electrical and thermal signals.
- Energy-Balance Modeling: A minimal energy-balance model was formulated for a 1 m trunk segment to test whether heat storage and atmospheric exchange alone could reproduce observed dynamics. The model included a term for effective soil coupling ( $K_s$ ) to account for potential vertical heat transfer.

Key Contributions

Integration of Bioelectric and Thermal Data: The study combines continuous thermal monitoring with spontaneous electrical potential (SP) measurements, proposing SP as a non-invasive, integrative indicator of slow hydraulic and thermal processes within the tree.
Quantification of Seasonal Lag: The research quantifies a persistent seasonal phase lag between electrical potential and thermal state, providing evidence of delayed system response.
Energetic Constraints on Thermal Inertia: By modeling energy balance, the study demonstrates that trunk thermal inertia alone is insufficient to explain observed seasonal buffering, necessitating an additional heat-transfer pathway.

Results

Thermal Buffering: Trunk temperature exhibited a smoother seasonal cycle with reduced amplitude compared to air temperature and remained consistently closer to deep-soil temperature. This buffering persisted across two consecutive annual cycles.
Electrical-Thermal Co-variation: SP showed pronounced seasonal variability that co-varied coherently with the trunk-soil temperature difference ( $\Delta T$ ). However, the signals did not reach seasonal extrema simultaneously.
Hysteresis and Phase Lag: Phase-space analysis revealed reproducible, closed hysteresis loops across individuals, indicating a delayed relationship rather than synchronous evolution. Instantaneous phase analysis quantified this delay as approximately 100 days (roughly one-quarter of the annual cycle) in five of the six trees.
Modeling Findings: The minimal energy-balance model showed that the ratio of trunk heat storage to atmospheric convective exchange was well below unity, confirming that thermal inertia alone cannot account for the buffering. Fitted effective soil-coupling coefficients ( $K_s$ ) were positive for most individuals, though magnitudes varied significantly, suggesting an additional vertically mediated thermal contribution.

Significance and Claims
The paper claims that its multi-year field evidence supports the hypothesis that mature tree trunks function as vertically coupled thermo-hydraulic systems. The observed thermal buffering is consistent with hydraulically linked heat transfer, where water movement redistributes thermal energy between the trunk and the stable deep-soil reservoir.

The authors posit that this coupling may stabilize the thermal environment of the vascular cambium and phloem, potentially influencing wood formation timing, metabolic activity, and hydraulic integrity during seasonal transitions. Furthermore, the study suggests that spontaneous electrical potentials serve as a valuable, non-invasive proxy for monitoring these slow, whole-stem dynamics.

The authors maintain a modest stance regarding mechanistic interpretation, acknowledging that the effective coupling coefficient ( $K_s$ ) integrates conductive, advective, and structural contributions, and that SP is an integrative signal rather than a direct measure of sap velocity. They conclude that while the results are consistent with thermo-hydraulic coupling, future work combining direct sap-flow measurements and radial temperature profiles is required to explicitly separate hydraulic and thermal contributions.

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