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How a Giant Tree Can Take Water In Through Its Leaves

Wait, What? Water Can Move From a Redwood Leaf Back Into the Tree

The familiar school diagram points one way:

soil → roots → xylem → leaves → atmosphere.

That is an excellent model for transpiring plants on dry days.

But when fog wets the crown of a coast redwood, the gradient can change. Water can enter leaves directly and move inward, sometimes producing measurable reverse flow in branches.

Fog also reduces the atmospheric demand that normally pulls water out of leaves.

fog wets foliage + humid air suppresses transpiration → leaf water status improves → external water enters foliage → internal gradients can reverse locally → canopy tissues rehydrate.

A giant tree can therefore receive drought relief from above as well as below.

Big Question: How can atmospheric water alter the hydraulic system of one of Earth’s tallest trees without first entering the soil?

Quick Answer

Coast redwoods, Sequoia sempervirens, occupy a climate where summer rain can be scarce but coastal fog is frequent. Fog helps in at least two distinct ways. First, humid foggy air lowers vapour-pressure deficit and suppresses transpiration, slowing water loss from leaves. Second, liquid fog water deposited on foliage can be absorbed directly through leaf surfaces. Experiments using detached and intact redwood foliage showed increases in leaf water content after wetting, while sap-flow measurements documented reversals consistent with water moving from wetted crowns toward drier internal tissues. Foliar uptake can improve leaf water potential and rehydrate canopy tissues. Fog drip can also reach soil, adding a separate root-accessible pathway. These mechanisms should not be collapsed into one claim: fog interception, transpiration suppression, direct foliar uptake and fog drip are different processes that can cooperate.

What You Will Learn

• Why tall trees face hydraulic limits.
• What vapour-pressure deficit means.
• How fog suppresses transpiration.
• What foliar water uptake is.
• How water potential can reverse local flow direction.
• What reverse sap flow reveals.
• How fog drip differs from direct leaf uptake.
• Why canopy height changes water stress.
• How isotopes and wetting experiments test fog pathways.
• Why “redwoods drink fog” is useful only after the mechanisms are separated.

Part 1 — Height Makes Water Transport Harder

A tall redwood must move water from roots to leaves tens of metres above the ground.

Gravity lowers water potential with height, and resistance accumulates along the hydraulic pathway. The upper crown can therefore experience greater water stress than lower foliage even when roots remain connected to moist soil.

Any process that rehydrates the crown directly can be disproportionately useful.

Part 2 — Transpiration Is Driven by an Atmospheric Gradient

Water evaporates inside leaves and diffuses through stomata toward drier air.

The drying power of the atmosphere is often described using vapour-pressure deficit, or VPD.

High VPD means the atmosphere can draw water strongly from leaves. Foggy air is close to saturation, so VPD falls and transpiration slows.

Part 3 — Fog Helps Before a Drop Enters the Plant

This is the first fog benefit and it is easy to miss.

Even if no fog water crossed the leaf surface, a humid fog event could reduce water loss by lowering the leaf-to-air vapour gradient.

That gives roots and internal storage more time to restore leaf water status.

Part 4 — Fog Droplets Collect on the Crown

Fog consists of suspended liquid droplets.

As moving fog encounters needles, twigs and branches, droplets collide with surfaces and coalesce. The enormous crown of a redwood is therefore a fog-interception structure.

Collected water can evaporate, drip toward the ground or remain in contact with foliage long enough for direct uptake.

Part 5 — Leaves Are Not Always One-Way Water Valves

Plant leaves are usually taught as sites of water loss.

But when external leaf surfaces are wet and internal tissues are sufficiently dry, the water-potential relationship can favour movement inward.

This is foliar water uptake.

The exact microscopic pathways can involve cuticular regions and specialised surface structures and can vary among species and leaf types. The robust redwood-level claim is direct uptake by wetted foliage.

Part 6 — Water Potential Determines Direction

Water does not “know” that xylem is supposed to carry it upward.

Direction emerges from potential gradients and pathway resistance.

On a dry sunny day, evaporation makes leaf water potential very negative, favouring root-to-leaf movement. During prolonged wetting, external water can raise leaf water status and change gradients within the branch.

Local flow can then slow, stop or reverse.

Part 7 — Reverse Sap Flow Is a Powerful Receipt

Researchers can place heat-based sap-flow sensors on branches to infer the direction and rate of water movement.

During crown wetting, coast redwood branches have shown reversed flow consistent with water entering leaves and moving toward drier internal tissues.

This is stronger evidence than merely observing wet leaves after fog.

Part 8 — Wetting Experiments Separate Uptake From Humidity

If a leaf simply sits in humid air, reduced transpiration can improve water status without direct uptake.

To test foliar absorption, researchers wet foliage directly and measure changes in leaf water content, water potential or mass while controlling other pathways.

Redwood foliage gains water during direct wetting experiments, supporting true surface uptake.

Part 9 — Fog Drip Is a Third Pathway

Some intercepted fog water coalesces and falls from the canopy to the forest floor.

That water can enter soil and later be absorbed by roots.

This is ecologically important, but it is not foliar uptake.

Fog pathway	Immediate effect
Humid air	Reduces transpiration demand
Liquid water on leaves	Can enter foliage directly
Fog drip to soil	Can later enter through roots

Part 10 — Why the Upper Crown Benefits

The highest foliage is farthest from roots and experiences strong solar and atmospheric exposure.

Direct wetting can bypass much of the long root-to-crown transport pathway and improve local water status where hydraulic stress is greatest.

That does not mean fog replaces roots. It means water can enter the plant through more than one boundary.

Part 11 — Fog Can Repair Hydraulic Function

Water stress can reduce hydraulic conductance and increase the risk of embolism in xylem.

Improving canopy water status during fog can reduce tension and help restore hydration. The details of embolism formation and repair remain an active area of plant hydraulics, so fog should not be described as a universal instant “xylem repair switch.”

The secure claim is that crown wetting improves leaf water status and alters internal flow.

Part 12 — Coastal Climate Makes the Strategy Valuable

Coast redwoods occur along a narrow Pacific coastal region where dry summers overlap with marine fog.

Fog therefore arrives when rainfall may be limited and atmospheric water stress would otherwise be high.

The timing of the resource matters as much as its annual quantity.

Part 13 — Why Climate Change Makes Fog Biology Important

Redwood water balance depends on interacting changes in temperature, drought, atmospheric demand and fog occurrence.

A change in fog timing or frequency can affect both direct water inputs and the hours during which transpiration is suppressed.

Predicting redwood responses therefore requires more than annual rainfall totals.

Part 14 — “Drinking Fog” Is a Good Hook but a Bad Final Model

The phrase is memorable because direct foliar uptake is real.

But a mature explanation separates four operations:

• intercept droplets;
• reduce atmospheric water loss;
• absorb some liquid water through foliage;
• deliver some fog water to soil through drip.

Only then can we ask how much each contributes under a particular weather event.

Someone Wet the Crown and Watched Water Move Backwards

Todd Dawson and colleagues helped establish fog as a major ecological water source in redwood forests, while later physiological work directly tested foliar uptake.

Researchers wetted foliage, measured leaf hydration and monitored branch sap flow. The appearance of reverse flow during wetting provided a striking mechanistic receipt.

observe fog → separate humidity from liquid water → wet foliage experimentally → measure hydration → measure flow direction → reconstruct the hydraulic pathway.

How Do We Know?

• Leaf-wetting experiments measure direct increases in water content.
• Water-potential measurements track hydration state.
• Sap-flow sensors detect changes and reversals in internal flow.
• Stable-isotope studies can distinguish water sources.
• Fog collectors quantify atmospheric deposition.
• Meteorological measurements connect fog with VPD and transpiration.
• Canopy studies compare upper and lower crown responses.

Observation vs Inference

Layer	Example
Observation	Fog wets redwood foliage during dry summer periods.
Measurement	Wetted leaves gain water and improve water status.
Measurement	Branch sap flow can reverse during crown wetting.
Mechanistic inference	Water absorbed by foliage can move into drier internal tissues.
Ecological inference	Fog can buffer summer hydraulic stress through several pathways.

Common Misconceptions and Repairs

Misconception	Better model
Water always moves root → leaf.	Direction follows water-potential gradients and can reverse locally.
Fog helps only by dripping into soil.	It also suppresses transpiration and can enter foliage directly.
Wet leaves prove foliar uptake.	Direct measurements of hydration and internal flow are needed.
Fog replaces roots.	Roots remain fundamental; fog supplements and buffers the hydraulic system.
All leaf uptake occurs through stomata.	Surface pathways are species-specific and can include non-stomatal routes.

Checkpoint Questions

1. Why does height increase hydraulic difficulty?
2. How does fog reduce transpiration?
3. What is foliar water uptake?
4. Why can internal water flow reverse?
5. What does reverse sap flow tell us?
6. How does fog drip differ from leaf uptake?
7. Why is the upper crown especially important?
8. Why is “redwoods drink fog” incomplete?

Apply It — Wet Leaves, Dry Soil

Two similar redwood saplings have equally dry soil. Plant A is exposed to humid air but its leaves remain dry. Plant B receives the same humid air plus liquid water on its leaves.

Predict which measurements could distinguish reduced transpiration from direct foliar uptake.

Answer Key

Open after attempting the question

Both plants may lose less water because humidity lowers VPD. Direct uptake in Plant B would be supported by greater leaf mass or water content, improved water potential beyond the humidity-only control, labelled-water movement into tissues, or reverse internal flow from wetted foliage. The control separates atmospheric demand from liquid entry.

Can You Explain WHY?

• Why can fog help before any water enters the plant?
• Why can flow direction change without changing xylem anatomy?
• Why is reverse sap flow strong evidence for foliar uptake?
• Why should fog drip and foliar uptake be measured separately?
• Why does tree height make crown wetting especially interesting?

Primary Science Bridge

• Plants need water.
• Roots usually absorb water.
• Leaves lose water to air.
• Humidity changes evaporation.
• Water can move through plant tissues.
• Experiments use controls to distinguish explanations.

Secondary / JC Resolution

School-scale idea	Higher-resolution science
Leaves lose water	VPD, stomatal conductance and transpiration
Leaves take up fog	Foliar water uptake and surface permeability
Water moves backwards	Water-potential reversal and hydraulic conductance
Fog helps trees	Canopy interception, fog drip and drought buffering
Tall trees struggle for water	Gravitational potential, path resistance and hydraulic limitation

Deep Science Window — Xylem Is a Pathway, Not an Arrow

Diagrams often put arrows on xylem because upward transport dominates under ordinary transpiring conditions.

But xylem itself does not impose one-way direction. If water-potential gradients reverse and the pathway remains conductive, flow can reverse too.

Deep Science Window — Environmental Water Changes Both Supply and Demand

Fog is unusually powerful because it can affect both sides of a plant’s water budget.

It lowers atmospheric demand and can add water directly. A resource can therefore help not only by increasing supply but also by reducing expenditure.

Evidence Boundaries

• Fog on leaves ≠ proof of uptake.
• Foliar uptake ≠ roots are unnecessary.
• Reverse branch flow ≠ the entire tree reverses simultaneously.
• One wetting experiment ≠ identical uptake across all redwood ages and canopy positions.
• Fog benefit ≠ one single pathway.
• Current fog dependence ≠ a simple prediction of future climate response.

Manual Summary — KNOW → CONNECT → EXPLAIN → APPLY → CHECK

KNOW: fog, VPD, transpiration, foliar uptake, water potential, sap flow, fog drip.

CONNECT: fog arrival → humidity rises + leaves wet → transpiration falls + liquid enters foliage → crown water status improves → internal flow can reverse.

EXPLAIN: fog changes both the atmospheric demand for water and the places where water can enter the tree.

APPLY: separate humidity-only and leaf-wetting treatments to identify direct uptake.

CHECK: keep fog interception, foliar uptake and fog drip as distinct mechanisms.

Research Sources and Further Reading

• Plant, Cell & Environment — Foliar water uptake by redwood and reverse sap flow
• US National Park Service — Redwood climate and fog