Leaves Remember the Air They Grew In

01The evidence

• The functional significance of the stomatal size to density relationship has been shown to depend on atmospheric carbon dioxide, with the trade-off shifting in ways that reflect developmental rather than instantaneous conditions.Science of the Total Environment
• Stomatal acclimation to CO2 is routinely observed in growth experiments, and the acclimated state persists after transfer, which is a memory effect by definition.Stomatal acclimation

02The tension

Vegetation models treat stomatal conductance as an instantaneous function of current CO2 and humidity. That works for short-term flux and fails for long-term change, and the failure is absorbed into parameter tuning rather than modelled as a lag.

03The theory

Stomata are formed during leaf expansion and their density is largely fixed once the leaf matures. The developmental decision is made in response to the CO2 environment at that time, so the mature leaf carries the conditions of its own construction.

The consequence is a one-generation lag. A leaf that developed at low CO2 has high stomatal density and continues to transpire as if the air were still low, even after CO2 rises. The canopy's conductance reflects the air of the previous growing season, not the current one.

The prediction is that canopy conductance should lag CO2 change by one leaf generation, and that this lag should be visible in flux records as a hysteresis loop rather than a single relation. It also predicts that plants grown at high CO2 and moved to low should behave like high-CO2 leaves until new leaves form.

The implication for models is that conductance should be a function of the CO2 at leaf formation, which requires tracking a memory state rather than evaluating a curve.

04What this is not

05Predictions

• P1Canopy conductance should lag CO2 by one leaf generation, producing hysteresis in flux relationships.
• P2Leaves transferred between CO2 levels should retain the conductance of their growth environment until replaced.
• P3A model with a memory state should outperform an instantaneous model on multi-year flux records.

06What would refute it

Conductance that adjusts to current CO2 within days, independent of leaf age, would restore the instantaneous model.

07What it would change

It would add a memory state to vegetation models, and change how forest carbon uptake is projected.