Sensitivity of mean canopy stomatal conductance to vapor pressure deficit in a flooded Taxodium distichum L. forest: hydraulic and non-hydraulic effects.

We measured the xylem sap flux in 64-year-old Taxodium distichum (L.) Richard trees growing in a flooded forest using Granier-type sensors to estimate mean canopy stomatal conductance of the stand (G _S ). Temporal variations in G _S were investigated in relation to variation in vapor pressure deficit (D), photosynthetic photon flux density (Q _o ), and the transpiration rate per unit of leaf area (E _L ), the latter variable serving as a proxy for plant water potential. We found that G _S was only weakly related to Q _o below 500 µmol m ^-2 s ^-1 (r ² =0.29), but unrelated to Q _o above this value. Above Q _o =500 µmol m ^-2 s ^-1 and D=0.6 kPa, G _S decreased linearly with increasing E _L with a poor fit (r ² =0.31), and linearly with lnD with a much better fit (r ² =0.81). The decrease of G _S with lnD was at a rate predicted based on a simple hydraulic model in which stomata regulate the minimum leaf water potential. Based on the hydraulic model, stomatal sensitivity to D is proportional to stomatal conductance at low D. A hurricane caused an ~41% reduction in leaf area. This resulted in a 28% increase in G _S at D=1 kPa (G _Sref ), indicating only partial compensation. As predicted, the increase in G _Sref after the hurricane was accompanied by a similar increase in stomatal sensitivity to D (29%). At night, G _Sref was ~20% of the daytime value under non-limiting light (Q _o >500 µmol m ^-2 s ^-1 ). However, stomatal sensitivity to D decreased only to ~46% (both reductions referenced to pre-hurricane daytime values), thus having more than twice the sensitivity expected based on hydraulic considerations alone. Therefore, non-hydraulic processes must cause heightened nighttime stomatal sensitivity to D.