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

Abstract

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 (r²=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 (r²=0.31), and linearly with lnD with a much better fit (r²=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.