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-yearold Taxodium distichum (L.) Richar d trees growing in a flooded forest using Granier-type sensors to estimate mean canopy stomatal conductance of the stand (G(S)). Temporal Variations i n 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 pl ant water potential. We found that Gs was only weakly related to Q(o) below 500 mu mol m(-2) s(-1) (r(2)=0.29), but unrelated to Q(o) above this value . Above Q(o)=500 mu mol 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 InD with a much better fit (r(2)=0.81). The decrease of Ga with InD was at a rate p redicted based on a simple hydraulic model in which stomata regulate the mi nimum leaf water potential. Based on the hydraulic model, stomatal sensitiv ity to D is proportional to stomatal conductance at low D. A hurricane caus ed an similar to 41% reduction in leaf area. This resulted in a 28% increas e in Gs at D=1 kPa (G(Sref)), indicating only partial compensation. As pred icted, the increase in G(Sref) after the hurricane was accompanied by a sim ilar increase in stomatal sensitivity to D (29%). At night, G(Sref) was sim ilar to 20% of the daytime value under non-limiting light (Q(o)>500 mu mol m(-2) s(-1)). However, stomatal sensitivity to D decreased only to similar to 46% (both reductions referenced to prehurricane daytime values), thus ha ving more than twice the sensitivity expected based on hydraulic considerat ions alone. Therefore, non-hydraulic processes must cause heightened nightt ime stomatal sensitivity to D.