Effects of Treated Effluent Discharge on Tree Growth and Water use in a Cypress Swamp

Abstract When municipal effluent is discharged into swamps, baldcypress trees ( Taxodium distichum ) generally respond with increased growth. This growth increase may be associated with increased transpiration (E), but if functional traits are also affected, they may modulate the response of E to environmental conditions such as atmospheric vapor pressure deficit (VPD). We measured tree growth and sap flux to assess water-use traits and daily E in mature baldcypress trees that received effluent discharge and compared them to nearby trees that did not receive effluent. Basal area increment and foliar N:P were higher in trees that received effluent. For any given tree size, E was 5.8 L day −1 higher in trees receiving effluent. Sap-flux-based whole-tree canopy conductance at the reference VPD of 1 kPa and its sensitivity to VPD were both positively related to foliar N:P, suggesting that effluent discharge shifted trees to higher water use when VPD was relatively low, but that water use was more sensitive to VPD. Our results suggest that when effluent discharge increases forested wetland productivity it can also increase E, but these effects may be diminished by high VPD.