Phreatophytes under stress: transpiration and stomatal conductance of saltcedar (Tamarix spp.) in a high-salinity environment

Background and aims

We sought to understand the environmental constraints on an arid-zone riparian phreatophtye, saltcedar (Tamarix ramosissima and related species and hybrids), growing over a brackish aquifer along the Colorado River in the western U.S. Depth to groundwater, meteorological factors, salinity and soil hydraulic properties were compared at stress and non-stressed sites that differed in salinity of the aquifer, soil properties and water use characteristics, to identify the factors depressing water use at the stress site.

Methods

Saltcedar leaf-level transpiration (E_L), LAI, and stomatal conductance (G_S) were measured over a growing season (June–September) with Granier and stem heat balance sensors and were compared to those for saltcedar at the non-stress site determined in a previous study. Transpiration on a ground-area basis (E_G) was calculated as E_L?×?LAI. Environmental factors were regressed against hourly and daily E_L and G_S at each site to determine the main factors controlling water use at each site.

Results

At the stress site, mean E_G over the summer was only 30 % of potential evapotranspiration (ET_o). G_S and E_G peaked between 8 and 9 am then decreased over the daylight hours. Daytime G_S was negatively correlated with vapor pressure deficit (VPD) (P?<?0.05). By contrast, E_G at the non-stress site tracked the daily radiation curve, was positively correlated with VPD and was nearly equal to ET_o on a daily basis. Depth to groundwater increased over the growing season at both sites and resulted in decreasing E_G but could not explain the difference between sites. Both sites had high soil moisture levels throughout the vadose zone with high calculated unsaturated conductivity. However, salinity in the aquifer and vadose zone was three times higher at the stress site than at the non-stress site and could explain differences in plant E_G and G_S.

Conclusions

Salts accumulated in the vadose zone at both sites so usable water was confined to the saturated capillary fringe above the aquifer. Existence of a saline aquifer imposes several types of constraints on phreatophyte E_G, which need to be considered in models of plant water uptake. The heterogeneous nature of saltcedar E_G over river terraces introduces potential errors into estimates of ET by wide-area methods.