Diurnal water relations and the salt content of two contrasting mangroves growing in hypersaline soils in tropical-arid Australia

Abstract

Diurnal water relations and salt content of the salt-secreting grey mangrove Avicennia marina (Forsk.) Vierh. and the non-secreting, spurred mangrove, Ceriops tagal (Perr.) C.B. Rob., growing on the arid north-west coast of Australia were examined in different seasons at two landward, hypersaline sites. Water use by Avicennia was similar at both locations despite different soil texture (sand loam and clay) and different community structure (closed, low woodland and low open scrub). Transpiration rates (T) of Avicennia were high (daily maxima 32–91 mg dm^-2 min^-1) despite high soil salinity (65–85‰). Avicennia displayed consistently higher T, higher stomatal conductance (SC) and lower midday xylem pressure potential (XPP) than Ceriops experiencing the same environmental conditions. T was greater, and mid-day XPP was lower in summer than in winter for both species. Differences in Tbetween co-existing Avicennia and Ceriops were related to differences in stomatal frequency. Increasing light, air temperature and vapour pressure deficit (VPD) over the morning were accompanied by increasing T and lowering XPP. T and XPP reached their respective extremes at the same time of day, usually at mid-day. Pre-dawn XPP of both species was consistently low (– 3 to – 4MPa) on account of high soil salinity, and decreased to low values (– 6 to – 8MPa) by mid-day. XPP always recovered close to pre-dawn values after sunset. Seasonal variation in SC was small. The diurnal patterns of SC for both species were similar on a given day, commonly displaying a gradual closure over the course of the day following a mid-morning maximum, or remaining low but stable over the day before closing in the evening. The relationship between diurnal SC and VPD was variable on different days within the same tidal recharge cycle and at different times of the year. Rates of salt secretion from leaves of Avicennia were high and the salt content of both species was high (overall range 0.7–1.8M in leaf sap and 0.2-0.6M in stem sap) in hypersaline soils (0.8–1.8M Cl^-). High T, seasonally stable pre-dawn XPP, consistently rapid recovery of XPP after mid-day and high rates of salt secretion were features indicative of readily available water supplies and salt stress avoidance in an environment where evaporation is up to ten-fold higher than precipitation and the water table is close to the surface and chronically hypersaline.