Riparian species are adapted to flooded conditions; however, saturated soils often compromise root function and reduce water availability as a result. Our study focused on understanding tradeoffs associated with potentially compromised roots within a single species. Salix mucronata is a species of willow that occurs predominately in riparian ecosystems in the Western Cape of South Africa. We examined five individuals of Salix mucronata whose stems were submerged and five individuals who were growing on an adjacent bank on the Holsloit river in the Western Cape of South Africa. We compared mid-day and pre-dawn water potentials to assess water availability and water stress. We also measured stomatal conductance over the course of a day. We compared canopy density by measuring leaf area index (LAI), and we compared leaf mass per area (LMA.) We also measured mechanical strength, specifically modulus of rupture (MOR) and modulus of elasticity (MOE). We predicted that if growing in water saturated soils compromised root function, we would see lower pre-dawn water potentials and lower stomatal conductance overall in individuals with submerged stems. We also predicted that LMA would be larger for individuals growing in water, and LAI would also be smaller because of water limitations imposed by compromised root function. We predicted lower MOR and MOE for individuals who were not submerged since lower water stress would facilitate larger vessel lumens.
Results/Conclusions
Our study found no significant differences in MOE, MOR, or predawn water potentials between individuals on land compared to those with roots submerged. However, our findings suggest individuals that occur in the river experience greater water stress than those that occur on land. Midday water potentials were more negative in individuals that were not submerged likely because of increased stomatal conductance which may increase gas exchange. Furthermore, lower canopy density in individuals with roots submerged suggest a reduced investment in aboveground biomass perhaps associated with decreased water availability. Greater LMA in individuals that occurred in the river may be associated with increased leaf thickness as a response to water stress from flooded roots. Our results suggest that while Salix mucronata may be able to tolerate flooded conditions, flooded individuals may be under more water stress than individuals growing adjacent on the bank.