Does increased salinity reduce functional depth tolerance of four non-halophytic wetland macrophyte species?

•Plant productivity declined as water depth increased in all four study species.•Depth tolerances decreased as salinity increased for the three emergent species.•We conclude that wetland salinisation could affect macrophyte depth zonation.

Rapid growth in height is an important mechanism used by many emergent wetland macrophytes to withstand water depth increases, particularly in species unable to maintain sufficient rates of photosynthesis and gas exchange for long-term survival underwater. However, increases in salinity can reduce growth rates and above-ground biomass production in non-halophytic macrophytes and this may reduce their inundation tolerance. We tested this hypothesis by comparing growth responses of Cynodon dactylon (L.) Pers, Paspalum distichum L., Eleocharis equisetina C.Presl and Bolboschoenus caldwellii (V.J.Cook) Soják at three depths (5, 20 and 60 cm) across four salinity treatments (200, 2500, 5000 and 10 000 mg L−1). Increases in depth had negative effects on the growth of all four species. The three emergent wetland macrophyte species (P. distichum, E. equisetina and B. caldwellii) grew more rapidly, produced more above-ground biomass, and/or maintained positive growth rates at greater depths in the lower salinity treatments than at higher salinities. The terrestrial grass species, C. dactylon, displayed negligible growth when waterlogged and where biomass decreased significantly with depth, there were no significant differences in biomass between the salinity treatments. We conclude that increases in salinity reduced the ability of the three emergent wetland macrophyte species to withstand increases in water depth. The potential depth ranges of these species are therefore likely to change within wetlands if salinisation occurs. Specifically, the habitat ranges of these species are likely to contract and shift towards the shallower, less-frequently flooded limits of their current ranges as salinity levels become limiting to growth.