A taste for salt: Physiological traits of mangrove species and communities vary non-linearly across a salinity gradient

Background/Question/Methods

The inter-relations between leaf traits, plant physiology and environmental conditions are central to understanding ecosystem responses global change. Although databases of plant traits have grown extensively in recent years, very little data exists on the response of plant traits to salinity. The Daintree River (Far North Queensland, Australia) supports a global mangrove diversity hotspot. We measured leaf gas exchange and leaf traits of 18 dominant mangrove species across the entire salinity range, from nearly freshwater conditions upriver to hyper-saline flats near the river mouth. Several species occur at multiple salinity levels, allowing us to distinguish between species-level and community-level patterns.

Results/Conclusions

Community-level gas exchange did not vary across salinity. Effects of salinity on the Leaf Economic Spectrum (LES) were entirely explained by a strong increase in Leaf Mass per Area (LMA) with salinity, suggesting no impact of salinity on photosynthetic biochemistry. The community average responses can mask individual species responses and reflect changes in community composition instead of physiological adaptations. Individual species responses show a non-linear response to salinity with greater rates of gas exchange at intermediate salinities. In contrast, photosynthetic water use efficiency showed a trend towards being greatest at the extremes of the range, being higher in the hypersaline flat and for some species upriver. Many species show a preference for intermediate salinities and the cross-community lack of response to salinity is primarily due to species turn-over across a salinity gradient.