Increasing fluctuations of soil salinity affect seedling growth performances and physiology in three Neotropical mangrove species

Micro-tidal wetlands are subject to strong seasonal variations of soil salinity that are likely to increase in amplitude according to climate model predictions for the Caribbean. Whereas the effects of constant salinity levels on the physiology of mangrove species have been widely tested, little is known about acclimation to fluctuations in salinity. Aims and methods The aim of this experiment was to characterize the consequences of the rate of increase in salinity (slow versus fast) and salinity fluctuations over time versus constant salt level. Seedling mortality, growth, and leaf gas exchange of three mangrove species, Avicennia germinans, Laguncularia racemosa, and Rhizophora mangle were investigated in semicontrolled conditions at different salt levels (0, 685, 1025, and 1370 mM NaCl). Results Slow salinity increase up to 685 mM induced acclimation, improving the salt tolerance of A. germinans and L. racemosa, but had no effect on R. mangle. During fluctuations between 0 and 685 mM, A. germinans and R. mangle were not affected by a salinity drop to zero, whereas L. racemosa took advantage of the brief freshwater episode as shown by the durable improvement of photosynthesis and biomass production. Conclusions This study provides new insights into physiological resistance and acclimation to salt stress. We show that seasonal variations of salinity may affect mangrove seedlings’ morphology and physiology as much as annual mean salinity. Moreover, more severe dry seasons due to climate change may impact tree stature and species composition in mangroves through higher mortality rates and physiological disturbance at the seedling stage

Seasonality effect on physiology and Growth performances of Caribbean mangrove tree species

Micro-tidal wetlands are subjected to strong seasonal rainfall variations that induce fluctuations in groundwater salinity. In Caribbean mangroves and swamp forests, top-soil salinity may fluctuate by 20 ‰ between wet and dry season. Moreover, climate change is expected to increase seasonal variation by decreasing the amount of rainfall during the dry season. Whereas the effects of salinity levels on the physiology of mangrove seedlings have been widely tested, little is known about the effect of seasonal variations on growth performances of mangrove and swamp forest trees. In this study, we monitored tree physiology responses to climatic seasonal variations on the 4 main tree species of coastal forested wetlands in Guadeloupe. Physiological leaf traits and percent loss of hydraulic conductivity have been monitored on 50 trees during a wet and a dry season in 5 study sites. In addition, variation in trunk circumference of 90 trees, water-table level and groundwater salinity were monitored monthly during 20 months. Decrease of water table and increase of salinity during the dry season were associated to a strong decrease in diameter increment for all species. Leaf water potentials, stomatal conductance and carbon assimilation decreased for all species during the dry season, A. germinans being the most resistant species. At the end of the dry season, all mangrove species reached high embolism levels, L. racemosa and A. germinans being respectively the most and the less sensitive species. These results suggest that stronger dry periods due to climate change may impact mangrove structure and species-specific composition, leading to a possible decline of L. racemosa abundance in Caribbean mangroves