Abstract
The continuous degradation of mangrove habitats has encouraged governments and multi-lateral agencies to undertake rehabilitation initiatives to foster the recovery and biodiversity of these areas. However, some rehabilitation initiatives suffer high mortality because of incorrect species-site matching and failure to recognize the ecophysiology of mangrove species. This study investigated the effects of salinity, water depth and inundation on the growth, biochemical stress responses, and ecophysiology of Rhizophora stylosa in greenhouse conditions. Propagules were cultured in aquarium tanks and irrigated with low (0 ppt), moderate (20 ppt), and high (35 ppt) salinity treatments. In the first setup, the seedlings were cultured in aquarium tanks and arranged on the top of a platform at different elevations, subjecting the seedlings to flooding with low-water (3–5 cm), mid-water (10–13 cm) and high-water (30–33 cm) levels for ten months. In another setup, the seedlings were cultured for 15 months at the low-water level and subjected to inundation hydroperiods: semi-diurnal, diurnal and permanent inundation for one week. These microcosms simulated emerged and submerged conditions, mimicking intertidal inundation that seedlings would experience. The results showed that salinity significantly affected the early development of the cultured seedlings with higher growth rates and biomass at low and moderate salinity than those at high salinity. Levels of reactive oxygen species (ROS) and antioxidant activities (AOX) were significantly lower in the emerged condition than those in an inundated condition. Inundation imposed a higher-degree of stress than that of the salinity effect, with prolonged inundation caused sublethal damage (chlorotic leaves). Furthermore, inundation caused the reduction of photosynthetic pigments and fluorescence, dependent on salinity. Extrapolating the ecophysiology of R. stylosa, this species had low tolerance to inundation stress (high ROS and AOX, reduced pigments). Translating this low tolerance to field conditions, in the frequently inundated areas (i.e., seafront mangrove fringes) that are subjected to longer inundation at spring tides, this species may suffer from oxidative stress, stunted growth and consequently low survival.
Highlights
Human exploitation and conversion of natural ecosystems are causing widespread habitat loss and degradation, which translate into a loss and decline in biodiversity and ecosystem services [1]
Integrating the ecophysiology in the mangrove forest ecology provided favorable conditions for the growth rate of cultured seedlings over 10 months (Fig 1B), whereas the water depth and the interaction of salinity and water depth had no effect based on the 2-way ANOVA (Table 1)
The optimum growth rate was observed in the moderate and low salinity cultured seedlings with average heights of 23.2±1.88 and 23.4±1.59 cm, respectively, whereas the high salinity cultured seedlings had an average height of 17.67±1.56 cm
Summary
Human exploitation and conversion of natural ecosystems are causing widespread habitat loss and degradation, which translate into a loss and decline in biodiversity and ecosystem services [1]. Rehabilitation programs have received significant interest as a tool to restore damaged habitats, the results include stories of mixed successes and failures [6, 7]. These efforts were often unsuccessful, because of the high mortality of the seedlings due to the failure to recognize the species-specific environmental tolerances and thresholds [5, 8, 9]. Located at the interface between the land and sea, the survival of mangrove seedlings are under the continuous influence of different environmental drivers salinity and tidal inundation. A gap in the knowledge remains linking the mangrove ecophysiology and biochemical stress responses with the environmental drivers
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