Mangrove forests are well known to be some of the most productive and important ecosystems on earth, performing critical roles in maintaining the diversity and abundance of marine and terrestrial life. Yet, mangrove forests are experiencing habitat loss greater than that facing tropical rainforests and coral reefs. In the last 20 years alone, at least 35% of the world’s mangrove forests have been decimated due to increased aquaculture and coastal property development, variation in hydrology, sea-level rise, and nutrient overloading (Valiela et al., 2001 & Ye et al., 2004).
Belonging to the halophyte plant family, mangroves are tolerant to levels of salinity that only 0.2% of the worlds plant species could survive (Flowers and Colmer 2015). Able to sequester more carbon than most other forest types, mangroves aid in climate control, provide crucial nursery habitat for many juvenile fish species and by controlling erosion during natural disasters, provides protection to many coastal peoples and property (Crase et al., 2013; Das, 2017 and He et al., 2007).
Found in tropical and sub-tropical regions (around 30◦N to 37◦S), mangrove forests are distributed along coastal wetlands and in estuary environments restricted to the intertidal zone (the area between tide marks) (Feller et al., 2010 and Ye et al., 2004). With global temperatures predicted to increase by an average 2–4°C and the atmospheric CO2 range to increase to between 467 and 555ppm by 2050, changes to mangrove distribution is expected (Mizanur Rahman et al., 2014). With a change in climate comes an increase in the variability of rainfall and weather patterns, a rise in sea level from arctic melts and a significant rise in ocean salinity (Eslami-Andargoli et al., 2010 and Feller et al., 2010). Therefore, understanding the complexity of mangrove ecosystems and their ecological value is an important challenge.
Mangroves often grow parallel to shorelines in generally predictable monospecific zones. This has been the subject of great scientific interest for decades (Allen et al., 2003; Bunt, 1996; Crase et al., 2013 and Snedaker, 1982). For example, Rhizophora stylosa is primarily distributed down-stream of estuarine systems, inhabiting the mid to low region of the intertidal zone. In contrast, the sister species Sonneratia alba predominantly persists down-stream of estuarine systems, inhabiting the lowest tidal elevations (Duke, 2006 and He et al., 2007). Contrary to this, a study in tropical Australia by Bunt (1996) found that although uniformity is often found along the open coastal areas for most common mangrove species, data was highly variable in riverine estuaries. Surveys appeared to show that species distribution in the shore parallel direction are not consistent across different locations so it was concluded that sequential ordering of mangrove species in the intertidal zones of tropical Australia should not be expected.
Numerous factors have been identified as playing key roles in mangrove distribution and zonation, including physiological adaptations to flooding and salinity, anthropogenic impacts, differential propagule dispersal and predation, interspecific competition, plant succession, hydrology, inundation and responses to geomorphological processes (Allen et al., 2003 and Snedaker, 1982). Of these proposed factors, it is the interaction between the physiological adaptation of the mangrove propagule and the varying levels of tidal inundation (hydroperiod) in open coastal and estuarine habitat that is the focus of this research.
Hydroperiod is a measure of time that a certain location is inundated and is influenced by factors such as elevation, tidal frequency and amplitude (Crase et al., 2013). The variation of tidal patterns depends on location and timescale with three broadly accepted categories of semi-diurnal (two high and two low tides each day), diurnal (one high and one low tide each day) and a “mixed tide” (two uneven tides a day or one high and one low tide each day) (Ye et al., 2004). Tidal variations and frequency of inundation expose mangrove species to a range of different conditions. In response to this, each species have evolved a wide range of adaptations to better ensure their survival (Allen et al., 2003). However, the evolution of these adaptations may also be restricting the distribution of each species. In an experiment by He et al. (2007) the flooding-tolerance of four mangrove species in the Beibu Gulf was measured. Different growth strategies among all four species were observed. Pioneer species, Avicennia marina and Aegiceras corniculatum appeared better adapted to a wide range of habitats while in comparison, R. stylosa and Bruguiera gymnorrhiza displayed an increase in stem elongation and biomass accumulation. Although all four species have evolved individual adaptations to survive the stress of flooding, each adaptation seemed to lead to a preference in habitat selection.
Propagule establishment among mangrove species is strongly influenced by the frequency and duration of inundation (Ball et al., 1999) Although persistent high water levels have been observed to decrease root/shoot biomass ratio in some species (Ye et al., 2004). Research conducted by Ye et al., (2004) simulated varying tidal patterns and measured the response of mature viviparous propagules of Kandelia candel and Bruguiera gymnorrhiza collected from Sai Kung mangrove forest in Hong Kong. Results showed that K. candel had faster establishment rates in prolonged high tide simulations, while the establishment rates of B. gymnorrhiza did not appear to be significantly influenced by tidal pattern. These results suggest that K. candel may have a greater ability to persist in lower tidal zones than B. gymnorrhiza. Both species develop short prop-roots but K. candel has a highly adapted root and oxygen transport system that allows them to avoid an oxygen deficit (Adame et al., 2010). The research that follows aims to investigate what role, if any, varying tidal patterns, hydroperiod (duration of inundation) and physiological adaptations play in the zonation of mangroves within the intertidal zone. It is expected that propagule establishment will vary between species and will depend on the adaptations that each species has evolved to withstand the effects of prolonged inundation and a variability of tidal sequences.
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Ye, Y., Tam, N. F., Wong, Y. S. and Lu, C. Y., (2004). Does sea level rise influence propagule establishment, early growth and physiology of Kandelia candel and Bruguiera gymnorrhiza? Journal of Experimental Marine Biology and Ecology, 306(2), 197-215.