171
and sea level curves for sites around the world, Ellison and Stoddart (1991) found
that mangrove ecosystems can keep pace with sea-level rise of 8–9 cm per 100 years,
are stressed at 9–12 cm per 100 years, and cannot adjust at rates above this level.
However, Snedaker (1995) pointed out that over the last 147 years, sea level rise in
Florida has been about 20 cm (double the rate of collapse predicted by Ellison and
Stoddart 1991), and mangrove systems in Florida have not collapsed and are even
expanding in some areas. Therefore, it is critical to account for site-specific expansion of mangroves in context to sea level rise.
Individual mangrove species have varying degree of tolerances to the period,
frequency, and depth of inundation. Mangrove zones are related to shore profile,
soils, and salinity, and changes in these can lead to alteration in mangrove species
composition. Different species may be able to move into new areas at different
speeds, making some species capable of accommodating a higher sea level rise rate
than others (Semeniuk 1994). He described how mangroves in Northwest Australia
colonize new substrates that become available through erosion, inundation, and
dilution of hypersaline groundwater of the salt flats. Mangrove zones displace the
adjoining zone as sea level rises. Varying tolerances of inundation and salinity may
result in changes in mangrove species composition with changes in inundation and
salinity due to sea level rise. For example, the SELVA-MANGRO model, an integrated landscape model, was used in Florida to predict species regeneration based
on probability functions of species and community tolerance to water level and
salinity (Ning et al. 2003). In persistently inundated soils, red mangrove seedlings
were favoured, whereas in irregularly flooded soils, white and black mangrove
seedlings were favoured. This difference in seedling survival may be more due to
different tolerances of inundation than to salinity. Although Rhizophora mangle
(red mangrove), Avicennia germinans (black mangrove), and Laguncularia racemosa (white mangrove) have different salinity tolerances, their differences are only
significant for salinities >50 psu (Menezes et al. 2003). R. Mangle has the lowest
salinity tolerance, about 70 psu, of these three mangrove species (Chen and Twilley
1998). Menezes et al. (2003) concluded that pore water salinity had little to no
influence on the tree species composition on the forest level. Therefore, the differences in seedling survival may be due to higher tolerance of inundation by R. mangle. Some scientists are exploring how different functional root types of mangrove
species respond to changes in elevation to determine if certain root structures may
be more or less vulnerable to sea level rise (Vicente 1989; Krauss et al. 2003). In
the Caribbean, Vicente (1989) noted that prop roots of Rhizophora mangle stand
higher above mean sea level than the aerial roots of Avicennia germinans which
protrude only slightly out of the mud. The author suggests that rapid sea level rise
may lead to local extinctions of A. germinans but have an insignificant effect on R.
mangle. Ellison and Stoddart’s (1991) work in Tonga also suggests that Rhizophora
communities are better positioned to survive rising sea level due to higher peat
accumulation rates beneath Rhizophora (5.3 mm/year) than Bruguiera and
Excoecaria (2.6 mm/year). The different rate of peat accumulation by different
species of mangroves was also confirmed by the present author after a rigorous
study (during 2014–2018) in the Chotomollakhali Island (88°54′26.71′′ longitude
and 22°10′40.00′′ latitude) of central Indian Sundarbans (Table 6.5).
Defensive Role of Mangroves against Sea Level Rise
and sea level curves for sites around the world, Ellison and Stoddart (1991) found
that mangrove ecosystems can keep pace with sea-level rise of 8–9 cm per 100 years,
are stressed at 9–12 cm per 100 years, and cannot adjust at rates above this level.
However, Snedaker (1995) pointed out that over the last 147 years, sea level rise in
Florida has been about 20 cm (double the rate of collapse predicted by Ellison and
Stoddart 1991), and mangrove systems in Florida have not collapsed and are even
expanding in some areas. Therefore, it is critical to account for site-specific expansion of mangroves in context to sea level rise.
Individual mangrove species have varying degree of tolerances to the period,
frequency, and depth of inundation. Mangrove zones are related to shore profile,
soils, and salinity, and changes in these can lead to alteration in mangrove species
composition. Different species may be able to move into new areas at different
speeds, making some species capable of accommodating a higher sea level rise rate
than others (Semeniuk 1994). He described how mangroves in Northwest Australia
colonize new substrates that become available through erosion, inundation, and
dilution of hypersaline groundwater of the salt flats. Mangrove zones displace the
adjoining zone as sea level rises. Varying tolerances of inundation and salinity may
result in changes in mangrove species composition with changes in inundation and
salinity due to sea level rise. For example, the SELVA-MANGRO model, an integrated landscape model, was used in Florida to predict species regeneration based
on probability functions of species and community tolerance to water level and
salinity (Ning et al. 2003). In persistently inundated soils, red mangrove seedlings
were favoured, whereas in irregularly flooded soils, white and black mangrove
seedlings were favoured. This difference in seedling survival may be more due to
different tolerances of inundation than to salinity. Although Rhizophora mangle
(red mangrove), Avicennia germinans (black mangrove), and Laguncularia racemosa (white mangrove) have different salinity tolerances, their differences are only
significant for salinities >50 psu (Menezes et al. 2003). R. Mangle has the lowest
salinity tolerance, about 70 psu, of these three mangrove species (Chen and Twilley
1998). Menezes et al. (2003) concluded that pore water salinity had little to no
influence on the tree species composition on the forest level. Therefore, the differences in seedling survival may be due to higher tolerance of inundation by R. mangle. Some scientists are exploring how different functional root types of mangrove
species respond to changes in elevation to determine if certain root structures may
be more or less vulnerable to sea level rise (Vicente 1989; Krauss et al. 2003). In
the Caribbean, Vicente (1989) noted that prop roots of Rhizophora mangle stand
higher above mean sea level than the aerial roots of Avicennia germinans which
protrude only slightly out of the mud. The author suggests that rapid sea level rise
may lead to local extinctions of A. germinans but have an insignificant effect on R.
mangle. Ellison and Stoddart’s (1991) work in Tonga also suggests that Rhizophora
communities are better positioned to survive rising sea level due to higher peat
accumulation rates beneath Rhizophora (5.3 mm/year) than Bruguiera and
Excoecaria (2.6 mm/year). The different rate of peat accumulation by different
species of mangroves was also confirmed by the present author after a rigorous
study (during 2014–2018) in the Chotomollakhali Island (88°54′26.71′′ longitude
and 22°10′40.00′′ latitude) of central Indian Sundarbans (Table 6.5).
Defensive Role of Mangroves against Sea Level Rise