Hydrological Classification, a Practical Tool for Mangrove Restoration

Abstract

Mangrove restoration projects, aimed at restoring important values of mangrove forests after degradation, often fail because hydrological conditions are disregarded. We present a simple, but robust methodology to determine hydrological suitability for mangrove species, which can guide restoration practice. In 15 natural and 8 disturbed sites (i.e. disused shrimp ponds) in three case study regions in south-east Asia, water levels were measured and vegetation species composition was determined. Using an existing hydrological classification for mangroves, sites were classified into hydrological classes, based on duration of inundation, and vegetation classes, based on occurrence of mangrove species. For the natural sites hydrological and vegetation classes were similar, showing clear distribution of mangrove species from wet to dry sites. Application of the classification to disturbed sites showed that in some locations hydrological conditions had been restored enough for mangrove vegetation to establish, in some locations hydrological conditions were suitable for various mangrove species but vegetation had not established naturally, and in some locations hydrological conditions were too wet for any mangrove species (natural or planted) to grow. We quantified the effect that removal of obstructions such as dams would have on the hydrology and found that failure of planting at one site could have been prevented. The hydrological classification needs relatively little data, i.e. water levels for a period of only one lunar tidal cycle without additional measurements, and uncertainties in the measurements and analysis are relatively small. For the study locations, the application of the hydrological classification gave important information about how to restore the hydrology to suitable conditions to improve natural regeneration or to plant mangrove species, which could not have been obtained by estimating elevation only. Based on this research a number of recommendations are given to improve the effectiveness of mangrove restoration projects.

Mangrove area and propagule number planting targets produce sub-optimal rehabilitation and afforestation outcomes

“…Individual species have differing tolerances to specific biogeochemical factors and gradients present across the intertidal area (Saenger, 2002). These include salinity, soil type, soil anoxia, sulphate levels, nutrient levels, pH, wave energy, temperature, light levels, inundation regimes (Alongi, 2009;Tomlinson, 2016;van Loon et al, 2016), tides and wind distribution of propagules and seeds (van der Stocken et al, 2012), and species-selective predation by herbivores (Elster, 2000;Sousa et al, 2003). Species therefore exhibit differing 'preferences' for elevation and location within the intertidal zone (Duke, 2006;Snedaker, 1982;Tomlinson, 2016).…”

“…These features greatly affect planting outcomes (Aung et al, 2011;Elster, 2000;Hashim et al, 2010;Kairo et al, 2001;Lewis, 2005). Duration of inundation is particularly important (van Loon et al, 2016).…”

Improved estimates of mangrove cover and change reveal catastrophic deforestation in Myanmar

“…Other studies have demonstrated the importance of reporting gross land cover statistics when evaluating change dynamics, including mangroves (Thomas et al 2017, Estoque et al 2018, Gaw et al 2018, De Alban et al 2019. This is because gross land cover change estimates provide essential information on transitions among land cover classes unavailable through net change studies, leading to a more robust analysis of the drivers of land cover change (Pontius et al 2004, Aldwaik and, which is especially important for relatively dynamic mangrove communities since they are amenable to rapid deforestation (loss) but can also rapidly regenerate (gain) when biophysical conditions are appropriate (Lewis 2005, Loon et al 2016.…”

Determining hydrological flow paths to enhance restoration in impaired mangrove wetlands

“…The modification of the topography may increase the strength of the tidal ebb and flow as topographic levels are reduced [33]. In Zone II, the hydrological gradient produced by tides and by the internal circulation caused by the wind spontaneously carved secondary tidal channels <1 m in width and <0.5 m in depth.…”

“…Some proposed using hydraulic modeling to simulate flooding to perform hydrological and sedimentological restoration [32], while others considered the presence of mangrove species to classify the hydrological patterns and to make decisions on restoration actions [33]. Moreover, strategies aimed to improve mangrove recruitment have been applied.…”