Coastal erosion is the herald of sea level rise. Mangrove restoration is promoted as ideal response to this problem: It’s cheap, does not harm the environment and mangroves are even natural land-builders. But can mangrove ecosystems grow in tandem with the sea? Opportunities and limits of this adaptation strategy are tracked for the South Pacific island region against the background of climate change.
Coastal erosion – The herald of sea level rise
South Pacific islands are exposed to a high erosion risk by sea-level rise; 16% of land area of South Pacific islands is in low elevation coastal areas (<10 m). Currently, 70% of the coast of South Pacific islands are erosive (Nurse et al., 2013).). Many Pacific islands have small land mass, so that people have limited space to migrate landwards (Gilman et al., 2006). 50% of the population live in coastal areas (Russell, 2009). Adaptation strategies against coastal erosion are hence vital for this region.
Paradigm shift in coastal engineering
Artificial coastal engineering methods like seawall got a bad reputation because they are expensive and have detrimental erosive effects on adjacent ecosystems (Cheong et al., 2013, Nunn, 2013). This lead to a paradigm shift in coastal engineering (Cheong et al., 2013; Duarte et al., 2013). In the last years, mangrove restoration is promoted as cheap ecosystem-based alternative: Gilman and Ellison (2007) calculated that a 12 months restoration project cost them only USD $2,150. Mangrove restoration therefore a tempting adaptation strategy for developing countries with limited financial resources.
How do mangroves protect against the ocean?
As intertidal habitat mangrove ecosystems are a natural buffer zone between land and sea. Mangrove ecosystems reduce the wave energy by 50% and stabilize shorelines with their roots (Alongi, 2008, Lewis, 2005). This protects against abrupt shoreline erosion after storm or tsunami events (Ellison et al., 2012). As mangroves are highly tolerant to fluctuating conditions, they colonize rapidly gaps after storm events (Alongi, 2008). Mangroves are also land builders: During tidal inundation and after storms and heavy rainfalls, substrates from adjacent coral reefs and rivers are carried to mangrove forests (Gilman et al., 2007). Mangrove roots slow the water movement so that fine clay and silt particles settle down the water column (Alongi, 2008).The accretion increases the local surface elevation. The short and mid time benefits of mangroves are uncontested. Optimistic scientists expect that mangroves allow islands to in rise ‘in tandem with the sea’ (Pala, 2014, p. 496).
Mangrove ecosystems under pressure
South Pacific mangrove forests occupy an area of 5,975 km2 with large areas in Papua New Guinea, Solomon Islands, Fiji and New Caledonia (Ellison, 2009). But they are under pressure. Overpopulation on capital islands lead to a rapid deforestation and pollution of mangrove ecosystems (Donato et al., 2012). Streets cut mangrove forests from freshwater input. Dredged sediments reduce the substrate source for mangroves (Allen et al., 2001). Moreover, artificial coastal control systems like seawalls reduced the longshore sediment transport so that coastal zones turned to deepwater habitat (Gilman et al., 2006). When mangroves did not recover naturally, the ecosystems need to be restored.
The success story of mangrove restoration
A success story of marine conservation is the Locally Managed Marine Area Network (LLMA) on Fiji Islands. Launched in 1997, the LMMA developed a decentralized village-based structure with over 250 marine protected areas. 25% of Fiji’s coastline are protected by this governance regime (UNDP, 2012). The project success relies on a good cooperation with local communities. Protected areas have an open-access regime with flexible rules for subsistence. Locals are involved in decision-making and in the conservation work. The project benefits also from the strong stewardship for nature of indigenous groups: Often LLMA areas are better protected than government-owned reserves. In 2002, the LLMA project was rewarded as case of best practice with the ‘Equator Initiative Award’ of the United Nations Development Programme (Techera, 2010).
Vulnerability of South Pacific mangroves to sea level rise
The natural pressure on mangrove ecosystems is growing with climate change. On long-term, mangrove sedimentation rates must exceed the rate of relative sea-level rise to hold pace with relative sea level rise. The sedimentation rate is defined as balance of net erosion relative to accretion rates (Gilman et al., 2007). When sea level rise is faster than the sedimentation rate, the seawards margins of mangrove forests continue to erode and retreat landwards (Figure 1).
Figure 1: Sea level rise relative to mangrove surface (Source: Gilman et al., 2006, p. 10)
Two factors make mangrove ecosystems of Pacific Islands vulnerable to sea-level rise: First, many have no significant sediment input from rivers or continental longshore drift (Alongi, 2008). Sedimentation relies on the supply of organic detritus and material from coral reefs (Gilman et al., 2006, Ellison et al., 2012, Alongi, 2008). Second, most Pacific islands have shallow slopes with tidal range of less than 1 m. Mangrove forests in Fiji, Marshall Islands, Federal State of Micronesia, Tonga and Tuvalu have today negative sediment budget balance. They experience a loss of land area. Only forests on Samoa and Solomon Islands have sediment budget balances over the projected sea level rise.
Mangroves’ response to climate change
Yet, it is uncertain how mangroves respond to altered climate conditions: On one hand, the plant productivity and thus the accumulation of organic material can increase with higher atmospheric CO2 levels and precipitation levels (Ellison and Fiu, 2010; Gilman et al., 2006). One the other hand, rising water temperatures can lead to temperature stress for mangroves which support only water temperatures up to 24°C (Ellison and Fiu, 2010). Either, waves deposit more organic material from adjacent ecosystems or degrading coral reefs cut mangroves from sediment supply (Gilman et al., 2006; Ellison and Fiu, 2010).
Strong regional variance of sea level rise
The Pacific Island region shows a strong regional variance of relative sea level rise: First, the Pacific Island region is characterized by the El Niño Southern Oscillation (ENSO). This natural ocean cycle causes a regional variability of sea level in the order of ± 20-30 cm (Nurse et al., 2013). Second, the region is shaped by strong tectonic plate movements: The Pacific plate subducts beneath the Australian plate to the east of Tonga, Fiji and Vanuatu and north of the Solomon Islands and Papua New Guinea (Ellison, 2009). Subsiding areas at the east of this margin experience a higher rate of relative sea-live rise than stable areas or uplifting regions (Ellison and Fiu, 2010). Due to the high regional variance, the appropriateness of mangrove restoration as adaptation strategy is very site dependant.
Climate smart restoration
Climate smart restoration must therefore select restoration sites carefully to gain long term success. Unfortunately, fieldwork is often rather an uncoordinated quick fix approach. Seedlings are planted like in gardens in the hope that it works against erosion. Stressors which inhibit the natural regeneration of mangroves are not removed. (Nunn, 2013). The selection of ‘climate smart species’ (Ellison et al., 2012, p. X) is also crucial for project success: Plants of the genus Rhizophora are more tolerant to the salinity of ocean water (Ellison, 2007a). This makes them on long term more resilient to sea-level rise. After several years, it must be monitored, if young trees are able to gain full habitat functions.
Adaptation in a situation of uncertainty
It is difficult to prognosis if mangroves of the South Pacific Island region can hold pace with sea level rise. Mangroves cannot reverse long-term erosion trends – especially not when worst case emission scenarios become true. But mangrove restoration is at least a low-cost mid-term adaptation strategy which gives time to retreat in a situation of uncertainty. The limits of this adaptation approach must be well communicated to local population to prevent maladaptation and a false sense of security.
© 2014 Heike Huntebrinker. All rights reserved.