Senior advisor of Wetlands International Indonesia in Bogor
As we prepare to celebrate International Day for the Conservation of Mangrove Ecosystems on 26 July, harrowing aerial pictures of large swathes of dead mangroves in the Gulf of Carpentaria, Australia, have emerged on news platforms around the world.
The massive dieback of mangrove forests may have been reported in the past but the latest Australian incident is considered unprecedented due to its mammoth 10,000 hectare (ha) scale and the apparent absence of natural or man-made disaster to cite as the cause. The size of the dieback in Australia is equivalent to the stretch of land from Ujung Kulon in Banten to Cirebon in West Java.
The case serves as a loud alarm for Indonesian scientists and policy makers. Indonesia shares many mangrove habitat characteristics with Australia, their importance for the national economy, and the fact that both countries disproportionately harbor the largest portion (23 percent) of the world’s mangrove forests.
It is worth contemplating the mangrove dieback in Australia and considering why Indonesia should prepare for the possibility of a similar disaster occurring in Indonesia.
Various studies have found factors that could cause mangrove death, including water pollution, chemical defoliant, pest outbreaks, hydrological triggers, and high or frost level temperatures.
How mangroves expand and die
Most of the mangroves in the Gulf of Carpentaria coast formed at the narrow fringe of an emerged flood plain, especially along the shores of tidal estuaries. Species that stand out and quickly grow in this environment are those with wide range salinity tolerance and high dispersal ability seedlings (propagule) such as Avicennia and sometimes Sonneratia.
These species have limiting factors; the pencil or knee-like pneumatophore (breathing roots) only grow to around 30 cm, meaning that persistent flooding above 30 cm could severely weaken or kill the species.
Species with small breathing roots are also sensitive to fine sediment and substances such as oil that could clog root pores, effectively suffocating the trees.
Species with high prop-up breathing roots like Rhizophora, also found in the region, withstand persistent inundation and sediment cover, but their expansion is bound by decreased seedling dispersal ability. The species is also less tolerant of extreme salinity change.
The combination of physical coastal features like those found in the Gulf of Carpentaria paired with mangrove root sensitivity makes the species prone to dieback. However, past cases did not lead to dieback on such a massive scale.
Initial assessment by scientists at James Cook University in Australia (JCU) suggests that the mangrove dieback occurred between November – December last year and may have been triggered by extremely poor rainfall and high temperatures in the region. This is explained by meteorology records which show the extreme conditions faced by the region in 2015 due to El Nino.
What is missing in the assessment is information highlighting that the harsh environmental conditions were coupled with an unusually long period of low tides that prolonged scorching sun and sediment exposure.
As the result, sediment temperature could have increased, causing hypersalinity and toxic conditions in stagnant water, turning the 700 kilometers fringe coast into a dangerous place for mangrove forests. Unfortunately, this scenario may only partially represent the situation in the region last year. Until now, scientists struggle to understand the exact mechanism that led to the dieback.
Should Indonesia be worried?
The massive mangrove dieback in Australia was rightly described by Professor Norm Duke of the JCU as the “unprecedented”. This is partly because current climate-mangrove research is predominantly focused on slow, large-scale mangroves disappearance triggered by rising sea levels and or their potential destruction due to storms and cyclones. The sudden-yet-large-scale dieback due to extreme rainfall, temperature, and tidal period, was not anticipated.
Understanding and preventing such tragedy is of paramount importance for Indonesia for two reasons.
Firstly, the same habitat characteristics are found all over Indonesia, particularly along the east coasts of Sumatra, north coast of Java, east coast of Kalimantan and south coast of Papua.
Although relatively rare and far smaller in scale, mangrove dieback triggered by similar mechanisms have been known to occur in Indonesia in the past, including the famous Cilacap case reported by Soerjanegara ( 1968 ). Therefore, it is hard to declare that the 10,000 ha mangrove dieback will be an isolated incident.
Secondly, mangroves are the backbone of US$ 3.2 billion per year brackish water aquaculture industries. The livelihoods of two million coastal communities across Indonesia directly depend on (MMAF 2015). The economic value skyrockets when other mangrove ecosystem services like coastal protection and carbon emissions are included in the calculation.
Indonesian scientists and policy makers should follow the situation in Australia, reposition mangrove-climate studies to anticipate sudden impact, speed up efforts to map vulnerable areas across Indonesia and revisit the government decision to shut down its 128 tidal monitoring station services.
The author, a senior advisor of Wetlands International Indonesia in Bogor, is currently researching mangrove management strategy for his PhD degree at the University of Queensland Australia. The views expressed are his own.
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