Figure 1: Mangrove Boardwalk at Sungei Buloh attached to the Wetland Centre allows visitors to observe mangroves (Map Source: NParks)

(Figure 1 Source)

The land and sea are drastically different entities. What happens when they clash along the coastlines? Harsh conditions ensue – tidal action, high water salinity, oxygen-poor soils, unstable mud and flooding risks. However, the mangrove ecosystem thrives at these intertidal zones. What are the special adaptations that make mangroves so resilient and important? We explored the mangroves of Sungei Buloh Wetland Reserve (SBWR) with our NUS GE3255 Aquatic, Riparian and Coastal Systems class in September 2023 (Figure 1).

Figure 2: (Left) Nipa Palm (Nypa fruticans) (IUCN: LC, Native), a palm adapted to mangrove ecosystems; (Right) Saltwater crocodile (Crocodylus porosus) (IUCN: LC, CR in SG, Native) (Photo Credit: Joan Tan 2023)

SBWR is Singapore’s largest mangrove ecosystem left. However, mangroves only constitute 0.5% of Singapore’s land now as compared to 13% in 1820s. To protect the remaining patches, SBWR was given nature reserve status in 2002. Now, SBWR is home to diverse flora and fauna (Figure 2), including mangroves with specialised adaptations.

Mangrove adaptations

Figure 3: (Left) Stilt roots of Red Mangroves (Rhizophora) (IUCN: LC, Native); (Right) Prop roots of Red Mangroves (Rhizophora) (Photo Credit: Joan Tan 2023)

Mangroves adapt to oxygen-poor soils by developing aerial roots. Pencil roots (type of pneumatophores) are another type of adaptation by Avicennia species that extend vertically from the water surface to absorb oxygen. To adapt to unconsolidated soil found in mangrove ecosystems, Rhizophora species develop stilt and prop roots. Stilt roots extend from trunk to ground while prop roots dangle from branches (Figure 3). Besides absorbing oxygen, they provide support for the mangroves, keep them upright against tidal action and cushion flood impacts.

Figure 4: Salt secreted by glands on Black Mangrove leaf (Avicennia germinans) (IUCN: LC, Not found in Singapore) (Photo Credit: Ulf Mehlig, 26 August 2006, Wikimedia Commons)

(Figure 4 Source)

Figure 5: Propagule of Rhizophora (Photo Credit: Joan Tan 2023)

Mangroves are also adapted to higher salinity levels via ultrafiltration and salt secretion. The roots of Sonneratia and Rhizophora species filter and exclude salts to reduce salt absorption while Avicennia species secrete salt through glands on leaves (Figure 4). Lastly, mangroves are adapted to reproduce effectively in this harsh environment. In Rhizophora species, the seedling and fruit form a long, green propagule (Figure 5). These propagules can drift in the water for a long time and germinate when vertically positioned.

Restoration

Mangroves are indeed valuable – no other ecosystems could survive at the coastlines like they do. They protect us against flooding and sequester carbon to slow down climate change. However, land use changes in Singapore and around the region have decimated mangrove ecosystems and it is crucial to ramp up mangrove restoration now. Vacant land near waterbodies can be used as mangrove replanting sites. Existing mangroves can be expanded by improving connectivity of surrounding waterbodies. Greater research can also improve our understanding of mangroves. For instance, previous research revealed that water conditions matter more than soils and seeds in determining successful mangrove restoration. Such findings allow us to refine restoration techniques. While restoration may be challenging, we should stand firm on our efforts and emerge above adversity to protect the environment, just like mangrove roots.

Written by: Joan Tan Yue Er