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Ganl. Bull. Singapore 70 (Suppl. 1) 2018 
the nutrients present in the water source and the flooding regime of the forest (Junk 
et al., 2011). For example, the nutrient-rich varzea forests in the lower Amazonian 
floodplains experience flooding caused by swollen rivers and have the highest species 
richness among wetland forests in the world (Junk et al., 2011). Alternatively, the 
vegetation of Brazilian blackwater igapo forests is dependent on heavy rains that 
determine the forests’ flood height and duration. Igapo forests generally have fewer 
tree species than varzea forests and few herbaceous plant species (Junk et al., 2011). 
Furthermore, floating plants are rare or absent in this forest type. Freshwater swamp 
forests are generally less diverse floristically if compared to dryland forests and, as a 
result, are dominated by one or a few tree species (Corlett, 2009). Often the dominant 
plant types are used to categorise freshwater swamp forests and the four main types 
recognized in Southeast Asia are (1) mixed swamp forest; (2) Melaleuca species 
(Myrtaceae) swamp forest; (3) Terminalia L. species (Combretaceae) swamp forest; 
and (4) Campnosperma Thwaites species (Anacardiaceae) swamp forest (Goltenboth 
et al., 2006). 
Floristically, freshwater swamp forests are not easily distinguishable from 
dryland forests at the taxonomic levels of family and genus (Whitmore, 1984). There 
are few plant species restricted only to freshwater swamp forest ecosystems and 
usually they tend to form clusters and have species-poor associations (Whitten et al., 
2000). The diverse assemblage of forest types are influenced by several environmental 
factors, including the wide variation of soil content as well as the degree of water 
inundation (Yamada, 1997). 
Depending on the degree of water inundation, a thin layer of peat may be 
present on the ground surface of freshwater swamp forests. However, the limited 
decomposition rate of organic matter often results in the development of a peat layer 
only a few centimetres thick (Whitmore, 1984). The slow decomposition rate is 
due to high phenolic concentrations in the leaves, which can be up to three times 
greater than those found in temperate forests (Coley & Barone, 1996). The high 
concentration of phenols is thought to be a response to the high levels of mammalian 
and invertebrate predation as well as fungal pathogens (Coley & Barone, 1996). The 
slow decomposition of organic matter under anoxic conditions causes the release of 
humic acid, which greatly lowers the pH of the water (Page et al., 1999; Goltenboth 
et al., 2006; Yule, 2010; Posa et al., 2011). This then affects the floral composition 
found in swamp forest streams and leads to specialisation of characteristics such as 
pneumatophores (Whitmore, 1984). 
The waterlogged nature of freshwater swamp forests creates soft, unstable, and 
anoxic waterlogged soil that may have led to the evolution of special root adaptations 
in freshwater swamp forest trees that are morphologically similar to those found in a 
true mangrove forest (Corlett, 1986). Special root adaptations such as pneumatophores 
are common within freshwater swamp forests and work by Corner (1978) noted that 
pneumatophores typically occur in five forms. For example, in the genus Sonneratia 
L.f., they grow as upright, elongated, conical pegs whereas in the species Lophopetalum 
multinervium Ridl., they develop as erect planks (Corlett, 1986). Pneumatophores help 
provide stability and aid in gas exchange in the anoxic soil conditions (Corner, 1978). 
