Borneo Peat Swamp Forests - Ecology

Ecology

About 62% of the world’s tropical peat lands occur in the Indo-Malayan region (80% in Indonesia, 11% in Malaysia, 6% in Papua New Guinea, with small pockets and remnants in Brunei, Viet Nam, the Philippines and Thailand. They are unusual ecosystems, with trees up to 70 m high - vastly different from the peat lands of the north temperate and boreal zones (which are dominated by Sphagnum mosses, grasses, sedges and shrubs). The spongy, unstable, waterlogged, anaerobic beds of peat can be up to 20 m deep with low pH (pH 2.9 – 4) and low nutrients, and the forest floor is seasonally flooded. The water is stained dark brown by the tannins that leach from the fallen leaves and peat – hence the name ‘blackwater swamps’. During the dry season, the peat remains waterlogged and pools remain among the trees.

Despite the extreme conditions the Borneo peat swamp forests have as many as 927 species of flowering plants and ferns recorded (By comparisom a biodiversity study in the Pekan peat swamp forest in Peninsular Malaysia reported 260 plant species). Patterns of forest type can be seen in circles from the centre of the swamps to their outer fringes which are made up of most of the tree families recorded in lowland dipterocarp forests although many species are only found here. Many trees have buttresses and stilt roots for support in the unstable substrate, and pneumatophores and hoop roots and knee roots to facilitate gas exchange. The trees have thick, root mats in the upper 50 cm of the peat to enable oxygen and nutrient uptake.

The lowland peat swamps of Borneo are mostly geologically recent (<5000 years old), low lying coastal formations above marine muds and sands but some of the lakeside peat forests of Kalimantan are up to 11,000 years old.

One reason for the low nutrient conditions is that streams and rivers do not enter these forests (if they did, nutrient rich freshwater swamps would result), they only flow out of them, so the only input of nutrients is from rainfall, marine aerosols and dust. In order to cope with the lack of nutrients, the plants invest heavily in defences against herbivory such as chemical (toxic secondary compounds) and physical defences (tough leathery leaves, spines and thorns). It is these defences that prevent the leaves from decaying and so they build up as peat. Although the cellular contents quickly leach out of the leaves when they fall, the physical structure is resistant to both bacterial and fungal decomposition and so remains intact, slowly breaking down to form peat (Yule and Gomez 2008). This is in stark contrast to the lowland dipterocarp forests where leaf decomposition is extremely rapid, resulting in very fast nutrient cycling on the forest floor. If non-endemic leaf species are placed in the peat swamp forests, they break down quite quickly, but even after one year submerged in the swamp, endemic species remain virtually unchanged (Yule and Gomez 2008). The only nutrients available for the trees are thus the ones that leach from the leaves when they fall, and these nutrients are rapidly absorbed by the thick root mat. It was previously assumed that the low pH and anaerobic conditions of the tropical peat swamps meant that bacteria and fungi could not survive, but recent studies have shown diverse and abundant communities (albeit not nearly as diverse as dry land tropical rainforests, or freshwater swamps) (Voglmayr and Yule 2006; Jackson, Liew and Yule 2008).