Swamp Things 1: Death, life and nutrient cycling in the peat swamp forest

Sean again here! For the past year and a half, Weng Ngai and I have been making regular trips to the Badas peat swamp forest of Brunei Darussalam for research as part of a collaboration between NTU ASE, the Institute for Biodiversity and Environmental Research (IBER) at Universiti Brunei Darussalam (UBD), and the Peatflux team from the Singapore-MIT Alliance for Research and Technology (SMART).

But what what’s a peat swamp forest, why’s it important, and what research are we up to anyway?

Me with Sylvia, JC, Nanette and Colton, in front of the Badas peat swamp transects

The quick summary is we’re studying the effects of disturbance on wood and leaf decomposition, as well as the associated saproxylic invertebrate communities in the Badas peat swamp.

But first, what’s a peat swamp?

Shorea albida trees, the monodominant dipterocarp species in the Badas peat swamp forest.

While temperate peatlands such as bogs are usually dominated by flora such as sphagnum moss, tropical peat swamp forests are characterized by towering trees.

Peat swamp forests are differentiated from other tropical rainforests by their waterlogged soil that slows down the decomposition of plant material. The lack of water and nutrient flow into peat forests causes plants to invest heavily in in chemical and physical defenses against herbivores, which also prevents or slows down decay. This results in a thick layer of acidic peat – the accumulation of of partially decayed organic matter. This matters a lot for a couple of reasons.

Carbon

The slow decomposition and buildup of organic material in peat soils and stabilization by waterlogged conditions traps carbon in the ground. In Indonesia, peatlands store about 30% more carbon than the biomass of all other forest types combined. This means that disturbance and destabilization of peat forests release disproportionately greater amounts of carbon into the atmosphere, exacerbating human-induced climate change.

Unique Biodiversity

The unique acidic and nutrient-poor environment in peat swamps have selected for a specific and in many cases, specialized communities of species that are adapted to those conditions. As such, many species of plants and animals that persist in the swamp are endemic, meaning they’re not found in other habitat types.

Hanging pitchers of Nepenthes bicalcarata. The leaching tannins give the acidic water in the swamp a dark, tea-like hue. The swamps are also sometimes referred to as blackwater swamps for this reason.

Swamp associated dragonflies such as the Pixie (Brachygonia oculata) are considered very rare in Singapore, but are relatively common in the Badas peat swamp forest.

From above, we can see that it’s important to conserve healthy peat forests in efforts to combat human-induced climate change and to prevent biodiversity loss. However, the peat forests face threats, mainly the drainage and clearance of swamps for agriculture such as rice and palm oil. We feel the effects on human health through phenomena such as transboundary haze caused by burning of the swamps in Borneo. Do peat swamp species also feel the heat when disturbed and is decomposition, and thus carbon storage, affected?

The fern and sedge areas of the swamp were once stands of trees, but those trees have been felled and the land was previously drained for agriculture, before being overtaken by regrowth.

To answer this question, we worked in 5 parallel transects along a degradation gradient in the Badas peat swamp, and in 3 plots per transect. Here, the outer edges of the swamp had previously been drained and burned, while forests closer to the center of the peat dome are more pristine.

Satellite view of the peat forest working area

Satellite view of the peat forest work area

Schematic of the transects and plots

In each plot, we conducted deadwood surveys where we identified, measured (length and diameter) and labelled fallen branches/trunks of 4 different size classes and 4 observable decay stages. For each piece of wood, we also took wood cores/shavings using a drill to calculate wood density. Measurements would be retaken for the same pieces of wood in a few years to estimate decomposition rate. Having measurements of wood of varying sizes and decay stages would allow us to plot decay vectors. This allows us to estimate decomposition rates at varying sizes/stages that would usually require a long time to observe if we were to just observe the decay of freshly fallen wood over time.

Sylvia measuring the diameter of a fallen tree with DBH tape

How we use a clothes hanger to inspect for cavities for accurate measurements of wood density.

Collected wood shavings are oven-dried before measuring weight, which can then be divided by the volume of the cylindrical drill core to obtain wood density. Since we underestimated the number of tube racks needed, we DIYed some out of cereal cartons. Wood samples were later brought back to singapore for chemical analysis as well.

To further examine decomposition and to investigate the roles of saproxylic invertebrates, we also wrapped leaf litter and blocks of relatively freshly fallen wood in mesh sheets with differing hole sizes. For these, we only used plant material from the monodominant tree species in the Badas peat swamp forest, Shorea albida. The wood blocks were cut into rough cubes of 20x20x20cm, to include possible effects from larger saproxylic insects. In each plot, blocks and litter of each mesh size were deployed in three treatments: Submerged in peat, resting at root level, or suspended about 1.5m off the ground.

The team with suspended treatments of leaf litter and wood blocks

Fenddy climbing a tree to suspend blocks

Weng Ngai deploying blocks for submerged treatment in the tip-up pool of a large S. albida tree.

Obtaining the woodblocks itself was a challenge. We initially thought of buying wood from local sawmills for the experiment, but S. albida is a protected species and the sawmills we visited were not selling wood from that species. Thus, the only solution was to find a freshly fallen S. albida tree in the swamp and cut it up to ensure the wood was all from the same tree for standardization. Thankfully, we were able to do this with great help from the field crew consisting Boy, Pudek, Maieng, Fenddy, Kelvin, Roni and Haji, as well as Jang, Watu and Azlan from the forestry department.

Weng Ngai identifying a suitable tree to extract wood blocks from

Moving the blocks was hard work - each freshly cut block weight about 6-8kg!

Jang making the lengthwise cut on the suspended section of the tree trunk

Sectioning the trunk for cutting into 20x20 blocks

The wood blocks were taken back to the lab at UBD and dried in an industrial drying oven, after which we measured their dry weight and volume (by sumerging them in a tank of water on a weighing scale) before wrapping them in mesh for deployment in the plots.

After this, we (or whoever succeeds us) will return in a few years to take re-measurements to estimate decay.

That’s all for now, but look out for part 2 where I’ll talk about invertebrate sampling, and what it’s like to work in the peat swamp!