Forests growing on peat
Most people think of peat as a feature of cold northern bogs. But there are also peat swamp forests in the tropics, with large areas in Southeast Asia (notably on the islands of Borneo and Sumatra), and others in the Amazon and Congo basins and in parts of Central America. In these places, a tall forest grows on top of a layer of peat, which is partly decayed plant material that has built up over thousands of years.
Peat forms where waterlogging keeps oxygen out of the soil. Dead leaves, wood and roots fall into standing water and break down very slowly because the microbes that would normally decompose them need oxygen. The material piles up instead of rotting away. The peat layer can eventually reach several meters in thickness, and in some places much more.
Why the water stays
Peat swamp forests often occupy low-lying areas behind coastal plains or between rivers, where drainage is poor and rain is plentiful. Some are fed mainly by rain and are quite acidic and low in nutrients, with water that looks dark, like tea, from dissolved plant compounds. The surface in some peat domes is slightly higher in the middle, so water drains slowly outward.
The peat itself acts like a sponge, holding a large volume of water compared with its solid content. That helps buffer floods and keeps the area wet through dry spells, as long as the peat remains intact.
A great deal of carbon in the ground
The carbon in dead plant material that would normally be returned to the air as carbon dioxide stays locked in the peat. Because the layer is deep, the stored amount per hectare can be larger than the carbon held in the trees above. Tropical peatlands cover only a small share of the planet’s land, but they are regarded as important carbon stores. Exact global totals are uncertain, and published estimates vary with how depth and density are measured.
This puts peat forests among the places that matter most for the global carbon cycle and for soil carbon storage, even though they are small in area.
Wildlife in a wet, acidic forest
The water is dark and acidic, and fish and other animals there are often specialized. Peat swamp forests can hold distinctive tree species, and many are home to threatened animals. In Borneo and Sumatra, for example, they are habitat for orangutans, among other mammals, and for specialized fish and birds. Because the forest is often quite different from nearby dryland forest, it adds to regional biodiversity rather than duplicating it.
What happens when peat is drained
Peatland is often drained to grow oil palm, pulpwood trees such as acacia, or other crops. Canals lower the water table so that roots can grow in the soil. The consequence is that air enters the upper peat layer, and decomposition begins in earnest. Microbes break down the stored organic matter and release carbon dioxide, which continues for as long as the peat stays dry. The process is slow compared with a fire, but it can go on for many years.
Drainage has other effects as well:
- Subsidence. As the peat dries and decays, the ground surface sinks, sometimes by a noticeable amount each year, which can lead to flooding of the farmland in the long run.
- Fire risk. Dry peat burns readily. Fires can smolder underground, are hard to put out, and may continue for weeks. In severe dry years, large fires on drained peatlands have produced widespread haze, which relates to the broader subject of wildfire smoke and its effects on health.
- Loss of forest and habitat. Clearing the trees removes the habitat and the plant material that feeds new peat.
Peat fires release a lot of carbon dioxide in a short time, and the emissions from drained peat are why these forests receive attention in climate discussions, far more than their area would suggest.
Protecting and restoring peat forests
Responses commonly discussed include protecting intact peat swamp forest, blocking drainage canals to raise the water table again (rewetting), planting native species adapted to wet conditions, and preventing fire. Rewetting can slow the carbon losses, but it does not quickly rebuild lost peat, since peat accumulates very slowly. Restoration is also complicated by land ownership, local livelihoods and the cost of monitoring.
Because of this, keeping peat forests wet in the first place is generally considered more effective than repairing them afterwards. Policies on forest carbon, including carbon accounting, increasingly treat peatland emissions as a separate item for this reason, though methods and rules differ between countries and programs.