There has been considerable attention paid to the lowland peats of Indonesia, including studies conducted in regions such as Riau, South Sumatra, and Central Kalimantan. However, the country's tropical highland peats have not received similar focus. Found in highlands in Sumatra, Sulawesi, and Papua, these peatlands remain poorly documented and understood.
One of these under-studied highland peats is the Toba peatland, located in the Humbang Hasundutan regency, on the southern side of Lake Toba. Despite being long-acknowledged, the Toba peatland has suffered extensive deforestation of its natural vegetation. Today, many of these areas are utilized for agricultural purposes, including horticulture, coffee, and rice cultivation. Most lands have been cleared, with some areas left abandoned or overrun by sedges and grasses.
These peatlands are primarily located in depressions, with remnants of past vegetation accumulating and remaining saturated over time. The composition of these peats predominantly originates from tree matters, with discernible large, undecomposed woods still visible. The thickness of the peat layers varies from 1 to 3 meters. However, areas that have been exploited are considerably thinner now, measuring less than 1 meter in places.
Notably, a significant area of Lintong Nihuta was previously mined for fuel. To safeguard these peatlands from further exploitation, the local government enacted a regulation in 2013. This regulation stipulates that peatlands deeper than 1 meter must not be utilized, thereby offering some level of protection to these valuable ecological areas.
Historical studies
The occurrence of peats on the Toba highland was noted by German botanist Franz Wilhelm Junghuhn in his publication Die Battaländer auf Sumatra (1847), where he encountered swamps with a coffee-brown coloured water.
The most comprehensive description was by Elisabeth Polak in her book Ueber Torf und Moor in Niederlandisch Indien published in 1933. She examined mountainous peats on the Toba Plain, Telaga Saat in West Java and Dieng Plateau.
Polak mentioned that raised bogs in the tropics are always forest bogs, which are not composed of sphagnum. However, above 1200 m sphagnum is very common.
The following are her description of the Toba peatland:
The Toba Plain (1400 m above sea level). The almost treeless Toba Plain extends around the large Lake Toba. However, it is soon discovered that this plateau was forested in earlier times, as the soil is rich in a type of resinous wood. Trunks, stumps, and root masses are dug up by the local population and used as fuel. The Toba Plain also has some "peat soils"; on closer inspection, these are former dried-up, humus-rich forest soils. The deforestation dating from relatively recent times was already noted by Junghuhn (1847) and later by Ruttner (1930). The steppe-like character of the current vegetation is therefore a secondary feature, resulting after the tree growth no longer protected the soil from drying out and the plants from direct sunlight.
The Toba Plain is considered heath-like and barren by both botanical travelers. Three types of bog-like formations have given rise to this idea:
1. Marshy areas where the ground is slightly sunken and moist, and where sparse scrub, grasses, Cyperaceae, orchids, ferns, and a lot of Nepenthes tobaica grow, with the soil layer completely occupied by Sphagnum. However, peat formation does not occur there; Sphagnum grows almost directly on hard rock.
2. Small ponds in the process of being filled in. Typical boggy areas often covered with Sphagnum.
3. A type of bare peat or humus deposits, partially cultivated as rice fields or covered with herbaceous vegetation. The latter are fossil forest soils, abundant with wood.
Lingtang Nihoeda
At Lingtang Nihoeda, there was a marshy area where a type of peat was visible on the surface. It turned out that the vegetation had been destroyed by fire; the dead layer had a resemblance to Sphagnum peat but was only 50 cm thick. New vegetation was already growing, including Sphagnum, ferns, and Juncus. The area is very moist and soft, and the peat-like mass is still slightly decomposed and has a pale yellow color.
Perhaps this represents an initial stage of peat formation, and the formation is still too young to show distinct peat accumulation and the accumulation of plant remains.
Si Gelapang
A relatively large swamp is found in Si Gelapang, near Si Borong-Borong. The bog is shallow and moist, with the soil layer occupied by Sphagnum. ... The humus layer is less than 30 cm thick. Rüttner (1930) suggests that these swamps formed after the forest was cleared, indicating that the short period of growth is responsible for the lack of peat formation.
Dolok Margoe
At the roadside near Dolok Margoe, there is a small pond with water that is white in color from the washed-out liparite tuff. The depth is only a few meters, and there is a floating mat of vegetation on the water's surface. Under the floating mat, wood remnants can be found, and small shrubs also grow interspersed among the herbaceous plants.
The floating mat is stable but sinks significantly when stepped on by new growth. It seems that the entire ecosystem is still in formation, and eventually, the pond will become completely overgrown. Perhaps a raised bog will form in that case.
Negri Djandi
The ladangs in Negri Djandi, Dolok Sanggoel District, are cultivated on peaty humus. Below the cultivation layer, there is approximately 30 cm of peat composed of reeds and roots, embedded in brown plastic humus. This is followed by a light yellow layer, which, according to the locals, can reach a thickness of about three men's heights. According to the claims of the Bataks, this layer has the highest heat effect, and they use the material as a household fuel for cooking. It is also applied and burned in the ladangs, as the ash is believed to improve the soil. The yellow color of this soil darkens quickly in the air and turns brown. When burned, this "peat" leaves behind yellow ash with many quartz crystals.
Jünguhn likely observed something similar during his journey to the Battalander (1847) when he described that "the clay layer is covered everywhere by a mostly three, often five feet thick layer of fertile, dark brown, lightweight, humus-rich soil. This genuinely vegetable soil, which is true, unmixed humus in many areas and represents the top layer of the plateau, cannot have originated from the progressive decomposition and further transformation of the clay layer beneath it without the involvement of organic-vegetable forces. Instead, it must have formed through the decay of vegetable substances, roots, and woody parts, in a calm succession of centuries, and therefore, its presence indicates the former cultivation of forests in these now bare and treeless grass plains, under the canopy of which the plateau may have been shaded for millennia before the all-transforming hand of humans brought about its demise."