3. FORMATION OF PEATS

3.1 Introduction
3.2 Environmental Factors
3.3 Stages in Development
3.4 Type of Vegetation

3.1 Introduction

Before starting to plan the reclamation of peatswamps it is wise to gain a proper insight into the mode of formation of the deposits and the conditions which have led to their development. The recognition of the present stage of natural peat formation is also very valuable for assessing its potential for agriculture. Discussion of the formation or genesis of peat soils is made easier by first making a distinction between the actual formation of the organic materials, and the process of their accumulation. The former is caused by biochemical processes, whereas the latter is mainly a direct function of the environmental conditions, the climate and ecosystems (peatswamps, bogs or mires) in which the peat is formed, and the climate.

Organic materials only accumulate under certain conditions. For peat to form it is essential that the production of biomass (organic materials) is greater than its chemical breakdown. Not all organic materials are classed as peat. For practical reasons litter, being a special type of organic material, is excluded from our discussion.

Peats are generally considered to be partly decomposed biomass (vegetation). They show a wide range in degree of decomposition. Kurbatov (1968) briefly summarizes 35 years of research into the formation of peat as follows: “The formation of peat is a relatively short biochemical process carried on under the influence of aerobic micro-organisms in the surface layers of the deposits during periods of low subsoil water. As the peat which is formed in the peat-producing layer becomes subjected to anaerobic conditions in the deeper layers of the deposit, it is preserved and shows comparatively little change with time”. According to this theory the presence of either aerobic or anaerobic conditions decides whether any biomass will accumulate and in what form. Distinction is made by Kurbatov (1968) between forest peat which is more aerated and therefore more decomposed, and peats formed under swampy conditions with strongly anaerobic conditions. In forest peat, lignin and carbohydrates appear to be completely decomposed so it generally has a low content of such organic compounds, whereas under swamp conditions peats are characterized by high contents of cutin and the presence of much unaltered lignin and cellulose (Table 3). Actually, Kurbatov’s forest peat is much the same as thick litter deposits.

Table 3 COMPOSITION OF SWAMP AND FOREST PEAT AS % DRY ORGANIC MATTER (source Kurbatov 1968)

Fraction	Swamp peat		Forest peat
	Carex-swamp 30% decomposed	Reed-swamp 40% decomposed	Birchwood 55% decomposed
Bitumen	3.3	1.1	8.8
Humic acids	32.2	33.6	52.2
Hemicellulose	15.0	8.6	1.0
Cellulose	3.5	3.7	0.0
Lignins	12.9	18.6	0.0
Cutin	11.9	5.2	16.0
Not determined	21.2	29.2	22.0

This Bulletin concentrates on the development of peat in swampy conditions, since most peats in the Tropics belong to this type. Anaerobic conditions, which prevent the micro-biological activity needed for the chemical breakdown of organic materials are generally assumed to be largely responsible for the accumulation of partly decomposed biomass in the form of peat. The anaerobic conditions are created by a specific hydro-topography whether marsh, swamp, bog or mire. Properties of such hydro-topographic units depend on many environmental factors, including climate, landform, local geology and hydrology, but they all have severe toxicity others have advanced theories of high sulphur and sodium content reducing oxidation. A high sodium content is also indicated as being responsible for the development of peat in the atolls of the Maldives (Hammond 1971).

Figure 1. Fundamental topo-hydrological situation for peatswamp development

3.2 Environmental Factors

3.2.1 Hydro-topography
3.2.2 Source and quality of water

The process of peat formation as a result of waterlogged conditions is called paludification. The major factors playing a role in this process are discussed below.

3.2.1 Hydro-topography

According to Moore and Bellamy (1974) peat growth is initiated if the water balance at a site is characterized by the equation:

INFLOW = OUTFLOW + RETENTION

INFLOW + PRECIPITATION = OUTFLOW + EVAPOTRANSPIRATION + RETENTION

Fig. 2. Profile of a ridge raised mire (source Moore and Bellamy 1974). The height of the component copulas depends in part on the area of the mire and in part on the climate

Systems producing secondary and tertiary peats are found only in climates in which retention values are high. Such conditions are frequently found in the cool wet temperate and boreal regions of Canada, Eire, Scotland and Northern Europe where peat is encroaching onto the hills forming blanket bogs. In wet equatorial and monsoon climates evapotranspiration is usually too great to cause the development of secondary and tertiary peats unless there is excessive rainfall, well distributed over the year, combined with favourable topography with a complete lack of drainage giving continuous wet conditions. Such conditions are found for example in the coastal lowlands surrounding the Sunda Flat (Malaysia/Indonesia) and in many of the other areas within the tropics listed in Tables 1 and 2. The topography is invariably basin-shaped with natural drainage being blocked by natural barriers. Common types of landscapes include:

i. Saucer-shaped inner parts of islands in river deltas, which are surrounded on all sides by natural river banks or incipient levées.

ii. Lagoons, which at their natural outlet are blocked by marine or riverine sediments.

iii. Cut-off meander bends (oxbow lakes).

iv. Fossil stream beds in braided river systems.

v. Small tributary valleys blocked by mineral or organic debris at their junction with the main river.

vi. Large coastal basins between major streams blocked to seaward by marine deposits (clays with mangrove vegetation, or sand dunes) and along the rivers by riverine deposits (levées).

vii. Depressions in river valleys separated from the main stream by random deposition of alluvial deposits caused by frequent and erratic stream bed changes that are often related to fast and intensive deposition of high silt loads.

The hydro-topography of tropical swamps on high ground as in central Africa (Rwanda, Burundi, and to a lesser extent in Kenya and Uganda) is largely influenced by recent volcanism which has blocked many interior valleys (Floor and Muyesu 1986). Some valleys are blocked by lava flows of very recent age and, because the lava is hard, the basins are difficult to drain. The age of these peat deposits is more related to periods of volcanic activity than to climate changes at the end of the glacial periods. Peat areas at high elevations are generally of small size because large alluvial depressions are rare.

The quantity and nature of the peat accumulating in a depression are very much related to depositional behaviour of the streams affecting the depression. If, for example, streams change their silt load, say seasonally, or there are other longer term fluctuations, the organic materials are contaminated with mineral deposits. Changes in the stream bed can also influence the actual site where mineral deposits accumulate. The author experienced conditions in South East Asia where deep almost pure peat is being covered by mineral deposits because of deforestation of the catchment. Deforestation causes erosion of mineral topsoil and increases the silt load of the river. It also increases the risk of flooding in downstream peat areas.

The admixture of mineral deposits with peat is highly significant for potential use and requires attention when undertaking reclamation.

3.2.2 Source and quality of water

Many peat researchers in the temperate regions hold the view that the mobility of the bog water is the most important factor controlling the edaphic conditions within a swamp (Kulczynski as quoted by Moore and Bellamy 1974, p.56). Before discussing influence of water flow, however, the properties of the water itself are briefly examined.

The type of vegetation and the characteristics of the developing peat depends strongly on the nature of the water which is feeding the ecosystem. Traditionally, eutrophic, mesotrophic and oligotrophic conditions are distinguished. Eutrophic conditions are characterized by neutral reactions (pH of 6-7) and a high content of minerals mainly calcium carbonate. Under oligotrophic conditions there are few minerals, calcium and magnesium are particularly lacking and the pH is low. Mesotrophic conditions are intermediate.

Water in a peat ecosystem can be either eutrophic, mesotrophic or oligotrophic depending on its source. But a gradual change from initial eutrophic conditions to oligotrophic conditions in the final stages of peatswamp development is very common. The sources of water and the swamps related to them can be subdivided into three groups (Kulczynski quoted by Moore and Bellamy 1974, p. 56):

Rheophilous type

These are swamps developing in mobile groundwater. In such cases water flows in from surrounding land and because it is enriched by cations leached from the surrounding soil the ecosystem is eutrophic and the developing organic soils are of the eutrophic type.

Transitional type

In this situation water no longer enters the system by surface flow but there is still some underground inflow from seepage. Amounts of incoming nutrients are therefore intermediate in quantity and the vegetation is poorer and less diverse than under eutrophic conditions. The resulting peat is mesotrophic in nature.

Ombrophilous type

Under these conditions water entering the system is derived only from precipitation and is therefore very low in nutrients. The water is acidified and lacks Ca, Mg and K, and consequently the vegetation is very poor giving rise to the oligotrophic organic peat soils which are extremely low in nutrients.

		Major Ions
		pH	HCO3	Cl	SO4	Ca	Mg	Na	K	H	Total
Hydrological Mire 1
Type	1	7.5	3.9	0.4	0.8	4.0	0.6	0.5	0.05	0	10.25
	2	6.9	2.7	0.5	1.0	3.2	0.4	0.4	0.08	0	8.28
	3	6.2	1.0	0.5	0.7	1.2	0.4	0.5	0.02	0	4.32
	4	5.6	0.4	0.5	0.5	0.7	0.2	0.5	0.04	0.01	2.85
	5	4.8	0.1	0.3	0.5	0.3	0.1	0.3	0.07	0.03	1.70
	6	4.1	0	0.4	0.4	0.2	0.1	0.3	0.04	0.14	1.58
	7	3.8	0	0.3	0.3	0.1	0.1	0.2	0.04	0.16	1.20
Extreme rich fen		7.7	2.3	0.2	0.4	1.8	0.9	0.2	0.02	-2	5.9
Transitional fen		5.8	0.9	0.1	0.03	0.9	0.02	0.05	0.01	-	1.9
Intermediate fen		4.8	0.6	0.01	0.06	0.6	0.03	0.08	0.01	0.02	1.4
Transitional poor fen		5.5	0.1	0.04	0.04	0.1	0.03	0.06	-	-	0.38
Intermediate poor fen		4.4	0	0.03	0.05	0.06	0.03	0.08	-	0.4	0.29
Extreme poor fen		3.9	0	0.06	0.07	0.07	0.02	0.05	-	0.13	0.40
Moss		3.8	0	0.04	0.13	0.04	0.05	0.09	0.01	0.16	0.50

¹ Types 1-7 indicate eutrophic to increasingly oligotrophic conditions
² - denotes less than 0.01 milli-equivalents per litre

3.3 Stages in Development

The various stages that can be distinguished in the development of peat swamps are illustrated in Figure 3 which is based on a model by Moore and Bellamy (1974) who in turn were much influenced by studies of mire ecosystems by Kulczynski. As already indicated the flow of water is extremely important for the type of peat developing, and since changes in water-flow signify the change from one stage to another we discuss the various stages in some detail.

Figure 3. Model of the succession of mire types (source Moore and Bellamy 1974)

Stage 1

During the initial deposition of peat material in flowing water there are two alternative conditions. In the first, there is a large flow of water bringing in an amount of sediment from outside. This, in combination with a slow rate of peat formation because of strong oxygenation of the system through the large influx of water, results in the production of a heavy sinking peat, and the water flow is concentrated near the surface. In the second, there is a small water flow and less material is added from outside so, with a faster rate of peat growth, a light, floating peat is produced and the water flows below a floating mat.

Stage 2

The accumulation of peat tends to canalize the main flow of water within the basin, leaving some areas (hatched in Fig. 3) which are subjected to the effects of moving groundwater during periods of excessive flow only. Again two alternatives are recognized: first, where the whole peat mass is inundated, and second where the peat mass is not inundated and is floating.

Stage 3

The continued vertical and horizontal growth of peat causes the largest part of the basin to be beyond the influence of inflow. Water supply is mainly restricted to rain falling directly on the swamp surface with some seepage from surrounding areas. Only those areas immediately lying along the main drainage tracts within the swamp may show a slow continuous flow.

Stage 4

Continuing peat growth leaves most of the swamp unaffected by moving water but inundation will occur when the water-table in the basin rises as a result of heavy rainfall.

Stage 5

The peat surface has risen so it is no longer affected by seasonal fluctuations of the groundwater. The dome-shaped peat surface possesses its own perched water-table fed by rainwater.

Although this model is based on numerous studies in western Europe and other temperate regions, it can be applied to tropical regions as is shown in schematic form in Figure 4 by an example of the successive stages in the development of deep peatswamps in coastal areas (Andriesse 1974). This is based on field information obtained from surveys in the Sarawak Lowlands, Malaysia, by Anderson (1964) and the author. Here too the development of primary, secondary and tertiary peats can be recognized, and a division can be made into topogenous and ombrogenous stages. Anderson (1964) also provides evidence of former islands of low elevation now completely covered by tertiary peat deposits.

Figure 4 illustrates that in a strong depositional environment, as is often found in a monsoonal or semi-arid climate, the evenly spread accumulating mineral deposits will slowly raise the floor of the basin and prevent complete blockage of drainage. In such cases peat development is either absent or found only in small depressions when favourable hydro-topographic conditions are present.

Many peat deposits in tropical areas show in cross-section the various stages described. The bottom layers are rich in plant species and are in general richer in plant nutrients than the overlying layers. There is generally a gradual impoverishment in the mineral content of the peat, particularly in the major elements, calcium, magnesium, potassium and phosphorus. Depth of peat is therefore an important indicator of fertility. The type of peat, whether topogenous or ombrogenous, gives clues to the fertility to be expected.

3.4 Type of Vegetation

Peatswamps can have very contrasting types of flora. The current vegetation, which is not necessarily the same as that of the past, generally reflects the age or stage of development of the peat and the climate. A vertical cross-section across a peatswamp reveals the succession of plant associations which must be regarded as the original materials of the peat. These layers from top to bottom could, for example, show the following succession: trees; shrubs; grassy perennials (sedge grass, saw-grass) forming a dense mat; large perennials protruding from shallow water and possibly still rooting in underlying mineral soils; rooted aquatics with floating leaves; floating aquatic plants, algae and plankton.

Figure 4. Stages in formation of peatswamps in South East Asia (source Andriesse 1974)

The vegetation layers commonly follow the stages in development recognized in the previous section. During Stage 1 (Fig. 3) algae, weeds and mineral deposits are produced. In successive stages as organic residues accumulate, conditions become more favourable for the growth of reeds, sedges and other perennials which retard water flow further. The diminishing influx of nutrients available for vegetative growth leads to impoverishment of the system and in the later stages of development only the more acid-loving plants are able to survive. The decomposing biomass produces inorganic and organic acids which tend to accumulate in the ecosystem as the neutralizing effect of calcium carbonate in the incoming water from surrounding land is no longer effective. Examples of acid vegetation include specific plant associations dominated by heath, sphagnum moss and many other acidophile plants.

There are numerous papers on the ecological and botanical aspects of organic soils and botanists have developed procedures to identify former vegetation associations by microtome analyses of peat fibres and pollen analysis.

It is beyond the scope of this Bulletin to provide detailed information on every possible vegetation type. It is well, however, to realize that present vegetation cover can be a sound indicator of the development stage of the peatswamp and that the vegetation of the various underlying peat layers can give major clues to the mode of peat formation and its relative richness of plant nutrients.

In classifying peat, use is often made of the nature of original material, this being either moss-like (Sphagnum), grass-like (sedges, saw-grass, papyrus), reeds, bush or forest. For reclamation purposes such distinctions are relevant and they are dealt with under the appropriate heading in Chapter 4.

In conclusion, notes are given on the rate of accumulation of peats. First there is no essential difference between the mode of formation of peatswamps and peats in tropical and temperate areas. In both cases climate plays a decisive role in the dynamics of the processes involved. Because of climate, the rate of build-up of barriers by silt in rivers is greatest in the tropics. Also, the much larger amounts of water generally passing through the tropical systems (rainfall of 4 000 mm compared with say 700 mm in a temperate region) and the seasonal differences in temperature regimes considerably influence water regime.

Apart from the difference in dynamic processes, the kinetics of energy influx and its dissipation are vastly different in the tropics when compared with temperate zones. This has a large effect on the rate of accumulation of peat because biomass production is many times greater than that in temperate regions. On the other hand oxidation and decomposition are also much enhanced in the tropics by higher temperatures. There are numerous studies on rates of peat accumulation and there appear to be many factors involved. Lucas (1982) in a review of a number of studies, indicates that it generally requires between 600 and 2 400 years for 1 m of peat to accumulate with an average of 1 500 years. These studies are mainly related to boreal and temperate climates and indicate varied conditions.

From studies by Anderson (1964) on the forest peat of Sarawak it can be calculated that the deepest layers of peat (4 300 years old) accumulated at a rate of 1 m in 214 years, those 3 900 years old accumulated at a rate of 1 m in 333 years, but those laid down in the last 2 300 years took 455 years for 1 m to accumulate. These figures indicate that peat in tropical climates accumulates at least 3 times as fast as in temperate areas. They also show that, as in temperate regions, the rate of accumulation is related to the stage in development. This is logical since, with increasing impoverishment of the ecosystem, biomass accumulation will be slowed down, and as a consequence also peat accumulation. Tropical peats in South East Asia appear to be mainly of the forest type. The vertical succession in the coastal lowlands (Anderson 1964) is commonly characterized by mangrove species at first (Stage 1) followed by transitional, brackish water communities in later stages. These change to true freshwater swamp communities which in turn are finally replaced by the a poor Shorea albida monostand on the ombrogenous raised peat domes.

Although forest peat is the rule rather than the exception in the coastal lands of the wet tropical belt this is not necessarily always so. As always the type of peat depends on the stage of development, site characteristics and climate. The dominance of forest-type peats in the tropical lowlands tends to be replaced by a Cyperacea type of vegetation (saw-grass, papyrus) when passing to a subtropical climate, whereas sedges and reeds develop almost anywhere depending on the hydro-topography of the site. Peats at high elevations in the tropics, say at over 2 000 m, are generally of a grassy and mossy nature. In Burundi/Rwanda, peat contains Sphagnum and attains characteristics of the peats of temperate regions.