Principales retos en la gestión de relaciones comunitarios

There are several schemes for classifying peat soils and peatland environments, which will be examined in detail in Chapter 5. However, it is important to clarify some of the basic distinctions that are widely recognised and adopted, particularly in the UK, as this is where the case study sites are located.

3.3.1 Overall divisions

There are multiple alternative classification schemes for peat, depending on, for example, where in the world you are researching or whether you are approaching the peat from a hydrological or biological perspective (Lindsay 1995; Haslam 2003, 57-103; Koster & Favier 2005). The definitions are by no means settled or agreed upon, and there are variations, for example between soil scientists, and biologists in the way they see things. Most follow a basic distinction between liminc and terrestrial peat, as outlined in Figure 3.1, but designate further subtypes, perhaps based on the

development sequence of the landforms, or the plant community it supports, depending on the emphasis of their respective discipline.

In essence, there are three types of peat, which form two ‘superclasess’ of peat soils.

Limnic peats are formed in lakes or slow moving water when organic matter is transported into the lake and falls to the bottom, gradually building up. Mires are the other ‘superclass’ and are either telmatic, where peat forms under swampy conditions with partially submerged vegetation, or terrestrial, where the peat forms at or above the high water level (Burton & Hodgson 1987). Mires with terrestrial peat are sometimes referred to as ombrogenous, as they are rain-fed. Topogenous and soligenous peats can be liminc or telmatic and may develop terrestrial raised bogs (Burton & Hodgson 1987; Koster & Favier 2005). These are formed due to the accumulation of water under the influence of local topography. A mire is also defined as ‘undrained virgin peatlands with living peat forming vegetation’ (Koster & Favier 2005, 161), though in the UK this can also includes peat in recovery, such as at Shapwick Heath.

With reference to the UK, Mires are conventionally split down into a further two classes, and three subclasses. Minerotrophic mires are fed by water supplied from ground water run off and rivers and they form both telmatic and terrestrial peat.

These are commonly called ‘fens’ in the UK. The water can be oligotrophic (nutrient poor), eutrophic (nutrient rich) or mesotrophic (in-between). Ombrogenous mires are fed by rain water and so are usually oligotrophic and form terrestrial peat. They split into two subclasses; raised bog, which forms in a depression and then grows above the ground level and blanket bog, which forms over a land surface where net

water input is greater than the drainage capacity, resulting in a waterlogged layer. In the UK the Soil Survey uses soil types, which are related to the mire types discussed above, but highly specific to local areas.

Koster & Favier (2005) give us four alternative main ways that mires are classified:

Ecological- based on nutrient content, plant communities or some other biological classification.

Geogenetic- based on the landforms the mire develops in, as outlined above- this is the basis of the scheme used for the lowlands in the UK

Hydrogenetic- based on a combination of position in the landscape, water input and peat forming processes related to it. There are 8 types, based on research in Germany.

Hydrogeomorphic- This scheme has a very long list of peat-bog types based on topography, water sources and formation processes. It is more descriptive than categorical.

As has been mentioned above, the Soil Survey of England and Wales, in their Gazette of lowland peat in England (Burton & Hodgson 1987) describe specific soils and layers using soil terminology and classifications, but they also have a classification scheme for mires, which is essentially the geogenetic scheme, with adaptations for the UK.

The Gazette is the main reference for lowland peat and mire locations in England, yet in archaeology a much simpler distinction is made, simply that between upland and lowland peat. In the MAREW report, this distinction was not explicit, but based on the schema used by the Soil Survey of England and Wales (Burton & Hodgson 1987) which has a cut-off between the two types at 200m OD; the soil survey only

considered lowland peat, which excluded ombrogenous mires almost entirely. This is not as arbitrary as it seems, it is the rough height above sea level at which rainfall inputs will exceed outputs via run off, through flow and evapotranspiration, thus distinguishing between ombrogenous mires and the lowland types which tend to be topogenous. This simple binary distinction is used throughout the archaeological literature, but particularly by English Heritage in terms of research frameworks and

planning (Darvill 1987; Howard-Davis et al. 1998; Olivier & Van de Noort 2002; Van de Noort et al. 2002a; Webster 2004; Hodgson et al. 2005).

Finally, it is important to remember that though slow to change, peat environments are dynamic systems, changing over time and in response to external pressures (Dincauze 2000, 335). Mire types almost never occur in isolation; it is possible that one mire type might overlie another, or represent a vegetational climax, as in the case of raised bogs over old lacustrine systems. Furthermore, particularly in the lowlands, distinctions between lowland peat and other wetland systems and sediments are not always straightforward; there is likely to be a zone of interaction between the two types of deposit, with complex interleaving of sediments.