Biological Characteristics

Types of Habitats – Peatlands


According to the categories established by the Habitats Directive, the existing peat complexes within the proposed territory could be differentiated, depending on their genetic and ecological characteristics into three main groups: "Blanket Bogs", "Raised Bogs" (high peatlands) and "Fens" (low peatlands). The distribution of each of these types of peatlands is clearly determined by mesoclimate and substrate conditions, so that in order to be fairly descriptive it could be useful to distinguish between the two major landscape units: peripheral mountain areas, compared to large plains and inland valleys.

A. Wetlands of mountain areas

The predominance of vertical forms characterising mountainous territories may limit the development of wetland environments where there is some morphological constraint that allows storage of water contributions of water, whether from rain, runoff, or phreatic. Such formations could be included under the general heading of topogenic wetlands. However, weather conditions can be determining for themselves in the appearance and development of certain wetlands, regardless of the morphological characteristics of the territory. This occurs in regions of high rainfall and especially where there is a balanced annual distribution, often offset by concealed precipitation (in a fog or dew form) during the months of lower contribution, which significantly reduce evaporation and promote the development of ombrogenous wetlands.

The ombrogenous or rainwater wetlands correspond exclusively to the so-called blanket bogs (blanket bog, cover). Since they are developed mainly from rainwater and concealed precipitation (dew, fog, etc.), they are peatlands separated from regional groundwater flow or runoff, and therefore, they constitute a highly distinct group from water recharge wetlands (Ramil - Rego et al. 1996a, 1996 b, 1996 c).

The arrangement of the northern mountainous ranges acts as a barrier against the progression of wet fronts from the ocean and confers this area a high rainfall. In anticyclonic conditions, mists and dew provide the mountain range with peculiar climate conditions, considerably softening the evaporation and favouring the persistence of wetlands. The identity of this fog layer is perceptible from the most inland areas of the Terra Chá as a cloud blanket that covers the highest elevations of the mountain, harbinger of warm and dry conditions. By contrast, it determines a rigorous climate in the mountains, characterised by a wet fog that covers most of the day, and by the constant wind.

The blanket bog areas that maintain a continuous deposition of peat, are now covered by a dense and homogeneous meadow with Carex and other Cyperaceae, among which Eriophorum angustifolium dominates, with the northwestern Iberian endemic Carex durieui. Both species give name to a characteristic association: Carex durieui - Eriophoretum angustifolii, which also includes different gramineous plants: Molinia caerulea, Deschampsia flexuosa, Agrostis curtisii, that contribute to form a cespitose mantle interrupted by the presence of isolated individuals belonging to Calluna vulgaris and Erica mackaiana, etc. It is less frequent the presence of Carex echinata, Hypericum elodes, Parnassia palustris, Viola palustris, Narthecium ossifragum, Drosera rotundifolia, etc.

The retraction of blanket bogs caused by weather changes, favoured the development of other types of vegetation in these mountains: broad-leaved, deciduous forests, dry scrubs, wet heaths (dominated by the Iberian endemic Erica mackaiana) and raised bogs.

The raised bogs are either uptake or discharge wetlands. That is, they have an hydrological source not exclusively from rain, although in part of the bog there can be subsystems (sphagnum moss bogs, bulges) whose development is exclusively ombrogenous. At the discretion of the Habitat Directive, these peat complexes are classified as raised bogs, because of position of the phreatic layer, higher than the one of the surrounding terrestrial environment throughout the year. Except for the ombrogenous bulges, rainwater contributions are less significant than those from the runoff, and groundwater emergences. The no dependence on rain enables these wetlands to appear in lower rainfall areas, but they are conditioned by the existence of geomorphological features that favour the uptake of groundwater or surface water (mixed wetlands), and prevent or hinder drainage.

On the basis of morphological criteria, existing discharge peatlands in the territory proposed have topographies that can vary between the Raised Bog or the Fen (low peatland) types, although in many systems peat topography is complex and favours the proliferation of transition stages between the two types. Within the general concept of the absorbing peatland, a typology based on genetic and morphological characteristics that includes several main types represented in the proposed territory could be established:

The plant communities present in the entire set of active raised bogs are varied, and are suited to different stages in the dynamics of the wetland. They existed from pioneering eras or stages, to stages characteristic of turfophile and hygroturfophile biotopes phases. The onset of peat formation may be related to the existence of puddles or areas that receive a certain influx of water, on which pioneering communities are developed: Sphagnum discontinuous tapestries (Sphagnum papillosum, Sphagnum auriculatum, etc.), which favour the establishment of hygrophilous graminoid plants, mainly Eriophorum angustifolium and Molinia caerulea, but also Rhynchospora alba, Eleocharis multicaulis, Juncus bulbosus, Drosera rotundifolia, Drosera intermediate, Narthecium ossifragum, etc. These pioneer formations, which are included within the association Drosero Intermediae-Rhynchosporetum albae (Fernández Prieto et al., 1987), are frequently attached to edges of temporary ponds, to places where superficial water contributions flows diffusely and areas degraded by erosive processes.

Other pioneer formations are assigned to the community of Sphagnum pylaisii. This community appears linked to more or less flooded, bare holes of turfophile wetlands or hygrophilous heaths. It is characterized by the dominance of Sphagnum pylaisii, species within the Iberian Peninusla, located almost exclusively in a few locations in this area: Cadramón, Veiga de Samarugo and Coto do Bispo, accompanied by Carum verticillatum and to a lesser extent by Molinia caerulea, Carex panicea, Eriophorum angustifolium and the endemic Arnica montana subsp. atlantica, Carex durieui and Ranunculus bulbosus subsp. Gallaecicus.

In mature stages, in which a genuine turfophile vegetation is developed, pioneer formations tend to be gradually replaced by denser communities dominated by a continuous carpet of sphagnum. In these environments, the periodical accumulation of plant debris causes the elevation of the surface of the wetland in relation to the original relief. Thus, we can see a complex microtopography comprising bulky tesserae, delimited by others more or less flattened or depressed. These bulky or bulten forms, which can reach over 200 cm of power in relation to the basal level of the peat and 30-50 cm with respect to the flattened areas, cover tens of square meters in the northern mountain ranges, and are locally called “inlló”.

The most extensive inllós are conditioned by the positive influx of phreatic water that, ascending by capillarity from the dead remains of Sphagnum, favour the superficial development of vegetation. As the inlló progresses in height, the plant growth becomes and more more dependent on rain, mist and fog contributions. This dynamics favours a gradual replacement of more hygrophilous plants by turfophile ombrotrophic species. On the surface of the inlló, there is dense and tight turf noted for its undulating configuration and for presenting maroon, pink or reddish colours, due to the dominance of Sphagnum capillifolium and Sphagnum subnitens. Besides, there are other abundant bryophytes: Sphagnum papillosum, S. tenellum, S. compactum, S. auriculatum, Odontoschisma sphagni, Kurzia pauciflora, Calypogeia spp., Leucobyrum juniperoideum, Racomitrium lanuginosum, Dicranum scoparium, Hynum spp., Pholia spp., etc., while spermatophytes are rather scarce: Carex durieui, Drosera rotundifolia, Drosera intermediate, Erica mackaiana, Calluna vulgaris, Myrica gale, and, sporadically, Salix repens , Salix and even Vaccinium myrtillus atrocinerea and Ulex gallii.

All the flat and curved peatland communities, with a predominance of hygrophilous Sphagnum, are included within the association Erico mackaianae-Sphagnetum papillosi. They are characterized by their homogeneity and the presence of the endemic sedge Carex durieui. When water oscillations are more pronounced and the thickness of peat is lower, the inlló develops dense carpets of mesohygrophilic Sphagnum interspersed between the peat and hygrophilic heather. These formations correspond to the association Erico mackaianae compacti-Sphagnetum, and give the inlló a characteristic green or yellowish-green colour due to the presence of Sphagnum compactum, Sphagnum tenellum and Sphagnum auriculatum.

A higher level of evolution is marked by the extension of these wetlands, progressively covering the outer areas of the socket, causing the interconnection of different peatlands through a mosaic of heathland (Gentiano pneumonanthe-Ericetum mackaianae) and other hygrophilous formations (Arnicetum atlanticae , Sphagnetum pylaisii).

The largest area of raised bogs is located in the granitic massifs of Buio and Toxiza, which by the action of socket alteration present a heterogeneous system of depressed areas, connected some, isolated others, where rainwater accumulates and the one from superficial and groundwater sources. These raised bogs of alveoli tend to acquire elevated topography in their mature stages, consisting of a chain of cords or bumps ("tetos"), depressions and small channels, whose spacial distribution leads to very characteristic topographies. In large granite alveoli peat accretion does not always forms a convex relief, but at times it tends to cover the entire basin, while reducing the entity of the chord system, forming raised flat bogs. However, bulging shapes abound in the flattened bottoms from the headers of the river valleys of the interior of the mountain system (valley bottom bogs). The widest representation corresponds to the source of Eume, in Veiga do Real or Chan do Eume, at the foot of the peak Xistral .

In the areas of higher altitude, raised bogs are often mixed with wet heaths often occupying ancient deposits of blanket bog, so the ecological transition with these is imperceptible in many cases. The old peaty sediment favours the management of wet heathland in areas of lower rainfall, but where hidden rain still has a relevant action. The raised bogs occupy small depressions, some of which were inherited from the glacier morphogenesis (glacier shutter bogs), but the peatlands that form in areas of rupture of the water table (hillside seepage bogs) are more abundant and extensive, which free the water previously collected by blanket bogs.

The set of existing wetlands in the foothills of the Range of Xistral constitutes a unique ecosystem, the last vestige of the landscapes that dominated the European Atlantic territory during periods of increased climatic stringency. The diversity of landscapes, habitats and species associated with these wetlands, as well as the traditional uses that man has made of these means, provide a great uniqueness to this mountainous territory, where the most biodiverse turfophile wetlands are found in the European SW nowadays.

In the rest of the mountain territories, due to its distance to the sea, wetlands always have a topogenous origin. Their number and distribution is strongly conditioned by the availability of morphology conducive to the accumulation of water, a fact which determines that their number and especially its extension in relation to the northern ranges is significantly lower. Thus, various types of peatlands (raised bogs and fens) and hygrophilous environments are, in these cases, humid islands within large areas of scrub and dry grasslands.

B. Wetlands of the plains and inland valleys

Opposed to the mountain landscape, this second landscape entity is characterized by the horizontality of its terrain. It encompasses a mosaic of plains and river valleys, where large horizontal spaces occur: valleys, flat areas and sedimentary depressions, structured around the upper and middle basin of the Miño. Its distancing from the coastal area causes seasonality of rainfall, so that it prevents the development of ombrogenous wetlands. But its morphology promotes the uptake, transport and storage of rainfall inputs, promoting the existence of a complex system of topogenous wetlands, whose genesis and operation dependent on superficial or underground water flows.

Excluding riparian environments, the remaining wetlands in the Terra Chá can be classified into two large groups according to the origin of the dominant water flow. The most abundant and widespread correspond to those fed by surface water, which basically depends on the flooding river channels spills and runoff contributions. The flooding becomes more superficial and temporary in the broad alluvial plains formed on impermeable substrates, mainly clays. After a flooding period, maintaining humid conditions is facilitated by the existence of a water table located in the proximity of the soil surface. Along with wetlands that have a direct dependence on surface water flow are others in which this contribution is reduced and offset by the groundwater flow.

The first step between the peripheral mountains and the inner depressions is marked by “bocarribeiras” (extensions of territory between the riverbank and the mountain), in contact with the wide central plain of the basin. In it we find little embedded channels of orders 3-4, feeding areas or large hygrophilous or hygroturfophile: "gándaras", "veigas", established in depressed areas in which impermeable sediments grow.

In the Terra Chá, the underground contributions usually have a local nature, a fact that determines the existence of semi-temporal hygrophilous environments or small ponds in which the conditions and aquatic communities are drastically reduced, or may even disappear in the summer. If the underground supply is of greater importance the wetland will become permanent, leading to the formation of shallow ponds and lagoons (25-75 cm) of stable nature, often surrounded by more or less extensive hygrophilous or even hygroturfophile environments.

Under undisturbed conditions, underground flow lagoons often present a fringe of marsh forest and formations of large sedges grown on organic sediments, which mark the transition with hygrophilous and climax forests of the territory. The presence of limestone material of clay nature between the Tertiary sediments of the basin, endow the waters of some wetlands with distinguishing characteristics, as well as with plant communities and animals of their own, which are internationally designated as "Fen". Most of the fens of Terra Chá correspond to small ponds rich in charophytes, with extensive coastal fringes consisting of marsh reeds of Cladium mariscus, along with bulrush (Typha latifolia). More externally we can find fringes of wet heathland dominated by Erica tetralix, Genista anglica, Erica vagans or, less frequently, by marshy forests.