Biological Characteristics

Types of Habitats - Forests


Under the heading of "forest" we could bring three types of tree formations. The first of these, the planedeciduous forests constitute the potential vegetation of a large part of the territory proposed. The other two, the sclerophyllous forests and alluvial forests, represent formations able to colonize marginal environments with similar and particular mesoclimatic or edaphic conditions, which allowed them to compete with the dominant deciduous forests. Actually, sclerophyllous formations are but small traces of more thermophilic forest facies. They are linked to the bottom of more temperate valleys where counted Mediterranean sclerophyllous and optimal elements remain: Quercus suber, Quercus ilex subsp. ballota, Arbutus unedo, Cistus populifolius, Genista falcata, Echinospartum barnadesii subsp. Dorsisericeum, etc. Instead, alluvial forests, which are a gradual transition between riparian vegetation and planedeciduous forests, have a relatively wide area of potential distribution in the floodplain of the middle basin of Miño

A. Deciduous forests

The current coverage of native deciduous forests represents less than five percent of the total area, which shows the degree of human disturbance in the area, given that this type of formations constitutes the potential vegetation of most of the territory.

The biogeographic position of the area, mainly divided between two different corological sectors, as well as the diversity of environments determined by altitudinal range, entails the existence of several forest types with differential characteristics (Rivas-Martínez, 1987; Izco, 1996). In the hills and mountain areas of the north, in the Galician-Asturian sector, the predominant climatophilic forest type is constituted by acidophilic oak woods of Quercus robur, although there are small points where the game of exhibition, topography, altitude, etc. allows the installation of Portuguese oak of Quercus pyrenaica and montane birch. The presence of formations called Castanea sativa is now reduced. In the past it was a species favoured and managed by man as a fruit tree, but its area has lately subsided by the harmful effects of root rot, as well as massive reforestation with fast-growing species .

The Galician-Asturian acidophilic oak woods typically have a well-developed tree canopy, basically comprised of Quercus robur, although sporadically Quercus petraea and Quercus pyrenaica can also be present. Trees listed as companion trees Castanea sativa, Betula alba, and, in areas of higher altitude, Sorbus aucuparia. The shrub layer is composed of juveniles of species that make up the canopy, as well as a number of shrubs or small trees such as Frangula Alnus, Corylus avellana, Pyrus cordata, Erica arborea, Ilex aquifolium, Crataegus monogyna, along with Arbutus unedo and Laurus nobilis in more thermal zones. In the case of tree canopies with a remarkable degree of disturbance, other species typical of bushland degradation appear (Erica arborea, Ulex sp., Cytisus scoparius, etc.). Below the canopy, a dense cover of herbaceous species is often found: Brachypodium sylvaticum, Holcus mollis, Pseudarrenatherum longifolium, Stellaria holostea, Anemone nemorosa, Euphorbia amygdaloides, Polygonatum verticillatum, etc., and pterophytes: Blechnum spicant, Dryopteris dilatata, Dryopteris affinis, Dryopteris aemula, Osmunda regalis, Athyrium filix – foemina, etc. Other common elements are some small-sized woody species, such as Vaccinium myrtillus or Ruscus aculeatus, and climbing species: Hedera helix and Lonicera periclymenum or, in warmer areas, Tamus communis and Clematis vitalba.

In Galician-Asturian and Orocantabrian Pyrenean oak woods, settled in oligotrophic soils on siliceous substrates, Quercus pyrenaica dominates the tree layer occasionally accompanied by other species such as Quercus robur and Castanea. Together they form a tree layer that in little disturbed forests, presents itself with a dense coverage. Late sprouting of white Pyrenean oaks and the fact that these formations occupy more xerophyte stations than previous oak woods, makes the floral composition of the undergrowth significantly different, with a smaller representation of pteridophytes and nemoral elements. As species of shrub Frangula alnus and Pyrus cordata predominate, while in underbush strata plants such as Physospermum cornubiense, Lathyrus linifolius, Holcus mollis, Melampyrum pratense, Stellaria holostea and Teucrium scorodonia abound.

The Galician-Asturian montane birch grow on siliceous oligotrophic soils. Physiognomically, they are characterized by being a bit thick tree formation and having a low height (12-14 m). The tree strata appears dominated by Betula alba and Sorbus aucuparia, with an occasional presence of other species such as Quercus robur or Ilex aquifolium. In the undergrowth are common, among others, Erica arborea, Vaccinium myrtillus, asturiensis Narcissus, Narcissus triandrus, Deschampsia flexuosa, Luzula henriquesii, Saxifraga spathularis, Melampyrum pratense, Blechnum spicant, Dryopteris aemula and Dryopteris dilatata. Their presence within the territory proposed is relegated to some shady valley headwaters, where the rigour of the climate and soil acidity restricts the development of oak woods.

In the central and southern part, within the domain of the Galician-Portuguese biogeographical sector, the most representative forest formations are, once again, acidophilic oak woods. Of course, with a peculiar floral composition, the differentiates them from the Galician-Asturian sector. Within these forests, it is possible to distinguish between those with a thermophilic trend, occupying the hill territories, and those in the mountainous areas, with floral elements characteristic of a less thermophilic nature. In locations where the slope and the hillside orientation induces a certain xericity in soils, forests dominated by Quercus pyrenaica appear, similar to those present in the northern area.

The hill-montanous Galician-Portuguese oak woods developed on siliceous substrates, and present the Quercus robur tree stratum as the dominant species, often accompanied Betula alba, Castanea sativa and Quercus pyrenaica. The bush stratum consists of shrubs or small trees as Frangula Alnus, Ilex aquifolium, Pyrus cordata or Crataegus monogyna, while underbush elements such as Saxifraga spathularis, Melampyrum pratensis, Omphalodes nitida or Euphorbia amygdaloides, stand out.

The Galician-Portuguese oak wood hills are located in the southern part of the territory, at points of lower elevation and general more tempered conditions than before. The main difference is then based on the abundance or presence of elements of a more thermophilic and/or xerophilic character, both within the tree stratum (Quercus pyrenaica) and bush (Laurus nobilis), as with respect to the underbush vegetation (Ruscus aculeatus) and lianas (Tamus communis).

The presence of oak woods is limited to some hill or montane sunny spots, of marked and well-drained slopes. They have a floral composition akin to oak woods, but with a better representation of certain species of heliophilous character, as some legumes (Cytisus striatus, Ulex europaeus), tree heath (Erica arborea) and bracken (Pteridium aquilinium).

Finally, in the southeast quadrant, and in the western foothills of the Cantabrian Mountains Range, hill-montaine oak woods appear, similar to the ones in the northern territory, hill-montane Pyrenees oaks and montane birches. In any case, the presence of the latter is almost incidental given the under-representation of the mountain side within the proposed territory. The hill-montane oak woods present as the main species of the tree stratum the Quercus petraea and hybrids thereof with Quercus robur and Quercus pyrenaica. The presence of other species such as Betula alba, Sorbus aucuparia, etc., depends on the edaphic conditions and the exposure of the slopes. The bush stratum species include Ilex aquifolium and Corylus avellana, along with ericaceous and leguminous plants, whose coverage depends on the density of the tree canopy. As frequent undergrowth species we have Luzula henrriquesii, Saxifraga spathularis, Oxalis acetosella, Deschampsia flexuosa, etc., and pteridophytes such as Drypoteris dilatata, Dryopteris affinis and Blechnum spicant.

Besides these tree formations, there is another group of formations with a more restricted distribution, for instance, hazels, holly trees, maple trees, etc. Their whole area is very small, and they are confined to marginal positions.

B. Alluvial forests

The central section of the Terra Chá includes the channels of larger entity of the territory represented by four sections of order four (Ladra, Parga, Támoga and Azumara) that, upon binding, form a single stream of order seven. River systems reach their greater complexity here, with channels that can exceed 60 meters in width, and delimited by strongly incised terrigenous edges, which usually exceed 50-70 cm and occasionally reach 2 meters of height. In turn, the channels often have beds of sand and gravel that favour the installation of a complex aquatic vegetation (Luronium natans, Nymphoides peltata, Ranunculus fluitants, perfoliatus Potamogeton, Potamogeton crispus, Callitriche stagnalis, Isoetes longuissima, Apium graveolens), and important invertebrate populations, among which various bivalve species (Margaritifera margaritifera, Anadonta cygnea, Unio tumidus).

The wanderings of the main river stretches through the soft, wide plains of Terra Chá has favoured the creation of meanders and abandoned arms, as well as various types of island structures like "chain of tree islands" consisting of alders or willows which take root directly or in the channel forming linear structures. These formations are collapsed and periodically altered by winter floods. More stable islands are the terrigenous islands, often originating from wandering meanders, such as the islands of Meilán, Seivane or Rábade, the latter 5 km long and 350 m wide.

Excluding strictly riparian vegetation, flood forests belonging to the classes QUERCO - FAGETEA and ALNETEA GLUTINOSAE. The most abundant and widespread correspond to those fed by superficial water, in which the flooding basically depends on spills from fluvial channels and on the runoff contributions linked to annual rainfall maxima. 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.