Trás-os-Montes and Beira Alta

Abstract

The Trás-os-Montes and Beira Alta region grosso modo includes the northeastern quadrant of Portugal as far as the Mondego River Platform, enlarged further north by neighbouring Spanish territories. Acid and phosphorus-poor palaeozoic schists intruded by variscan ganitoids are the prevailing lithological types. Its relief is characterized by the widespread occurrence of uplifted planation surfaces, the majority of which are between 600 and 900 m in altitude, dissected by deep river valleys, sometimes interrupted by larger tectonic basins. The supratemperate or supramediterranean, mainly sub-humid to humid, plateaus are the climactic domain of climatophilous, mesophilous, deciduous mesoforest of Quercus pyrenaica. Old forests are not only species-richer, but share complex, forest-dependent vegetation mosaics. On the Mondego platform prevails a Potential Natural Vegetation of Q. robur subsp. broteroana forests. The mesomediterranean upper dry to lower sub-humid valleys and basins harbour the most remarkable forests of the study area: one association of semi-deciduous forests of Q. faginea subsp. faginea, and six associations of perennial oak woodlands, three of them dominated by Q. suber, and the other three by Q. rotundifolia. Heathlands and gorse heathlands are the most conspicuous substitution stages of Q. pyrenaica and Q. robur subsp. broteroana Querco-Fagetea sylvaticae woodlands; Cistus shrublands are seral of Quercetea ilicis evergreen and semi-deciduous woodlands. Also diverse and with a clear-cut bioclimatic control are riparian and tempori-hygrophilous forests. The geobotanical interpretation of regional grassland vegetation complexes requires particular caution. They include several oligotrophic grassland types, dominated by therophytes, Agrostis sp. pl. or Nardus stricta, among other species. Mesoscale physiography, tree shading, grazing and hay cutting have a strong effect on hay-meadow vegetation complexes. Recent changes in hay-meadow management promote grass abundance and the extension of Arrhenatherion grasslands. The mafic and ultramafic metamorphic rocks are, simultaneously, the most singular lithologically, and the habitat of the most remarkable phytocoenosis of the study area. Herbaceous weed, ruderal and road vegetation types reflect the widespread oligotrophy of the regional soils. This chapter ends with a brief resumé of the series, and geoseries of the Trás-os-Montes and Beira Alta.