The Sicó/Alvaiázere Special Protection Zone comprises an area of 31,678 hectares, added in 2000 to the Natura Network, the highest point of which stands at 618 metres.
This area classified as limestone is characterised by the diversity of that same bed rock, where the karst surface forms, such as clints, dolines and canyons, form a fascinating landscape, creating a network of caves and reflecting pools.
One of the largest areas of oaks (Quercus spp.) in the country can be found on this territory, in good condition, as well as a considerable patch of holm oaks (Quercus rotundifolia). Rupicoline habitats are of particular note, boasting an abundance of orchids (Orchidaceae spp.) and juniper species, including the Junus valvatus, endemic to Portugal, making the Sicó/Alvaiázere one of the most important sites for the conservation of the calcareous flora. On the banks of the Nabão River it is possible to find, in good condition, rupicoline woods consisting of poplars (Populus nigra var. betulifolia), willows (Salix spp.) and common alders (Alnus glutinosa).
Also on the Nabão lies one of the few places where brook lamprey (Lampetra planeri) can be found, but, due to the limestone relief, it is bats that predominate in the Area, from the greater mouse-eared bat (Myotis myotis), to the greater horseshoe bat (Rhinolophus ferrumequinum) and the common bent-wing bat (Miniopterus schreibersi).
The area is mostly occupied by a forest population, with woods consisting of maritime pine (Pinus pinaster) and oak (Quercus spp.), but there is also an agricultural area, dominated mainly by olive groves and the breeding of sheep and goats, which is essential to the production of Rabaçal Cheese (PDO).